NATIONAL COMPANY, INC.
HRO - COMMUNICATION RECEIVERS - "The Cream of the Crop"
PART 1 - Design and Production History ~ The HRO Models
PART 2 - Serial Numbering Analysis ~ Dating by Serial Number
Engineering Changes ~ HRO Coil Details, PS Details, Accessories
PART 3 -
Servicing the PW-Gear
Drive & the PW-D Micrometer Dial
Alice Bourke was a reporter for the Chicago Tribune along with being very active in ham radio.
NATIONAL COMPANY, INC.
"The Cream of the Crop"
|Without a doubt the finest creation to come from National Company, Inc. was their HRO Communications Receiver. Introduced in October 1934 (first production run January 1935 with first deliveries in March 1935,) the HRO featured incredible performance capabilities coupled with an anachronistic, almost scientific instrument appearance that certainly appealed to the technically-minded ham. The host of accessories necessary for complete operation included a power supply, a speaker and four plug-in coil sets. The HRO main tuning dial was unlike any other - a non-illuminated micrometer device that displayed numbers behind small openings as the dial was rotated. Each of the coil sets had graphs that could be correlated to the micrometer readout to determine tuned frequency. Much of the HRO's engineering seemed to contradict everything that was happening with contemporary receiver design. Why did a receiver that seemed to defy then-modern communications receiver evolution become such a favorite of hams, the military and commercial users? By providing absolutely the best low noise front-end resulting in high sensitivity coupled with an incredibly well-designed tuning system along with tremendous bandspread capability, giving the user the ultimate advantage when it came to working rare DX or coping with challenging band conditions. - H. Rogers, April 2007|
History of the HRO Design
The HRO design owes much to its predecessor, the AGS receiver. The AGS was developed to fulfill a contract with the Department of Commerce for modern receivers for airports in 1932. The initial receiver designed for the contract was designated the RHM. It is likely that Herbert Hoover Jr. and his design team were involved to a certain degree in the electronic design of the RHM while National provided the mechanical design and assembly. The final product was a high quality, high performance receiver built from the best parts available at the time. The accuracy of the Type-N vernier dial was excellent and the receiver's sensitivity quite good. Since the contract was only for a handful of receivers, National decided to also produce the RHM as a civilian communication receiver called the AGS. Though expensive, National felt there must be a market for a high performance receiver, even during the Depression. Within a short time, National upgraded the AGS with different tubes and different calibration procedures in an effort to make the receiver easier to produce. By late 1933, the ham version of the AGS, the Single Signal AGS-X, made its appearance. The AGS-X had a crystal filter, amateur bandspread coils (optional) and a front panel adjustable BFO. At nearly $300, these receivers were for affluent enthusiasts and very few were sold. By 1934, AGS coils for 10 meter operation were being offered. Meanwhile, the commercial users (mainly airlines) were complaining about the design quirks of the RHM-AGS. Three coils were difficult to handle during a band change and the receiver had to be turned off to facilitate a coil change since there was no standby switch. Additionally, the Type-N dial was beginning to slip on these receivers as the lubrication dried up on the rack and pinion drive to the tuning condensers. A redesigned AGS appeared for commercial users, designated the AGU, featuring a coil assembly that carried all three coils as a unit for easy band changing. The vernier dial was changed to a National Type-BX (similar to the dial used on the SW-3.) Again, tube line up was changed but it was becoming apparent that the design was aging rapidly and a new receiver was going to be necessary.
|In fact, some of the airlines never used the AGS-AGU receivers because they felt the design was not sophisticated enough for their requirements (Transcontinental and Western Airlines in particular.) A modern superheterodyne with double preselection was needed. Herbert Hoover Jr. was selected by the Bureau of Aeronautical Commerce to coordinate the job of designing a replacement for the AGS receiver. He was living in Pasadena, California and teaching part-time at Cal Tech. Hoover, of course, contacted James Millen at National Co., since the creation of a "sophisticated" design was going to require the expertise that National had gained building the AGS receivers. Hoover setup a lab in his garage, employing Howard Morgan from Western Electric Co. and a few of his technicians to develop the new receiver circuitry. The new receiver would be a team effort with engineers working on the project on both coasts.|
In order for the "new" AGS replacement to meet the requirements of commercial users, Millen and Hoover thoroughly examined the shortcomings of the AGS/AGU receivers and additionally, examined all of the necessary requirements for an ultimate communications receiver. The two design teams - James Millen, National's Chief Engineer and General Manager, headed the mechanical design team which was located on the East Coast in Malden, Mass. While on the West Coast, Herbert Hoover Jr. headed the electrical design team. Both men agreed that the greatest sensitivity and a low noise floor came from using plug-in coils, eliminating the many types of signal loss found in most bandswitching circuits at that time. Also, to keep other problems (like hum and heat) out of the receiver an external power supply would be used. Maximum efficient operation of each tube stage would keep the number of tubes to a minimum and resulted in a good signal to noise ratio and low drift due to reduced thermal problems.
Double shielding would be used on the coils for frequency stability and the coil set would be located at the bottom of the receiver, away from heat. For the additional ham market, a bandspread option on the plug-in coils that had been popular with the SW-3, FB-7 and the AGS and would be continued with the HRO - National was not going to exclude the very profitable ham market. Dial accuracy was a difficult problem to solve and required tight specifications on many of the components used in each receiver. >>>
| The tuning condenser drive used a spring-loaded split-gear
driven by a spring-preloaded worm gear eliminating any
backlash. The dial itself was based on a Sperry Gyroscope design that
had the main dial rotate an internal readout dial via an elliptic hub.
The 0-500 readout dial was viewed through the top window of the external dial's five
windows and, with ten revolutions, the micrometer dial mechanism had the
equivalent resolution of a linear dial 12 feet long. Accuracy of the
reset ability would be incredible. More design necessities were double preselection on all coil sets, a front panel adjustable BFO, a crystal
filter, a S-meter and a front panel stand-by switch (B+ switch.)
Towards the end of development, Millen personally
delivered a prototype HRO to Hoover's lab in Pasadena for final
revisions and final testing. At this time, a drift problem turned up
when the HRO was used in the bandspread mode. The corrections utilized
combinations of brass, steel and aluminum mounts for the trimmers in the coil
assemblies to reduce the problem. However, with the resolution the
micrometer dial had in bandspread a minor drift of a couple of
kilocycles meant that drift showed up as maybe five or six divisions on the micrometer dial.
The drift problem was reduced to an acceptable level but it could not be eliminated due to the
extreme resolution the HRO had in bandspread. The HRO
was designed to use mostly parts that National made. Very few purchased parts were
utilized for the HRO. As National stated, "The National HRO receivers
are not an assembly of broadcast receiver parts, they are completely
designed from antenna to output." In all,
the HRO design leapt ahead of any other receiver being built in
Prototype HRO Receivers - When National published their bulletin No.240 (a catalog that was inserted in the October 1934 issue of QST) it contained details on the HRO and revealed artwork of a "prototype" HRO (which is shown in a photo above.) The "HRO Prototype" receiver did have a micrometer dial but note that the knob is the type used on National's Type N dial (used on the AGS.) The receiver had AGS style knobs installed. Also note the prototype had a "Volume" control and a BFO switch on the front panel but no front panel BFO adjustment. Also, the coil sets only have one graph which implies that they don't have the band spread function. Absent also is an S-meter switch. Though the artwork of the interior shown in Bulletin No.240 doesn't show the internal BFO knob on the BFO coil can, it was shown in subsequent artwork and photos of other prototypes. These prototypes were never production receivers and were built for testing and development purposes, for artwork or for subsequent photographs.
How the HRO became the HRO and not the HOR
The original published story for the origin of the HRO designation related that all of National's inter-departmental paperwork for the receiver project was stamped "H.R.O." which stood for "Hellva Rush Order" since the time table for the receiver development was a "rush order" type of project. For many years this was the story related in National advertising and it sounded believable. However, after James Millen left National in 1939, he corrected the story as follows:
The original development paperwork was usually marked "H.O.R." - for "Hell Of a Rush" but during the finalization phase, someone at National decided they didn't want their new receivers to be referred to as "HORs" so the letters were rearranged and became HRO - then the "Hellva Rush Order" story created to explain the HRO designation.
Well,...that's Millen's story anyway.
Photo right shows James Millen around the time that the HRO was being developed.
The HRO Models
When first introduced, the receiver was the "HRO." No other designation was necessary since it was the only version available. The receiver used nine, 2.5vac heater tubes providing two RF amplifiers, two IF amplifiers, separate Mixer and Local Oscillator functions, a Duplex Diode-Pentode for AVC, Detector and First Audio Amplifier functions, a Beat Frequency Oscillator and a pentode Audio Output Amplifier. The separate power supply utilized a type 80 tube rectifier that actually brought the tube total to ten.
Typical of most communications receiver manufacturing, progressively later models will have several minor improvements added as production continued. During early production both the rack mounted versions with 3/16" thick aluminum panels (finished in a textured "crackle finish" that resembled leatherette) were produced along with the steel sheet metal table model (painted with standard wrinkle finish.)
A specific power supply wasn't available for the early HRO receivers, so many receivers were sold with the FB-7 power supply, type 5897AB with the data plate emblazoned "Designed Especially for the FB-7." Many rack mounted power supplies were also produced for the commercial users. Speakers were not specifically available in a table cabinet but National advertising stated that a "loud speaker could be provided, if desired." The first advertised speaker shown was a rack mount version.
Shown in the photo to the right is a rack mount HRO from the first production run with the serial number of D-65 with its matching serial number coil set. Note that this receiver has all of the features that are found on the earliest production. D-65 has the pearl-button push switch for the S-meter and the small red "NC" dial pointer mounting screw. Also obvious are the white background frequency charts on the coil sets. The S-meter has the first type scale used with the pink colored NC diamond with white letters. The S-meter scale is 0-5 with "PLUS" indicated in the range over 5. This S-meter scale was only used on the first two production runs. The German Silver plated micrometer dial was used on the first five production runs. Notable is the leatherette finish on the panel and on the coil set panels (this finish was used on the rack mount receivers up until around 1940.) The four coil sets are matching serial number originals. Also, note the lack of a finger lift projection on the lid of the dust cover which was standard for rack mount receivers. This receiver was probably built around January of 1935.
Close examination of the HRO D-65 revealed that it must have been sent back to National for upgrading, probably around late-1936 or early-1937. The upgrades included a pilot lamp installation, BSW terminal strip for remote standby (included lengthening of the chassis opening and dust cover cut-out as part of the installation,) a set of new knobs for RF Gain, AF Gain, Phasing and C.W. Osc (knobs have the tall boss) and circuit upgrade to the 2B7 Detector, AVC and 1st AF amplifier to conform to the later model HRO. Since the S-meter and pearl button switch weren't changed, the receiver wasn't sent back to National so late as to have these problem-prone parts replaced. All of the rework is factory quality which suggests that D-65 was returned to National for the upgrades. Note that the pilot lamp has been removed and the hole filled to make this first production run receiver appear more like an original example. The top of the chassis is still very original and the underside of the chassis has some replacement components that suggest normal repairs and not any type of rebuilding effort. Fifteen of the paper wax capacitors are still original Micamold brand.
photo above: HRO sn: D-65 (rack mount,) first production run built
in January 1935
||The first HRO production run built a little less than
100 receivers. The second production run, run-E, was going to build
substantially more receivers. The total number of E-run HRO receivers is
a little less than 200 receivers with the highest reported serial number
from run-E being E 178. Very few changes were incorporated into this
second run of receivers. The location of the S-meter zero adjustment pot
was changed from the front left top of the chassis to near the audio
output tube shield and the antenna-ground terminal insulator. The
S-meter itself sometimes has minor differences in the housing style used
during run-E. The Audio Gain control and 2B7 grid bias circuit changes
may have been installed in the later part of run-E. The plated PW-D, the
S-meter scale, the pearl push-button S-meter switch, the small red <NC>
diamond dial pointer, the white charts, the lack of remote standby and lack of a pilot
lamp all remained the same as run-D receivers.
Shown in the photo to the left is the second production run HRO E-50. This receiver is a table model version so the front panel is made of sheet steel and is the same width as the cabinet. Table model cabinets and front panels were painted black wrinkle finish. Coil set panels were normally painted to match the receiver panel they were to go with. HRO E-50 spent most of its existence in Alaska where for a time it was used by the CAA whose technicians extensively modified and repaired the receiver. Today, E-50 is a complete and authentically restored example of the second production run of HRO receivers.
HRO E-50 is profiled in more detail in the Restoration section in Part 4 this web-article.
|Shown in the photo to the right is an early third production run
receiver, serial number F-16, another rack mount version probably built
in April 1935. With the third run, a red jewel pilot lamp was now
mounted to the right side of the panel between the Selectivity control
and the PW-D giving users a visual indication that the receiver was
turned on. Note that the S-meter scale was changed with the new scale
having the NC diamond in orange with the "NC" in black letters. Also,
the "PLUS" that was under the arc above 5 was
eliminated. This scale was used from run F to run H. Another S-meter change was the elimination
of the pearl push-button switch which was now replaced with a black metal push-pull
operated switch. This allowed users to have the S-meter operational
without having to hold a button in while tuning an AM signal. During CW
reception the S-meter could be turned off. Also, with receivers that
were built during the early part of the
third run, there still was no remote standby capabilities. Remote
standby was added later during run-F. Additionally, the small red
<NC> emblem for the dial pointer is still being used with this
particular receiver but its use was becoming sporadic beginning in the
latest part of run-E. Many F-run receivers will have the newer style
dial pointer that was a much stronger part. Still in use with early
third run receivers were the white background calibration charts on the
coil sets although the black charts replace the white charts during
run-F. With run-F, the coil set contact buttons were enlarged and raised
for better engagement with the finger contacts within the coil bay.
F-16 has not been restored,...it was "detailed." The process is covered in the Restoration section in Part 4 of this web-article.
||Shown to the left is fifth production run HRO, H-103, which is another
table model so it has the standard
sheet metal cabinet and front panel that is entirely painted black
wrinkle finish. Several changes are apparent when looking at this HRO
receiver built in September 1935. The black
background frequency charts on the coil set panel were introduced during
the third production run (run-F.) Moisture may have caused
staining on the white charts that prompted the change to black charts.
Gone is the small red <NC> emblem as part of the dial pointer. The
replacement is a much stronger, "one-piece" screw that has a diamond
shaped head. The small red <NC> emblem was fragile since the small
diamond head was machined so thin that it easily broke if
over-tightened. H-103 still has the early-style short boss bar knobs and
the R-5 S-meter that was introduced in run-F. The other changes aren't
so visible. Remote standby was introduced during run-F. A two terminal
strip was added to the rear chassis to allow remote switching of the B+.
The remote switching initially paralleled the front panel B+ switch but
this didn't allow using the front panel B+ for coil changing operations
depending on the remote relay. Within a few production runs, the remote
standby was rewiring to be a series switch that allowed use of the panel
B+ switch regardless of the remote relay set-up. H-103 is profiled in detail
in the Restoration section in Part 4 this web-article.
By the end of the first five production runs (D, E, F, G & H,) the HRO had several "tweaks" made to its design. These changes were for the most part subtle with values of a few resistors changed slightly, the wiring of the Detector-1st Audio biasing and AF Gain control revised, some changes to the material that was used a insulation mounts between each section of the tuning condenser, addition of a remote standby circuit, replacement of the pearl push button S-meter switch and the addition of a visual power on indicator.The sixth production run, run-J, would bring several visually obvious changes to the HRO. Gone would be the plated PW-D dial in favor of a painted lacquer version. The cabinet would also now feature improved ventilation with louvers on the side and enlarged holes on the rear. The S-meter scale was changed from the old QSA 0-5 scaling to the RST scaling of 0-9. The S-meter housing was changed from metal to bakelite. These "J-run" receivers were advertised as the "1936" HRO and by February 1936, the "HRO" became the "HRO Senior."
The HRO's design was much more advanced than any other communications receiver available in early 1935. James Millen, using his monthly QST letter to hams, had the ham interest piqued and both commercial users and hams rushed to buy the first HRO production available. Of course, the HRO performance became legend quickly and, if the receiver was affordable to a depression-era enthusiast, it was purchased. The production runs were very small and by the introduction of the HRO Senior in 1936, about 1100 HROs had been produced within the initial five production runs.
Features Found on the Earliest HRO Receivers - Shown below are some of the external parts and manufacturing variations that are found only on the early HRO receivers, that is, the receivers built in 1935. Photo "A" shows the German Silver plated Micrometer dial. This dial was used for the first five production runs (D, E, F, G & H.) Photo "B" shows the "NC" emblem that is used with the dial pointer. This emblem was exclusively used on the initial production run (run-D.) It was then used for most of run-E and sporadically on runs-F and G alternating with the "diamond" emblem (see HRO Senior below for close-up photo of the later <NC>.) Photo "C" shows the pearl push-button S-meter switch that was used only for two production runs, runs-D and E, along with the "raised-rounded flange" S-meter only used for the E-run. Additionally, the S-meter shown has the early 0-5 S-Units scaling that was used on runs D & E. By run F, the "PLUS" was dropped and the NC diamond was changed to red with black "NC." Photo "D" shows the white background frequency graphs that were used for three production runs, runs-D, E and F. Photo "E" shows a small dial-knob with the short boss (left) used on run-D through part of run-J. The small dial-knob to the right has the taller boss used from production run-J-on. Photo "F" shows the black painted chassis that was found on production runs-D through L. This photo also shows the round IF and BFO shields (cans) that was used from runs-D through P. Photo "G" shows the early cabinet ventilation holes that were used from production run-D through H. (photos B and C are by Gary Halverson K6GLH of his HRO SN: E-159)
The HRO was an expensive receiver priced at about $200 with all of the accessories necessary for operation. In the January 1936 issue of QST, James Millen indicated that he had been in contact with many hams who were HRO owners - either through letters or actual visits to their ham stations. He came to the conclusion that most hams didn't use the HRO to its full capabilities. Most hams never used the Crystal Filter. Many never switched the coil sets to amateur bandspread. Most hams were on CW at that time and never used the S-meter. Millen also indicated that some letters from hams actually inquired if they could order the HRO - minus specific circuits they believed they wouldn't need - at a reduced price, of course. Millen thought that offering a budget-priced HRO that eliminated these unused circuits and parts might be a good seller. Certainly, it would be a way for the Depression Era ham on a strictly limited budget to get an HRO. National announced the reduced-cost HRO in February 1936, dubbed "HRO Junior." Although the official announcement was in February, the existence of HRO Junior SN: J-125 indicates that National was building the Junior in late-1935. There also is a Junior that was built for American Airlines with the serial number J-248. The implication is that the Junior was available somewhat before the official Feb.1936 announcement in QST. >>>
photo right: HRO Junior sn P-116 from August 1936 with its original P-116 JD coil set installed. Note that this receiver has a gun-metal gray lacquer on the PW-D micrometer dial. Some PW-D dials will have a dark bronze color lacquer paint. Note that the pilot lamp location is different than the HRO Senior. Also, the location of the phone jack is moved since there is no S-meter switch (or S-meter.) Also obvious is the centrally located frequency graph on the coil set. Since the Junior coils didn't bandspread only one graph was necessary. Later (mid-1938 on) HRO Junior receivers will have the National ID tag in the upper right corner of the front panel but the receiver is identified only as "HRO" - "Junior" is not on the tag.
|>>> The HRO Junior didn't have a crystal filter. This
modification required that the Crystal Filter assembly be replaced with
a standard IF transformer. Elimination of the S-meter also eliminated
the meter push-pull switch and meter adjustment pot circuitry. Naturally, a
different front panel was going to be required since the meter wasn't
used. National also moved the location of the pilot lamp slightly to the
right (vertically inline with the RF Gain control) and moved the phone jack slightly to the left
(vertically inline with the AF Gain control.) The Junior used the
same chassis as the regular HRO, obvious because the S-meter pot
mounting hole is present as are the mounting holes for the Crystal
The coil sets for the "Junior" did not have the bandspread feature and
were identified with a "J" proceeding the regular coil set letter designator.
Since "J" coils couldn't be set to bandspread only one tuning
chart was needed and it was centrally mounted on the coil panel. The "Junior" was usually offered for $99 from most of the discount
dealers but this price only included one coil set. Millen recommended
equipping the "Junior" with two coils sets, the JA and the JC for the CW operator giving coverage of 20 and 10 meters on the JA coil and
40 and 80 meters on the JC coil. For the AM phone op, the JA and JD coils gave
160 and 80 meters (JD) and 20 and 10 meters (JA) - at that time (pre-WWII) 40 meters was a CW only band.
Of course, a purchaser could order as many coils as they wanted although
the idea of the Junior was to keep the cost down by utilizing only what
was going to be absolutely necessary. >>>
photo left: The chassis of HRO Junior P-116 showing the absence of the Crystal Filter and S-meter. Note that the chassis is punched for the S-meter adjustment pot which was located next to the Antenna-Ground terminal insulator. This Junior has the gray chassis paint which started to be used in early 1936. The IF cans had just changed to the square type with the "P" production run.
>>> The HRO Junior was not a "reduced cost" HRO in the sense that it would have been built using cheaper parts. Rather, the Junior used all of the standard Senior parts and assembly techniques - just the S-meter, the Crystal Filter and the bandspreading coil sets were not used. The cost was less than the standard HRO because certain circuits and parts were not installed - circuitry and parts that some hams or other users might find superfluous to their needs. Since the Junior had almost everything a Senior had it was natural that National offered to upgrade a Junior after purchase (at a later date - perhaps when the owner had saved up the money) for a very reasonable price. Perhaps some Juniors were sent back to National for conversion to an HRO Senior though there doesn't seem to be much evidence that it happened too often. A new front panel would have been required along with the Crystal Filter assembly and S-meter with the S-meter switch. Then there was the problem of the J coil sets. Although the conversion was offered, it must have been done at a "loss" for National, who probably realized that very few Juniors would ever be returned for the conversion to Seniors.
The HRO Junior wasn't very popular and today it is quite a rare set in its original configuration. Most thirties-era hams probably realized that purchasing the Junior wasn't that "good of a deal." The HRO Senior was around $170 with four bandspreading coil sets included. Most dealers would sell the HRO Senior with some money down and monthly payments to follow. The Junior was $99 with one general coverage coil set. Just adding the three coil sets to provide full frequency coverage would run the price up to about $150 and then these coils wouldn't bandspread. The price would be even more if bandspreading coils were desired. For only a few dollars more, the HRO Senior provided many more features. Power supplies and speakers were considered extra accessories for both types of receivers. >>>
photo above: "In 1936,..." an early advertisement for the Junior's introduction. The artwork example (prototype?) shows the plated PW-D that was probably never used in production. The earliest example reported, J-125 from late-1935, is equipped with a gun-metal lacquer finished PW-D.
|>>> Most "Juniors" encountered today are really RAS or RBJ Navy
receivers,...rack mounted HRO Juniors with some modifications for military use.
Like Millen had observed with the hams, the Navy found their radio operators had little use for
an S-meter, Crystal Filter or Bandspread coil sets. Apparently, American
Airlines also felt the Junior was more of what they wanted and some rack
mount HRO Juniors were supplied to them. These HRO Juniors are
identified with double "AA" preceding the HRO, i.e., AA-HRO.
photo right: HRO Junior SN: Y217 from mid-1938 showing how the ID tag only has "HRO" as the receiver type. This fine example belongs to Geoff G3YVF who supplied the photo.
From February 1936 on - coinciding with the official HRO Junior introduction - the standard HRO was afterward referred to as the "HRO Senior." The Senior suffix is usually applied to receivers built from 1936 to 1941 (starting with run-J, which was advertised as the "1936 HRO" even though the receivers were built in late-1935.) From 1936 to just before the WWII, the HRO went through several minor physical changes. Most noticeable came with run-J. This was the change from the German Silver plated PW-D micrometer dial to a black lacquer dial. The black paint color on the PW-D micrometer dials varies from dark gunmetal gray to dark bronze. There doesn't seem to be any chronology to the variability of the subtle shading which indicates that the paint mix variability was just part of the production process at National. Under intense light the PW-D will show its true color shading (as in the photos shown) but in normal room illumination most of the paint shadings appear black. The inner dial readout also changed from black numerals on silver to white numerals on black paint. The cost savings were realized by Doehler (the manufacturer of the PW-D) in that they could now use castings that had pits and other defects that would be "filled" by the lacquer paint. Also, since the index lines were going to be filled with paint, they were widened slightly so the paint fill would appear the correct width when finished (the earliest painted dials had the narrow index lines.)
Other changes were to the chassis color, probably to conform with the gray paint that was being used used on the new 1936 NC-100 receivers. This cost savings was realized in that only the small quantity SW-3 receivers continued to have the black chassis paint. Starting in January 1936, most of production now used gray paint for all chassis.
By mid-1936, the IF transformers were changed to different adjusting screws with slots instead of hex heads and the shields were changed to square rather than the earlier round shields. Again, the cost savings was that the the HRO would conform with the NC-100 series, although better quality IF transformers were probably also involved in the changeover. >>>
photo above: HRO Senior sn: N-130 was built in July 1936 (10th production run) and has the features found in the early HRO Seniors. Most noticeable is the bakelite housing, white face S-meter (that is non-illuminated on early versions) and the painted PW-D. Note that the paint on this PW-D is the dark gunmetal gray lacquer. The cabinet (from run-J on) has side louvers and large vent holes in the rear panel.
|>>> In the December 1936 issue of QST, National ran an ad on the back-inside
cover showing a rack mount HRO finished in gray "leatherette" finish.
The photo below shows a table rack designated MRR and a combination speaker
and coil storage box designated SPC. The ad states that the rack mount
HRO is available, "Your choice of finish, either rich grey or black
leatherette." (note the British spelling of gray.) A close-up of the
receiver shown in the ad to the left. This ad ran several times in QST from 1936 up to around 1938.
Note the bound QST magazines on the book shelf. The gray wrinkle
finish was also an option on the table model HRO (serial number V-27 is
original gray wrinkle finish panel and cabinet from 1937.) Besides this
advertised color option, HRO receivers have been found with original
paint in blue (special orders to RCAF Canada) and smooth machine
grayish-blue (possibly other Canadian users.) Other colors probably
exist since National would do special orders and had the in-factory
Another minor change at the time was to the remote standby terminals which now placed the "BSW" terminals in series with the panel B+ switch requiring a jumper be installed across the terminals if the receiver was used without a remote T-R switch. This seemingly minor change actually now provided the operator of a transmit-receive station the ability to remove the B+ independent of the T-R switch - a function required for safe coil set removal and installation.
The S-meter had been a Marion Electric Instrument Company bakelite housing meter with a non-illuminated white scale with 0-9 S-units in black and the NC diamond in red. By 1937, this S-meter became an illuminated meter. The next change (early 1938 - possibly by run-W) was to an illuminated light-yellow scaled meter. The S-meter also added "db above S-9" to the scale in red printing.
Finally, an identification tag was added to the upper right corner of the front panel in late-1938. The tag identifies the receiver as "TYPE HRO" - Senior is never shown on the tag (the same is true for the 1938 and later HRO Junior.) This was the first time that the actual receiver model is physically identified in any manner. This completed the physical evolution of the HRO from 1936 to 1938. From 1938 up to WWII, the HRO looked the same except that some very late HRO Seniors will have a bar knob for the selectivity adjustment. >>>
photo left: Gray HRO Rack Mount from 1936 QST Advertisement. In the 1941 National catalog, the HRO-R, SPC, PS, MRR combination was referred to as the "Deluxe HRO" or HRO-C
By the later thirties, most of the HRO receivers were being equipped
with 6.3vac tubes. One of Millen's QST letters (1937) had pointed out
that the 2.5vac heater tubes were preferred by National since the 6.3vac heater
tube were subject to producing modulated hum. In 1939, the
697 power supply was introduced featuring a 6.3vac heater winding with
sufficient current to operate the HRO and packaged in the "dog house" style
cabinet. At this time, the older 2.5vac tubes were all but eliminated from the
production HRO receiver. Millen also recanted his former opinion of the 6.3vac
tubes in 1939, when the 697 power supply became available. There was
some speculation by former National employees that the actual reason for
the preference for 2.5vac tubes was that National had over-stocked the
2.5vac filament winding HRO-type power transformers and these really
weren't useable anywhere else. >>>
>>> The story goes that National kept up the 2.5vac tube performance "myth" active until they depleted their over-stocked transformers. Many HRO owners did re-tube their older 2.5vac HRO receivers with 6.3vac tubes. More than likely this was due to tube availability rather than performance improvement.
About this time, May 1939, James Millen resigned from National. The reason sited at the time was so Millen could form "James Millen Manufacturing Company" but other factors may have been involved. See "HRO Production & Engineering Changes" section (1939) below for several various reasons sited for Millen's departure from National.
|Features Found on Later Pre-War HRO Senior Receivers - Probably the most obvious difference between the early HRO receivers and the later HRO Senior is the lacquer finished micrometer dial that made its appearance with production run-J (1936). The painted micrometer dials are shown in photo A and B. Photo A shows the gunmetal gray lacquer and Photo B shows the dark bronze lacquer. Photo C shows the early bakelite housing S-meter that is non-illuminated at first but later was illuminated and has the S-9 scale on a white meter face. These meters were being installed by run-J and are used up to around run-W (early 1938.) Photo D shows the later Marion Electric bakelite illuminated S-meter that was installed beginning with run-W (early 1938.) Typical of the later Marion S-meters, this example shows considerable fading of the red scale, "DB OVER S-9" and the scale face itself has darkened somewhat from the original light-yellow color. Also, note that this photo shows the black pull-switch that replaced the pearl push-button with run-F (1935.) Photo E shows the "raised diamond" pointer-mounting screw that started to replace the "NC" emblem mounting screw as early as production run-E (1935) but was intermixed with the "NC" emblem until run-H. Photo F shows the black background coil frequency graphs that were installed during the latter part of the third production run (run-F.) Photo G shows the improved ventilation of the new cabinet introduced with run-J (1936.) Photo H shows the early gray painted chassis with round IF-BFO cans and the fiber board Antenna-Ground terminal mount (gray paint beginning run-L, 1936.) Photo I shows the later gray chassis with square IF and BFO cans (square cans beginning run-P.) Also, note in Photo I the polystyrene Antenna-Ground terminal mount which was introduced very late in production, around run-suffix F (1939.) Note: photos A, C and H are of N-130 (1936). Photos B, D, E, F, G and I are of 463-K (1940.)|
The HRO for Airways Communication
|Since the initial design of the HRO was to replace the
AGS-based receivers used by some Airways companies, it's not surprising
that the HRO found its way into companies like Braniff, Eastern Airlines and American
Airlines. From 1935 up to 1937, National provided Airways companies with
HRO receivers for use at airports and for airway navigation uses. In
1937, National introduced the RCD, a receiver based on the new NC-100
moving coil receiver that National had introduced in 1936. The RCD
provided specific airways features like squelch, relay controlled speaker selection and
audio bandpass filters and single audio amplifier output sections designed for either 600 Z ohm
lines or 20000 Z ohm speakers that had internal output transformers. No
S-meters, no crystal filters and most importantly, no external power
supply. Undoubtedly, the Airways companies found the HRO, with its
numerous idle coils and separate power supply, somewhat
difficult to install and maintain. The RCD and its successors were
self-contained receivers only requiring a loud speaker or telephones.
After WWII, National upgraded many earlier versions of their airway
receivers. In 1947, the last moving coil airways receiver based on the NC-240CS
(the RCR) was released.
Shown in the photo to the right are the HRO receivers that were used at Eastern Airlines' Miami Radio Station. This station was basically the aeronautical communications to various airplanes providing weather, flight information and other necessary communications to "in flight" aircraft. Note that there are "banks" of HRO receivers but notice the three receivers near the rear-most operator. The top receiver is the HRO Junior. Note that while the HRO Seniors all have nickel PW-D dials, the Junior has a dark painted dial. Date is probably 1936. Photo is from George Sterling's "The Radio Manual - Third Edition" dating from the late-thirties.
The HRO During WWII
Prior to WWII, the Navy was buying some HRO receivers for various uses. These receivers generally will have a National Audio Output transformer installed in the chassis area behind the S-meter and adjacent to the antenna terminals. This area of the standard chassis already had mounting holes and lead thru holes for an audio transformer, implying that National anticipated some customers requiring this option. This would especially be true for many commercial users as well as the military. In fact, some National catalogs do mention that any audio output configuration could be provided. The Navy wanted 500 ohm Z outputs for their requirements and the National transformer usually installed is a National Type S500. Also, these pre-WWII Navy HROs will usually have an "anchor" ink stamped somewhere around the chassis or chassis mounted parts. Other than the audio configuration changes, the pre-WWII Navy HROs are standard production types and even have the standard serial number placement and format which implies that the Navy purchased them "as needed" rather than by a large quantity contract.
Perhaps the HRO's most famous use during WWII was in England where banks of HROs were set-up as intercept receivers at various sites. Generally, most reception stations were separated from the transmitting sites and the decoding sites were separate from either the reception or transmitting sites. At first, Britain couldn't buy the HRO receivers directly, so various methods were used to purchase the receivers. Usually, British officials in the US on business would purchase an HRO receiver from a dealer and hand carry it back to England. This rather tedious method lasted until Lend-Lease was passed at which time then the British were able to have a steady supply of HROs direct from National. Many of the HROs sent via Lend-Lease were identified with a double letter prefix to the serial number, e.g., PP or AJ, etc. It also appears that single "P" prefixes with numbers above 150 seemed to indicate HROs destined for duty overseas.
The HRO-M & HRO-MX - The HRO Senior was given a few engineering upgrades at the end of 1941 or early in 1942. The most obvious was the modification of the crystal filter to use an internally mounted crystal, thus eliminating the "easy to remove" crystal that plugged into the top of the filter assembly. There was always questions that arouse regarding the plug-in crystal in that the "air gap" required for the crystal seemed to be interpreted as something "loose" and "rattling around" in the crystal. The new design crystal eliminated that unwarranted concern and didn't change how the Crystal Filter operated. This type of crystal filter had been introduced in late-1940 with National's NC-200 receiver. After that change, National referred to the new HRO Senior as the HRO-M or MX. During WWII, the HRO-M was improved with ID rings added to the AVC and B+ toggle switches. Many HRO-M receivers will have a 0-1mA scaled S-meter made by Marion Electric. Initially, these mA meters were only for the receivers being sent to England but later many military HROs had this meter installed. The S-meter "pull-switch" was replaced with a black-finish, ball-handle toggle switch. The HRO-M used the same glass tubes as it predecessor, the HRO Sr.
photo right: Inspection tag from WWII HRO-M sn PP-988 showing the various signatures and dates for the operations to complete the receiver. The HRO-M is owned by G3UWP- Robin Pickering, who found the tag wedged between the chassis and the cabinet of the receiver. Full dates show June 30, 1943
photo above: The U.S. Navy RBJ-2 receiver featured 50kc to 400kc and 480kc to 30mc coverage with nine coil sets.
The RAS, RBJ & RAW, etc. - The U.S. Navy wanted a simple to use receiver and National supplied HRO Juniors in fairly large numbers as the RBJ, RAW and the RAS (possibly other designations were also used.) The RAS was a special receiver that had a 175KC IF to allow tuning through 400KC to 500KC range without interruption. The lower end of the frequency coverage was 190kc. The RAS models also have special coil sets for 175KC IF operation and are identified with a special number code. The RBJ individual coils (four in each coil set) are also apparently different than the standard HRO because National assigned them a different number code.
The Navy receivers were normally rack mounted and usually were supplied with anywhere from five to nine coil sets that were housed in a coil storage box that was rack mounted. The RAS came with seven coil sets while the RBJ was supplied with nine. Unlike the HRO which used letters to ID the coil sets, the RAS and RBJ used a number ID, 1 to 7 for the RAS and 1 to 9 for the RBJ.
The power supply was a rack mounted type and the power cable from the receiver was a shielded cable (even though cloth covered.) Loud speaker panels were generally not included in the rack since nearly all Navy operations required headset reception by the radioman. During the RBJ, RAW and RAS production the plug-in coil set panels were changed from 3/16" thick aluminum to 1/8" steel panels. Small ID tags are mounted between the frequency graph and the logging chart for coil identification. In fact, all of the Navy HRO receivers have a multitude of data plates on the coil box, the power supply, the receiver and each coil set. Unfortunately, over the years many of these Navy HROs have been stripped of their data plates making actual identification somewhat difficult without a close inspection.
photo above: The U.S. Coast Guard version of the HRO Senior was designated as the RC-105 and the matching loudspeaker was designated as the R-115. The set shown belongs to Brian KN4R who supplied the photograph.
|Other Designations - The U.S. Coast Guard also purchased HRO receivers with designations of RC-105. These are HRO Seniors and probably date to shortly before WWII and into the beginning of WWII. Additionally, the British designated some of their HRO-M receivers as R-106. To further add to the confusion, the U.S. Army Signal Corps also identified some of their HRO-M receivers as R-106. The Signal Corps versions have a specific data plate mounted in the upper right corner of the panel. There was an HRO-12-S that operated on a 12vdc battery system with the 1286 power pack which may have been built for Canadian use. The U.S. Navy also had the RDG which was a scanning receiver that interfaced with a panadapter. The RDG used plug-in coil sets that are identical to the HRO (even using the same individual coil identification numbers.) There are certainly many more designations and variations. During WWII, the many variations of the HRO and its accessories were necessary for the various uses the receiver was put to and for the various end-users of the receiver. Some end-users modified their HRO receivers to their specific needs and these receivers were sometimes given new designations determined by the end-user.|
|The HRO-5 & HRO-W - Near the end of WWII, the HRO was upgraded to all octal metal tubes (except the 6V6GT "glass" audio tube) and most of the components became JAN standard values. This receiver was dubbed the HRO-5. The HRO-5 was identified as HRO-W if it was going specifically to the Signal Corps. They are virtually the same receivers. The HRO-W will have a military data plate installed in the upper right corner of the panel that specifically identifies the receiver as "HRO-W" and the contract number is "49906-PHILA-45-03." The HRO-5/W receivers generally have a Marion Electric S-meter that is non-illuminated and has a white metal 0 to 1.0mA scale. The National manual is not very specific about the S-meter and some HRO-5/W receivers have turned up with the standard amber S-meter scale installed. The common belief was that the all white scale mA meters were exported to England but there were many exceptions to this and the HRO-W is commonly found with the mA meter. Additionally, the S-meter ball-handle toggle switch was replaced with a "bat handle" toggle switch. The HRO-5/W cabinet was changed to have no ventilation holes in the rear panel of the cabinet or louvers on the sides. The HRO-5/W was sometimes powered by the 697W heavy duty power supply. Sometimes receivers were given a heavy moisture and fungus proofing for severe service depending on the intended location. Audio output transformers are shown as optional for the HRO-5 in the National manual but are shown on the HRO-5 schematic. The HRO-W didn't have an audio output transformer installed in the receiver. With the HRO-5/W, coil sets were changed to have a large aluminum plate with silk-screened frequency graph and logging chart along with the coil set identification mounted onto the front panel of the coil set. All HRO-W receivers reported so far are from the WWII "K" production run in 1945. All HRO-5 receivers appear to be from the WWII "J" production run in 1945.|
photos left& below: The Signal Corps version, the HRO-W sn: K-127, from mid-1945. All HRO-W receivers were heavily coated with MFP which is a yellow lacquer with a fungicide added. Even the knobs are MFD'd. Note the gold appearance of the knob skirts and the olive drab appearance of the PW-D dial which is due to the heavy MFP coating. Under the lid is silk-screened data with a place for stamping an application date. In this receiver's case, the date is JUL 29, 1945. Also note the use of metal octal tubes that began with the HRO-5/W receivers, the non-ventilated cabinet and the silk-screened coil ID plate.
|HRO Copies from Other Countries - National Co., Inc. published a pamphlet in 1964 that celebrated their 50th anniversary in business. Inside that pamphlet, National mentioned that both Germany and Japan had produced "knock-offs" of the HRO. Shown in one of the photos in the pamphlet was a technician testing one of the German HRO copies. Also shown was one of the Japanese copies. Both photos are shown below. The Germans built at least two HRO copies during WWII, the Korting KST and the Siemens R4. After WWII, a few other copies were built in Eastern Germany. Later versions of the East German copy used Czech tubes and Russian resistors.|
A National engineer testing one of the WWII German copies of the HRO.
Note the HRO-5 receiver to the left of the German HRO. Also, note the
incredibly huge General Radio 805-C Signal Generator being adjusted.
photo right: A Japanese copy of the HRO from WWII. Note that the nomenclature is entirely in Japanese. These copies were not as "literal" as the German copies.
Both photos are from "National's Anniversary Photo Album" - published in 1964 for National's fifty years 1914-1964 in business.
Kingsley Radio Company - K/CR/11 aka AR7 - In addition to Axis-created copies of the HRO, some of our allies also created "knock-offs." Probably the best known HRO "knock-off" is the AR7 receiver built during WWII by Kingsley Radio Co. of Melbourne, Australia. The Australian Army referred to the AR7 as "Reception Set No.1" while Kingsley's original designation was K/CR/11. Though the AR7 has a micrometer dial and uses plug-in coil sets, that's about as far as the HRO copying went. The AR7 uses eight tubes (plus two in the original PS) and covers 138kc up to 25mc using five coil sets. Rather than using a separate Mixer and Local Oscillator, the AR7 uses a Converter Stage. The tubes employed are standard "American" tube types. The receiver uses a stainless steel overlay on the front panel. The Royal Australian Air Force used the receiver with the panel unpainted while the Royal Australia Army receivers have the front panels painted green. Interestingly, the S-meter on the AR7 works "backwards." Full scale is "0" and mechanical zero is "9." Additionally, the micrometer dial also works "backwards" (when compared to the HRO) with 0 being the highest frequency tuned and 500 being the lowest. All Army AR7 receivers were installed in a small "table" rack with a rack mounted power supply and a rack mounted speaker. The audio output impedance was approximately 1750 Z ohms and 600 Z ohms and the panel jacks provided both audio outputs. Probably around 3500 AR7s were produced. After WWII, the AR7 was used extensively in airports around Australia as a communications receiver. All AR7s for post-war airport use were totally rebuilt and many were modified for CODAN, a type of squelch circuit. Some of the receivers had the LO coil removed from the coil sets and a Crystal controlled oscillator installed for "fixed frequency" operation. It was common to find a "wall of AR7 receivers" (sometimes up to 30 receivers) in Australian airports post-WWII. See "Collector's Gallery" in part 4 for the Amalgamated Wireless Australasia AMR-100 "HRO Copy" from K6DGH.
photo right: Kingsley Radio Co., Melbourne, Australia - K/CR/11 or AR-7 receiver. This is SN: 01 805 and it was used by the Royal Australian Air Force. Note that the coil graphs are also stamped stainless overlays. Two graphs are used to improve the accuracy of the graph. The coil sets are general coverage only. RAAF tag is missing but all five plug-in coil sets are still with this receiver.
The Post-WWII HRO Receivers
At the end of WWII, almost all manufacturing had been for the war effort since 1942. Most companies were ready to start civilian production by September 1945. National, like most other radio companies, offered what had been late WWII receivers as the initial, post-war product line. The first post-WWII receivers offered were standard WWII HRO-5/W models with general coverage coils and a aluminum silk-screened panel with graphs and ID mounted to the coil assembly. These coils sets will be ID'd as JA, JB, etc., to indicate they are general coverage only. Since the military HRO-5/W came with nine coil sets, this may have been offered with the civilian HRO-5/W (at an extra cost.) It's also likely that some of the civilian HRO-5/W power supplies were the heavy duty 697W. See 1946 Radio Shack Boston ad below showing the HRO-W being offered for $217.35 including power supply and four coil sets. Note that the other five coils sets are offered at an extra cost. No wonder it's so hard to find an HRO-W with its complete original nine coil sets. Additionally, the power supply shown in the ad's artwork is the standard 697 and not the heavy-duty "W" version. As expected, the coils sets are "J" version non-bandspreading types. Note that the meter shown is the 0-1mA Marion Electric that was standard for the HRO-W. In a later ad (April 1947 QST,) Radio Shack offers the HRO-W complete in its original wooden crate with all nine coil sets and two power supplies, the 697 and the 6vdc vibrator supply, all for $229. The ad further states that "supplies are limited."
Updates from National seem to come at a leisurely pace and by early 1946 the HRO-5 had been upgraded to have the A,B,C and D coils feature the bandspread function. National designated this receiver the HRO-5A. National replaced the white scale 0-1mA scaled meter used in the HRO-5/W with the round S meter with yellow-amber scale made by Marion Electric (as used on the pre-war HRO Senior) although some HROs were also fitted with a white S meter scale with the "NC" diamond logo (probably left-over stock from earlier manufacturing.)
|The photos to the right show an HRO-5TA1 probably built
around the spring of 1946. Although this receiver appears to have the
National tag installed back in its proper place in the upper right
corner of the front panel, note that the S-meter scale is an earlier
style scale from pre-WWII but it is installed in the typical WWII Marion bakelite meter case. Note on this version that the Crystal Filter is
still the older style with the close spacing of the two small dials and
with the Selectivity control as the upper dial.
Looking at the photo far right of the chassis it is obvious that this receiver is still using WWII "left-overs" in the IF transformers and BFO coils. Note that the last IF transformer shield and the BFO coil shield are MFP'd. The BFO coil shield still has the WWII ID printing on it. The tuning condenser is the WWII style. Also apparent is the gray painted chassis. >>>
photos right: from eBay
>>> Though difficult to tell, the Noise Limiter is still being built onto a small chassis mounted on top of the main chassis. Note the cable routed across the chassis going to the NL chassis. The cable exits the chassis to the right of the 6V6 audio output tube.
|The "last version" of the HRO-5TA1 was introduced in the late-summer of 1946. It
featured a new Noise Limiter circuit that was now built onto the
receiver chassis which was now cadmium-plated. This NL limiter circuit
is slightly different with minor wiring and component changes when
compared to the NL circuit used in the earlier HRO-5A1 receivers. At the same time, the Crystal Filter was changed to a more conventional circuit
that National had used on the pre-WWII NC-200 receivers and on the WWII USN RAO receivers featuring a six position switch controlling
Selectivity and a variable capacitor to adjust Phasing. The position of
the two controls were interchanged and the spacing between the two
controls increased when compared to the earlier HRO Crystal Filter.
Another change was to the S-meter which became a Marion Electric meter
with a square housing (to conform with the meter used on the new
NC-240D.) Additionally, on the "last version" HRO-5A1 receivers the serial number
format is changed from the old letter designation for production run
identification to a three digit number identification. The specific
receiver is identified with a four digit number which results in a seven
digit serial number that was then relocated to the rightside-top of the
chassis about midway back.
Although dealers began offering earlier versions of the HRO-5A1 as early as July 1946,
the receivers are shown with round S-meters. National didn't feature the
"last version" HRO-5A1 on a back cover ad in QST until February 1947. The
construction of these later HRO-5A1 receivers is very consistent and
used all new parts in the assembly. All of the "last version" HRO-5A1 receivers were built on production runs 184
(reported serial numbers confirm this.) National also offered the
HRO-5RA1 rack mount version of the receiver.
photo right: Inside a "last version" HRO-5TA1 (sn: 184-1054) showing that the chassis is now cadmium plated (post-summer of 1946 units,) two more tubes are added next to the left side of the tuning condenser for the chassis mounted Noise Limiter. Also, note the NATIONAL decal has returned and is installed under the lid. These decals began to be installed on cabinets very early in production - by run-G (1935) but were not applied to the cabinets during WWII. Also, note rear wall of cabinet has no ventilation holes as the pre-WWII HRO Senior cabinets did.
Receiver" - The HRO-5C was considered a "Deluxe Receiver"
because the total package included the HRO-5RA1 receiver, the SPC panel
loudspeaker, power supply and spare coil set storage unit. A chassis-built Model 697
power pack was mounted on top of the rear part of the coil box directly
behind the loudspeaker. The coil storage bay had spaces for five coil
sets. With one coil set in the receiver plus five in bays, coil storage
capacity was six coil sets. This usually was sufficient for standard
coil sets D, C, B and A in addition to the most common optional coils
sets, E and F (AM BC band and 160M general coverage.)
The receiver and the SPC were both mounted in a
MRR table rack that's 29 inches tall. The MRR included two side trim panels to
cover the rack mounting screws and three stainless steel trim strips that mounted above
and below the receiver and at the top of the SPC panel. The receiver included
a one-piece and easily removable dust cover. While later dust covers,
such as those used with the HRO-50/60 that entirely covered the rear of
the receiver, the early dust cover just covered the chassis and allowed
easy access to the rear chassis terminals.
The HRO-C "Deluxe Receiver" was advertised as early as 1941 when it
included the rack mount version of the HRO Senior, the SPC and the MRR
rack. The same trim pieces were included in this pre-WWII version. Prior
to the HRO-C, rack mounted receivers with spare coil set storage and
loudspeaker panels were available but didn't seem to have a specific
designation. Early versions shown in advertising don't have the trim
The HRO-5C was available from 1946 up to mid-1947 when the HRO-7 was introduced. These deluxe combinations were later available in the HRO-7, HRO-50 and HRO-60 versions of the receiver. The HRO-60R Deluxe is shown in the section on HRO-60 receiver further down this article.
HRO-5C Deluxe Receiver SN 184 0009 has undergone a complete rebuild and I've written-up the process. Includes several photos. The write-up can be found in Part 4 of this HRO web-article.
|Are the "Last
HRO-5A1 Receivers Actually HRO-6s too? - In late-1946 to early 1947, a short lived HRO-6 was produced, supposedly with an
"improved" Noise Limiter, but its physical appearance is
identical to the "last version" HRO-5A1. Some National manuals will have
an advertisement in the back pages for the HRO-6. These ads date from
1946. Very few HRO-6 models were sold and it is seldom encountered
today. It's known that HRO-6 receivers were built on production run-184, the same production run as the
"last version" HRO-5A1
receivers. Note in the "HRO Serial Number Log"
(in Part 2 of this article) that the two HRO-6 and nine of the HRO-5A1 receivers are all
from run-184. Interestingly, five of the HRO-5A1 receivers are
apparently higher serial numbers than the highest reported HRO-6
serial number. Also, one HRO-5A1 serial number is within the HRO-6
serial number range of reported numbers. It appears that for a very
short period of time, during the middle of run-184, the HRO-6
designation was used on only a small quantity of receivers. The HRO-6
serial numbers may seem to be inter-mixed with HRO-5A1 serial numbers
but there is an obvious gap on HRO-5A1 receivers from about serial
number 184-0300 up to about serial number 184-0700. Until some other
HRO-5A1 receivers are reported within that range one might assume that
about 400 receivers were designated as HRO-6. However, with only two
reported serial numbers it would seem that the total HRO-6 receivers is
probably less than
|It might be possible that National considered
identifying the "last version" HRO-5A1 receivers as the "HRO-6" since
these last HRO-5A1 receivers incorporated so many changes when compared
to the earlier HRO-5, HRO-5A and the highly variable (construction-wise)
versions of the HRO-5A1 receivers (that is, those HRO-5A1 receivers
that weren't built on run-184.) In comparing photographs of the HRO-6
to the "last
version," run-184 HRO-5A1, there are no apparent differences and perhaps
the ID tag was the only change. Perhaps the "improved" NL that is
mentioned so often as the "difference" was the fact that these "last"
HRO-5A1 receivers had the NL built onto the chassis rather than the
separate small NL chassis used in early HRO-5A1 production. Checking the
NL schematic that was included as a separate page in the early HRO-5A
manuals and comparing it to the receiver schematic that is in the HRO-5A1 manual, there
are several minor changes to the NL circuit. When the "improved" NL is
sited as the HRO-6 difference, that argument doesn't take into account
the run-184 HRO-5A1, which does have this "improved" NL, where-as the
earlier HRO-5A1 (not on run-184 versions) doesn't. This seems to confirm
that there weren't any differences in
the HRO-6 and the "last" HRO-5A1 receivers built on run-184.
photo left: Top of chassis of the HRO-6, identical to the HRO-5TA1
photo above: ID plate from the HRO-6
|Someone at National may have "jumped the gun" issuing the HRO-6 designation since
basically the run-184 HRO-5TA1 (or HRO-6) looked very much like all of
the proceeding HRO models, that is, a black wrinkle finish box. The
intermixing of model designations within a production run is odd and
points to an internal communications problem probably between marketing
and engineering. National had certainly started
design work on the "new" post-WWII HRO receiver (the future
HRO-7) which was going to have significant changes to
its external appearance, fitting a new model receiver designed after
WWII. The "Higher-ups" at National didn't want the late-1946 to early-1947 (1947 model year)
prospective customers to think that National was still just "revamping"
the old HRO as their new model receiver. Maybe to keep the "older" looking HRO-6 from being mistaken for
National's "new post-WWII design," the designation of HRO-6 was quickly
dropped and the ID plates returned to the HRO-5TA1 or HRO-5RA-1
designation. That left National with having to designate their actual
"new" 1947 HRO as the HRO-7.
By stopping the HRO-6 designation, the introduction of the HRO-7 would then have been seen as National's "new post-WWII design" and that their "new" receiver would have a "new" look, completely different from the older HRO receivers. Another factor was that the HRO-7 seemed to be behind schedule. It would have been expected that its announcement would have happened in October 1946 for the "1947 Model Year." However, the HRO-7 wasn't announced until ten months later, in August 1947. Thanks to Chris Dockery for all HRO-6 photos. Chris owned this HRO-6 at one time and, luckily, kept some photos.
HRO-50, HRO-50-1 and HRO-60
photo above: HRO-50T from 1950. Note the gray plastic grips on the smaller knobs while the PW-D is sporting the black HRK-style knob. Earliest receivers had a gray HRT-style knob on the PW-D. This HRO-50T is SN 280-0194
The HRO-50 and HRO 50-1 - The new HRO-50 was going to bring the HRO design into the mid-twentieth century. The separate power supply was first on the list of things that needed to go. The new HRO power supply was built-in though on a separate chassis that was bolted behind the main receiver chassis and thermally insulated from the receiver chassis by an asbestos sheet that was also continued up to the top of the vertical divider panel to provide insulation from the heat from the rectifier tube and the audio tubes. Relying on the micrometer dial versus graphs was also gone. Now an illuminated linear slide-rule dial would provide direct frequency readout. Changeable plastic scales were mounted to a front panel controlled, rotating drum, providing band-in-use scaling. The PW-D dial was now just for logging purposes. An Antenna Trim control on the front panel was finally incorporated. Auxiliary sockets were provided for the optional 1MC/100KC crystal calibrator and NBFM adaptor. Also, a Local Oscillator Trim that allowed front panel calibration for the best dial accuracy (using the crystal calibrator - either the optional one or an external calibrator.) Voltage regulator, push-pull audio, 8Z ohm and 500Z ohm audio outputs - almost everything necessary to update the old HRO. The plug-in coils had to remain along with the PW-D micrometer dial otherwise it wouldn't even have been an HRO.
The HRO-50 was introduced in 1950 and was followed quickly by the HRO-50-1 that added an extra IF amplifier and, to improve selectivity, double IF transformers were used in last two IF stages (three dual transformers that included a dual IF output transformer to the detector tube.) With 12 tuned IF transformers, the receiver was very selective. The HRO-50-1 was produced in 1951 through 1952.
Early versions of the HRO-50 had grey grip knobs with bright aluminum skirts. Even the PW-D used the gray HRT-style knob on the earliest versions. Later versions will have gray knobs and the PW-D will have the standard HRK-style black knob. All HRO-50-1 versions have all black knobs. If the receiver is a table cabinet it will be labeled as "HRO-50T" or "HRO-50T-1." Rack mounts substituted an "R" for the "T" and are so-marked.
The USCG contracted with National to supply HRO-50 receivers as the R-460/UR. The contract was issued in 1949. R-460 receivers are black wrinkle finish and have a data plate mounted in the upper right corner of the front panel. Coil panels are not painted but have a chemical treatment something like iridite leaving the metal with a light gold coloring.
Photo below left shows the chassis of the HRO-50T. Note the standard IF transformers used in the two IF amplifier stages. Also note that the two IF amplifier tubes are 6K7 tubes with grid caps. In the photo below right is the HRO-50T-1 showing the three IF amplifiers with the dual IF transformers that includes the input transformer to the detector stage. Also note, the 6SG7 tubes (no grid cap) for the second and third IF amplifiers. This receiver is also equipped with the optional NBFM adapter.
The final evolution of tube-type HRO receivers was introduced in 1953 - the HRO-60. With 18 tubes, double conversion and a current regulator on the oscillators, it was the final evolution of the tube-type HRO. Double conversion was utilized above 7MC when using coil sets B and A or any of the A prefix bandspread coils available. Bandspreaded 40M required the C coil set which wasn't double conversion. The HRO-60 accessories included coil sets J, H, G, E and F, providing general coverage from 50kc to 430kc and 480kc up to 2.0mc. Additionally, an AC coil set provided bandspreaded 15 meter coverage. AD covered 6 meters. There were two other "A prefix" coil sets, the AA and the AB, both of which bandspreaded in the 27mc up to 35mc ranges. The HRO-60 was available from 1953 up to about 1964. At the end of production, the selling price had escalated to an unimaginable $745.00!
As with all of the HRO receivers, rack mount versions were available that featured a 30" tall table rack that allowed mounting of a coil storage unit and a rack mounted speaker. These rack mounted speakers with coil storage were available as early as with the HRO Senior receivers and, with the introduction of the HRO-50, the type SC-2 coil storage and speaker combination became available. Special "dust covers" were installed on these later rack-mount receivers that fit over the side panels and are held in place with two knurled thumb-screws. The HRO-60R is only labeled as HRO "Sixty" with no specification as to its rack mount construction (the same is true for the table cabinet.) When using one of the rack mounted speaker set-ups one will notice somewhat of a reduction in the bass response due to the lack of any true enclosure for the speaker. This is typical of most rack mounted speaker set-ups.
The HRO-60 first conversion oscillator frequency was changed during production. Early versions of the receiver used 2010kc while later versions use 1990kc. At least two versions of the manual were published with early manuals referencing 2010kc as the first conversion frequency and later manuals showing 1990kc. In fact, I have an original HRO-60 manual that has a hand written note saying "1995kc - per National", so certainly there was much confusion over what frequency was used in any particular receiver. While aligning any HRO-60, it's easy to input a specific amplitude signal at 2010kc and adjust the two first conversion transformers for maximum output. Then measure the output of the receiver exactly - use an audio output meter connected to the 500Z ohm output. Next, input the same exact amplitude signal at 1990kc and readjust the first conversion transformers for maximum. Measure the output of the receiver exactly. Whichever alignment frequency results in the highest output level is the correct frequency for that receiver. The correct frequency should be marked on the two first conversion transformers for future alignment reference. >>>
>>> With the direct readout dial on the HRO-60 it is possible to achieve very accurate tracking. You'll find that it is tedious work since each LO low-end adjustment will require coil set removal, then making a slight adjustment followed by reinserting the coil set to test the adjustment. This will most likely require readjusting the high-end trimmer and then checking the tracking again. This process is repeated until tracking is accurate. With patience it's possible to have the HRO-60 (or HRO-50) dial achieve a very accurate readout but you will have to realize that its resolution is somewhat limited.
- The HRO-60 is often times berated as less of a receiver than
its predecessor, the HRO-50 but this is mainly from hams who are
looking for maximum bandwidth for AM signals. The early HRO-50
used two rather conventional IF amplifiers that provide a
typical "bell curve" passband which many AM ops find pleasing to
listen to. However, the HRO-60 and the HRO-50-1 were trying to
cope with the crowded band conditions of the fifties and sixties
communication was the goal. The additional IF transformers and
IF amplifier stage provided a narrow passband with steep skirts
resulting in very selective tuning. An excellent Crystal Filter
circuit also helped with difficult QRM. Operating a rebuilt and
correctly aligned HRO-50-1 or HRO-60 is a pleasure - QRM is
rarely (if ever) a problem and the receiver is still a
competitive performer at any frequency. However, that isn't to
say that the early HRO-50 isn't a capable performer that is able
to handle QRM with its Crystal Filter and provide excellent
audio when conditions allow for it.
photo left: The HRO-60R Deluxe with the SC-2 doors open showing the ten coil bays. I've owned this HRO-60R twice. The first time was in 1990. I traded it a couple of years later for an early National receiver. Around 2003, I found out where this receiver had gotten to and was able to purchase it back. Over the past several years, I've located most of the optional coil sets besides the standard D, C, B and A coil sets. Included are coil sets J, H and G covering 50kc up to 430kc. Coil sets F and E covering 480kc up to 2050kc. Other optional coil sets are AC for 15 meters and AD for 6 meters.
photo above: The HRO-60R chassis. The smaller IF transformers are for the first conversion set-up. Note the two dual IF transformers - the HRO-50-1 used three dual IF transformers. The rear of the chassis has the Calibration Oscillator installed but a NBFM module is not installed.
This HRO-60R had spent many years in a basement in New York City. The chassis "spots" are a result of the somewhat humid environment.
|Cold War Use of HRO-50R and
HRO-60R Receivers by Sweden - The Swedish Royal
Board of Telecommunications made extensive use of the National HRO-50R
and especially the HRO-60R receivers when building radio monitoring and
point-to-point receiving stations in the late-1950s and through most of
the 1960s. The system was to provide civilian contingency communications
that could link various regional governments together and to the main
government in case of a nuclear conflagration.
The transmitters were 5KW output and operated in RTTY or CW Morse modes. The receivers chosen for the stations were first (earliest) the HRO-50R and by the late-fifties, the HRO-60R. Most of the receivers were installed in pairs to operate in space diversity. Since RTTY was the main mode used, increased receiver stability was necessary. The frequency stability was provided by external synthesizers, the Philips Impulse Governed Oscillator, the IGO.
Main communications routes usually had 2 or 3 circuits. Branch communications routes had only 1 circuit.
Most of the equipment was high-maintenance in nature and, in reality, the system had very low usage. The system was decommissioned in the late-seventies.
The fate or even the disbursement of the equipment was an unknown until one complete station was discovered intact in the 1990s. The equipment from that station was put into a museum in Sweden and a portion of the equipment is shown in the photo to the right.
Thanks to Karl-Arne Markstrom SMØAOM for this information (and the photograph) that reveals an interesting use of the National HRO-50/60 receivers.
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