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Western Historic Radio Museum
 

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 ~ Restoration Write-ups (2)
 
 PART 4 - Restoration Write-ups (4)  ~  Collector's Photo Gallery HRO Receivers

  Competition Comparisons  ~  Performance Evaluation

by:  Henry Rogers WA7YBS

W9DXX, Alice Bourke, Chicago, IL ca.1935 -  from Frank C. Jones "Radio Handbook" 1936.

This interesting photo shows a very early HRO receiver. Note the pearl push button for S-meter operation and the white background graphs on the coil sets indicating that the receiver is either from the first or second production runs (1935.) Of interest is the PEAK Pre-selector to the left of the HRO and the 1934 AGS-X receiver to the right. Note the Hammond Cathedral Clock on top of the HRO coil sets. Also, the homebrew transmitter again the wall.

Alice Bourke was a reporter for the Chicago Tribune along with being very active in ham radio.

NATIONAL  COMPANY,  INC. 
HRO  -  COMMUNICATION RECEIVERS

"The Cream of the Crop"

PART 1

History of the HRO Design

AGS Receiver

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.

photo above: The original National Co. Airport Communication Receiver - the 1932 RHM sn: 8
 

photo above: Artwork for the HRO Prototype published in National Bulletin No.240 showing the receiver with AGS style knobs, a Volume control, a single coil frequency graph, no S-meter switch and many other differences from the actual production HRO receiver. Bulletin No.240 was published in the October 1934 issue of QST

HRO Design

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.  >>>

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 1934.


Delayed Release - James Millen had earlier started a monthly "open letter" to all hams, published in each issue of QST. Usually the letter told about projects that National was working on or sometimes interesting problems were written about. Generally, the letters were used for fomenting ham interest in new National products. Millen announced that National was working on a new receiver in the October 1934 issue of QST, however the HRO wasn't even close to release yet. In the December 1934 issue, Millen told his ham audience about the instability problems facing the new receiver and that the release was delayed until January 1935. Millen candidly related that the problem would not have been noticed except for the accuracy of the micrometer dial. Apparently the problem was more difficult to solve than anticipated for the January release date was also missed.  Production HRO receivers were being built as early as January 1935 but these units weren't shipped out until all of the "bugs" had been worked out. HROs finally began shipping to awaiting customer orders around March 1935. The first HRO receivers were eagerly purchased by National fans because they knew the receiver was going to be the best available since James Millen had been keeping all of the hams informed of the HRO's progress with of his monthly open letter to hams that was published in QST.

photo above:  This 1935 ad is from Leeds, a New York radio dealer, showing the high regard that was accorded the new National HRO. This artwork shows the production receiver with the exception of the knob on the BFO can and the S-meter scale.

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
 

HRO sn: E-50 (table model,) second production run built in February 1935

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.

F-16 has not been restored,...it was "detailed." The process is covered in the Restoration section in Part 4 of this web-article.

photo above: HRO sn: F-16 (rack mount,) third production run built in April 1935
 

photo above:  HRO  sn: H-103 (table cabinet) was built in September 1935 (5th production run.) By this time, the receivers had remote standby and black background coil charts. The small, red <NC> dial pointer had been replaced with a small diamond head screw. The "H" production run was the last run of receivers to use the plated PW-D, the 0-5 scale S-meter (the later style 0-5 scale is on this receiver) and the early-style cabinet. This photo shows the typical texture of the original, two-part wrinkle finish.

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 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)

A

B

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G

HRO Junior

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 Juniors SN: J-16 and 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 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 & right: An early HRO Junior with the serial number J-125 (Oct-Nov 1935) and is from the first run of Juniors. The JA coil is installed. It's interesting to note that this very early Junior has the painted PW-D installed. In the chassis photo note the round IF cans and the black chassis.  photos by: Charles Lindsey

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. An early example reported, J-125 from late-1935, is equipped with a gun-metal lacquer finished PW-D.

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.

HRO Senior

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. It's also possible that "mold wear" might have caused the widening of the index lines over time.)

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.

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

 >>>   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) in Part 2 of this write-up for several various reasons sited for Millen's departure from National.

photo above:  HRO Senior sn: 463-K was built in 1940 and has most of the "late pre-war" HRO Senior characteristics. Most noticeable is the illuminated S-meter with amber scale (these scales were originally a light yellow color but darken due to exposure to UV or bright light.) Also note the name plate in the upper right of the panel showing "TYPE HRO" and the PW-D that has the dark bronze paint.

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D

E

F
 

G

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I

The HRO for Airways Communication

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. Some serial numbers using the "P" prefix used a very large letter "P" and then the standard size numerals (when the SN was located near the AF output tube.)

The HRO-M & HRO-MX - The HRO Senior went into "war production" almost unchanged at first. The earliest HRO receivers sent to England were the glass tube, six volt tube heater versions that still retained the plug-in crystal on top of the crystal filter housing. This type of HRO was renamed the HRO-M, especially if was for Lend-Lease to be sent to England. There were some engineering upgrades in mid-1942. The most obvious was the modification of the crystal filter to change to an internally mounted crystal, thus eliminating the "easy to remove" crystal that plugged into the top of the filter assembly. There had always been questions regarding the plug-in crystal in that the "air gap" required for the quartz crystal seemed to be interpreted as something "loose" and "rattling around" inside the crystal. The new design of the crystal filter and the crystal itself eliminated that unwarranted concern and didn't change how the Crystal Filter operated. After that change, National referred to the new HRO version with the internally mounted crystal as the HRO-MX. During WWII, the HRO-MX was improved with ID rings added to the AVC and B+ toggle switches. Many HRO-MX 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 all military HROs had this meter installed. The S-meter "pull-switch" was replaced with a black-finish, ball-handle toggle switch. Both the HRO-M and MX used the same glass tubes as it predecessor, the HRO Sr. Some HRO-M and HRO-MX versions will have that designation on the ID plate (but many ID plates just indicate "HRO.")

photo right: Inspection tag from WWII HRO-MX sn PP-988 showing the various signatures and dates for the operations to complete the receiver. The HRO-MX 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

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.

To add to the confusion, the U.S. Army Signal Corps also identified some of their HRO-M receivers as "R-106" with the hyphen being the subtle difference in the designations. The Signal Corps versions have a specific National Company data plate mounted in the upper right corner of the panel showing "R-106 / HRO" as the receiver designation. The Signal Corps R-106 receivers were divided into several versions that referenced whether the receivers were Mk.I which was the "M" or Mk.II which was the "MX." Also, the HRO-5 was sometimes tagged with the R-106 designation with a Mk.III suffix and sometimes the tag was mounted to the left of the PW-D.

The stenciling on the lid of SN:P861 is "G. 13/8/44" and that looks a lot like a date of 13 August 1944. It seems like a late date for this early HRO-M receiver but the stenciling was certainly added in the field and possibly after the receiver had gone to the depot for the Antenna Trim addition. The information on the B/C No.2 DF Stations seems to run out into 1945 and beyond.

photo above: HRO-M Reception Set R106 sn: P861, stenciled "G.3-9" on the receiver and on the coils and with "SER.No 52" on the data plate. When found, the matching serial number coils for the receiver were stowed in a canvas bag and included the F, E, JD, JC, JB and JA coil sets. These are all general coverage only coil sets that were typical for WWII. The non-original round knob is a field or depot installation of an antenna trimmer control. There is a U.S. Army Signal Corps acceptance stamp on the back of the receiver but the receiver has many characteristics of British use and modifications, including the use of "cheese head" screws in some locations that were reworked. The tag indicates "D.F. No.2" designating this R106 as part of a B/C No.2 DF Mobile Hut as shown in the drawing to the right. Drawing is from the website: https://www.qsl.net/pe1ngz/army/vehicle/dftrailer/html

photo above: Close-up of the data plate on the R106

Manual Error SC R-106 - The Signal Corps manual for the R-106 has a significant problem with its detailed description of how to remove, disassemble and reassemble the PW-D micrometer dial. The procedure is totally wrong and completely different than the National Co. instructions on PW-D assembly. The SC procedure will cause problems rather than allow successfully completing any PW-D adjustment task. How this section ever got into the manual is a mystery. Additionally, there was a supplement to the manual with a PW-D instruction sheet with the same erroneous procedure. DO NOT USE the Signal Corps instructions for working on the PW-D. The correct method of removal, disassembly and reassembly is in Part 3 of this HRO write-up,  "Servicing the PW-D." This correct procedure was also in some of the National Company manuals.

I wanted to keep P 861 as original as possible but I also wanted to fix all of the mechanical problems and do a thorough cleaning but that required significant disassembly. Straightening the cabinet was very easily accomplished once the bent areas could be accessed for "body working" procedures. During disassembly, it was noted that the S-meter glass was very loose and "rattling around" in the housing. Dismounting and disassembling the meter revealed other problems. Someone applied glue to secure the glass but this probably didn't last long and only managed to really "glue" the backing ring to the inside wall of the housing making proper adjustment impossible. The only solution was to use the internal meter mechanism (surprisingly, in very nice condition) and transplant it into a good condition HRO S-meter housing. Close inspection of the AF Gain pot repair indicated that it wasn't a depot job and additionally the potentiometer shaft was hopelessly stuck (even applying heat and oil couldn't break loose the stuck shaft.) A good condition vintage replacement pot was installed. Oily dirt was all over the chassis. This is easy to remove using WD-40 as a cleaning solvent. After cleaning, Glass Plus was used to remove the WD-40 residue. The cabinet was also greasy and needed the same treatment. Knobs were reconditioned using WD-40 as the cleaning agent. Each coil panel was cleaned. Inspection of the wiring under the chassis revealed that the speaker terminal wiring had been moved to the BSW terminals and the two wires that were connected to the BSW terminals were shorted together using an insulated twist "wire nut."  The actual speaker pin jack was broken and repair would require replacing the entire terminal board and that's riveted to the chassis. I don't think the use of a wire nut for the BSW wire connections and the use of a red crimp-type wire splice used on the AF Gain pot (red) wire were the products of depot work. It looks like the Antenna Trim and the S-meter toggle switch replacement are definitely "depot quality" work. The other changes appear to be post-war hamster repairs. Luckily, the minor hamster damage was very limited and easily repairable.

The R106 was reassembled without any further issues. I've tried to keep P 861 as an original example that has only been cleaned with most of the mechanical problems repaired. (Refurbishment completed: April 28, 2022)

photos left& below: The Signal Corps version, the HRO-W sn: K-127, from mid-1945. The data plate is stamped with the number "85." 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.

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. 

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.)

photo above: The HRO-5TA1 sn: 184 1054. This is a very late version of the receiver (it's the highest reported SN in run 184 for HRO-5A1 receivers) featuring the square S-meter made by Marion Electric which was used to conform with the S-meter used on the NC-240D receiver. Also, this receiver has the later cadmium-plated chassis and the six-step Selectivity control for the Crystal Filter operation. Note the increased spacing between the two Crystal Filter controls. These "last version" HRO-5A1 receivers have the improved Noise Limiter circuit.

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.                      

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.

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 stainless trim pieces or the side rack-screw covers. Shown in the inset photos, left is the 1946 Marion Electric S-meter for National receivers. Note that besides the square mounting flange, the scale now has "S UNITS" and "DB OVER S-9" located over the scale. The font used for the numbers and letters is a smaller, thinner type of font. Compare to the 1940 Marion Electric S-meter shown in the inset photo right.

The HRO-5C was available from 1946 up to mid-1947 when the HRO-7 was introduced. These deluxe combinations were later available with the HRO-7, HRO-50 and HRO-60 versions of the receiver. The HRO-7C and the HRO-60C are shown in the sections 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.

HRO-6

photo left: HRO-6 SN: 184-0697

photo left: Top of chassis of the HRO-6, identical to the HRO-5TA1

photo above: ID plate from the HRO-6

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.

photo left: Bottom of chassis of the HRO-6. This example has been re-capped.

HRO-7

HRO-7T in Light Gray-Silver - the Standard "T" Paint Color

The 1947 model year production was running very late for National. They had missed the introduction of the new 1947 models that normally happened in October, 1946 meaning that they had also missed the 1946 Christmas sales of new 1947 models. National had been trying to "use up" all of their WWII surplus parts and assemblies during the 1946 production year and that certainly must have had an impact on the design of "completely new" models. National seems to have "caught-up" by early 1947 and started introducing new models at the beginning of 1947. The late start resulted in National introducing several new models at different times all through 1947. In August of 1947, National revealed a seemingly total "make-over" for the old HRO. The new HRO-7 had a completely restyled cabinet with slightly rounded corners, a smooth light-gray with a slightly silver undertone paint job, levers for easy coil removal, S-meter viewed though a panel "cut-out," different knobs that were round with light gray grips, a light gray PW-D. However, at first glance, the HRO-7 chassis looked very familiar and it appeared that the HRO-7 circuit was basically the same as the late HRO-5TA1. But a closer look at the chassis would reveal there were several circuit and accessory changes, even some tube changes, the most apparent being the two seven pin miniature tubes added, the 6C4 LO and the 0A2 VR tube. Also, an adapter socket was added to the rear chassis for use of a NBFM adapter or a crystal calibrator (a crystal calibrator would have to be a kit with onboard on-off toggle switch since there wasn't any way to switch off the B+ just to the Accessory socket.) A Tone control switch was added to the HRO-7 circuit although it was just a toggle switch that selected a 3000pf capacitor to ground on the 1st AF amplifier output to the 6V6 to "roll off" the higher audio frequency response. Since the Tone switch was located where the AVC switch had been on all earlier HROs, the AVC switch was relocated to the right of the PW-D. The PW-D index dial was painted a light gray and the knob was changed to a gray HRT knob. A switch was added to the RF Gain control that disconnected the S-meter if the RF Gain was reduced from 10 to a lower setting.  >>>

While the HRO-7 looked like a new receiver on the outside, lifting the lid did reveal a chassis that looked very familiar. At a cursory glance it appeared almost identical to the HRO-5A1 chassis. But, a closer examination revealed several, subtle changes. For example, there was a butch plate riveted on the chassis where the LO tube had been and two seven-pin miniature tube sockets installed there. These were the 6C4 LO tube and the 0A2 voltage regulator tube that was only used for the LO plate voltage. The 6SQ7 duplex diode-triode was changed to just a 6H6 dual diode for the detector and AVC. The Noise Limiter circuit was changed to eliminate the 6J5 Noise Amplifier and use just a 6H6 dual diode rectifier as the balanced noise limiter. An "off" switch was now provided for the new Noise Limiter function. The first audio amplifier was now a pentode 6SJ7 instead of the triode in the formerly used 6SQ7. The HRO-7 front end, other than the LO change, remained the same using two 6K7 tubes as the RF amplifiers and 6J7 as the Mixer. The IF amplifier still used two 6K7 tubes. The audio output tube remained a 6V6. The BFO remained a 6J7. The Crystal Filter remained the same as had been used in the HRO-5A1 receiver. Being aware of the circuit changes for the HRO-7 required looking under the chassis which nobody really did. Even then, the circuit changes are so subtle most users thought under the chassis had remained the same as the HRO-5A1. But, the HRO-7's performance was improved with better stability in that the LO plate voltage regulation, a better noise limiter circuit, an improved detector/AVC and a better first AF amplifier arrangement and improved audio output capabilities. Sensitivity was spec'd at better than 1uv (certainly believable.) The HRO-7's introduction did spell the end of the rectangular, black wrinkle finish box. The HRO's exterior had remained basically unchanged for the past 12 years.

The HRO-7 was also offered in a rack mount configuration, the HRO-7R that featured a 19" wide black wrinkle finish front panel. The HRO-7R was also available as the deluxe HRO-7C that incorporated the HRO-7R receiver into a 29" tall MRR rack with the SPC coil storage unit with loudspeaker and 697 power supply. The MRR rack was fitted with side trim pieces to cover the receiver and SPC mounting screws and chrome trim strips were also provided. The HRO-7R was very different in appearance from the table model sporting a black wrinkle finish panel, an olive-black PW-D and knobs with black grips.  >>>

Shown above is the HRO-7C, comprised of the HRO-7R with black wrinkle panel with SPC speaker panel with coil storage and the MRR table rack. Owned and photo by Chris Dockery.

Other than the 6C4 LO, the 0A2 VR tube and the missing ANT-GND terminals, the HRO-7 chassis looked very familiar (at least on top.)

1947 Artwork for the HRO-7T and the MCR 8" Loudspeaker is shown to the right

Black Wrinkle HRO-7T SN: 189 0396 - Probably a Commercial End-User Request - From the serial number 189 0396, this HRO-7T was the 396th receiver from the HRO-7's first production run. That would seem to have the actual build happening in October or November of 1947. Black wrinkle for the HRO-7T receiver WASN'T listed as an option in National's 1948 catalog. Black Wrinkle WASN'T shown as an option for the HRO-7T in the months of August through December 1947 QST ads. The most likely explanation for the existence of a small quantity of HRO-7T receivers painted Black Wrinkle would be a special order requesting that paint color for a specific end user. But, who would that end user be? Almost certainly it had to be either a commercial user or a military user. One guess could be airport ground stations or some type of commercial monitoring use where several receivers would be part of the equipment used and the standard gray-silver paint would have been a visual distraction. The HRO did have a well-known history of use by various airlines and airports. The HRO also had a long history with the military and a small sample run for evaluation might have been a possibility, especially during the post-WWII military upgrades to radio equipment that was going on at the time. But, there aren't any military data plates or military acceptance stamps on the receiver or any other indications of military use. In fact, the receiver looks like it was used very little. However, commercial or military users generally wanted rack mount receivers, so the table model would be an uncommon choice for those users (not unheard of, but uncommon.) It's also possible that the special order for a small number of Black Wrinkle HRO-7T receivers was to use the table model receiver in conjunction with the already standard Black Wrinkle HRO-7R rack mount versions and have both the "T" and "R" receivers match. Since Black Wrinkle on the HRO-7T appears to have never been an advertised option, the only explanation is a small special order but the "why" and "who" are a mystery at this time.

So far, the only other black wrinkle HRO-7T I've seen is on the radiomuseum.org website and that is just a photograph with no specific details provided. The photograph of the radiomuseum.org receiver is shown in the photo to the left. My black wrinkle HRO-7T is shown in the photo below. As can be seen, both receivers have exactly the same appearance. The existence of another black wrinkle HRO-7T with the same unique combination of wrinkle finish and smooth finish panels seems to indicate that these receivers came from a small quantity special order for a specific end user that requested the special paint job. Because of the small quantity built these Black Wrinkle HRO-7T receivers are quite rare.   photo left: The Black Wrinkle Finish HRO-7T shown on the radiomuseum.org website (note that coil set "A" is installed in this receiver)

Accessories - Although I haven't found any National Company literature that mentions a HRO-7T in Black Wrinkle, the associated accessories must have also been painted to match. The standard supplied D, C, B and A coil panels would have been black wrinkle (or use HRO-7R coil sets, serialized and aligned to the specific receiver,) the 697 power supply should have also been black wrinkle (that was the standard color when the 697 was supplied with the HRO-5TA1) and the MCR loudspeaker would have been black wrinkle (if the end user required a loudspeaker.) Perhaps the special order included the "complete set of accessories" which all would have been provided in black wrinkle.

HRO 7T "Black Wrinkle Finish"  SN: 189 0396

Unfortunately, there weren't any other matching coil sets, there wasn't a 697 power supply (but I have one) and there wasn't a MCR loudspeaker. There were five other HRO-7 coil sets included in the deal but they were all "orphan coils" that were the standard gray-silver finish. A wooden three bay coil box was also part of the deal. Interestingly, as mentioned, if HRO-7R coil sets could be found, they would have the correct black wrinkle panels, or these gray coil set panels could be repainted black wrinkle finish. The HRO-7T does have the typical, two-part wrinkle pattern that has a secondary pattern along with the primary wrinkle. This two-part wrinkle isn't available anymore but modern VHT black wrinkle would provide a very good match for the color but the texture is a single pattern, fine wrinkle. At this time, VHT is the only wrinkle finish paint that I know is easily available. The repaint would have to be done in two parts with the back of the coil panel smooth matte finish black and just the front of the coil set panel in wrinkle finish. More on this in the Restoration Section. The rebuild of this HRO-7T is written-up in detail in the Restorations Section in Part 4 of this Four Part HRO article.

"W1MW" - Pencil writing on Coil Set D

HRO-50, HRO-50-1 and HRO-60

The HRO-50 and HRO 50-1 - The new HRO-50 was going to bring the HRO design into the mid-twentieth century. No more separate power supply that was cable connected to the receiver. The separate power supply was certainly the 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.) A voltage regulator, push-pull audio, 8Z ohm and 500Z ohm audio outputs (audio output transformer in the receiver) - almost everything necessary to update the old HRO. The plug-in coils had to remain, but at least the switching from GC to BS was easily accomplished with small switches on the coil assemblies (no more moving those four little screws.) Of course, the PW-D micrometer dial had to be included otherwise it wouldn't even have been an HRO (it was certainly the most elaborate logging scale ever included on a receiver.)

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.

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.

HRO-50

HRO-50-1

The HRO-60

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. There was also an ADX coil that covered 35mc to 50mc. 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! And, if $745 in 1964 dollars is equated to 2024 dollars the HRO-60 price would be $7400! Wow!

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.) It doesn't seem that National used the traditional "C" suffix to identify the HRO-60R/SC-2/MRR-2 combo or that they even advertised it as available (I'm using the traditional "C" suffix since it does positively identify the HRO-60R/SC-2/MRR-2 combo.)

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 quite a bit of confusion over what frequency was used in any particular receiver. If you are really concerned, it's possible while aligning any HRO-60 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 probably the correct frequency for that receiver. The correct frequency should then be marked on the two first conversion transformers for future alignment reference. However, only coil sets B and A along with the A "bandspead only" coil sets are operating in double conversion and a conversion frequency alignment error would only affect the tracking slightly and would probably go unnoticed anyway. Besides, when the B and A coil sets are then aligned any tracking error can be adjusted out. >>>

photo right: 1955 HRO-60C Deluxe  SN: 393 0255, rack mount HRO-60R receiver in MRR-2 table rack with SC-2 8"speaker & coil storage unit. This MRR-2 has its original side trim pieces installed. This HRO-60C also has the proper receiver enclosure that slides on the side panels and covers the top and bottom of the chassis and is held in place with 2 thumbscrews. The rack mount SC-2 8"speaker won't provide the kind of impressive audio experience that the HRO-60's push-pull high fidelity 6V6s are capable of. A separate large speaker with an efficient enclosure can be used to showcase the HRO-60C high fidelity audio capabilities. 

The HRO-60C Deluxe - SN: 393 0255 - with the SC-2 doors open showing the ten coil bays.

>>>  With the direct readout dial on the HRO-60 it is possible to achieve very accurate tracking (but that doesn't necessarily equate to tremendous resolution.) You'll find that alignment 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 an accurate readout at known transmitting station frequencies or a "marker frequency" but you'll have to realize that the limited dial resolution won't allow "to the kilocycle accuracy" at any random tuned frequency. The PW-D was there to provide a very accurate logging dial for precise frequency reset ability and a heterodyne frequency meter could be used if extremely accurate frequency measurement was needed.

Performance - 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 conventional IF amplifiers that provide a fairly wide IF passband that does benefit the high fidelity push-pull audio amplifier section of the receiver. However, the HRO-60 and the HRO-50-1 were trying to cope with the crowded band conditions of the fifties and sixties where successful communication was the goal. The additional IF transformers and IF amplifier stage provided a narrow 3.5kc IF 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 also a very capable performer that is able to handle QRM with its Crystal Filter and can also provide excellent high fidelity audio when conditions allow for it.

History - I've owned this same HRO-60C twice. The first time was in 1990. I traded it a couple of years later for an early National FBX receiver. Around 2003, I found out where this receiver had gotten to and was able to purchase it back. During the time the HRO-60C was gone it had been recapped with all 715P Orange Drops. Over the past two decades, I've been able to find most of the optional coil sets besides the standard D, C, B and A coil sets it already had. 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. I have all of the plastic dial scales except for Band J and Band AD. The receiver has always had its original Crystal Calibrator module.

photo above: The HRO-60C (R) chassis with the dust cover removed. 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 (a useless device anyway.)

This HRO-60C had spent many years in a basement in New York City. The chassis "spots" are a result of the somewhat humid environment reacting with the cadmium plating used on the chassis.

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.

CONTINUE TO PART 2