Western Historic

RADIO  MUSEUM

NATIONAL CO., INC.

 HRO Communications Receiver

 "The Cream of the Crop"

 ~ Production History ~ Serial Number System ~Dating by Serial Number ~ Accessories

~Production and Engineering Changes ~ Restoration Suggestions & Performance Evaluation

by:  Henry Rogers W7HTR

 W9DXX, Alice Bourke, Chicago, IL ca.1935 -  from Frank C. Jones "Radio Handbook" 1936. This interesting photo shows a very early HRO receiver with a Peak Pre-selector to the left and a 1934 AGS-X receiver to the right.

History of the HRO Design Origin

The National 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 was a high quality, high performance receiver built from the best parts available. 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 communications 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 as 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. 

Above:  This 1935 ad is from Leeds, a New York radio dealer, showing the high regard that was accorded the new National HRO

The 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 kinds of noise (like hum) out of the receiver an external power supply would be used. An added benefit of the external supply was the additional heat was not in the receiver cabinet. 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 few kilocycles meant that drift showed up as several 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. HRO receivers finally began shipping 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 by way of his monthly open letter to hams that was published in QST.

The published story for the origin of  the HRO designation related that all of the inter-department paperwork was stamped "H.R.O." which stood for "Hellva Rush Order" since the time table for the project was a "rush order." James Millen later 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 decided they didn't want the receivers to be referred to as "HORs" and the letters were rearranged and became HRO - then the "Hellva Rush Order" story created to explain the HRO designation.

Photo:  HRO  sn: H-103 was built in September 1935 and has most of the characteristics of the early receivers

The HRO from 1935 to 1941

The HRO receiver was first available to hams around March 1935. This was still an austere economic time with Depression recovery taking place slowly. The HRO was an expensive receiver, priced at nearly $200 with all of the necessary accessories - and this was the price from the major discount dealers, like Leeds of New York. Most communications receivers at the time were selling in the $100 to $130 range (the Patterson PR-10 sold at discount houses for $70) making the HRO one of the most expensive choices for the radio amateur. Yet the HRO was popular and sold well. Most of the dealers would take a small down payment with monthly payments in order to actually have a buyer for almost any of their stock. Hams knew the HRO was worth the price, with performance that eclipsed the competition of 1935. In fact, the HRO was so well designed that very little engineering upgrades were required for several years. Most of the changes during the thirties are to physical aspects of various components - either to make the part better or to make it cheaper (or both.) Depression economics did affect the design of the HRO cabinet. It is simple and inexpensive to build - only held together with a modest number of sheet metal screws. Certainly, when the nickel plated micrometer dial became a painted unit in 1936, economics were the main influence to the decision. When it came to the engineering side of the HRO, only a few component values change slightly over the years. One significant component change was the elimination of the National-made resistors. These must have been very expensive to build, test and label. Not surprisingly, purchased resistors from Centralab show up around 1937. National didn't mass produce the HRO either - each production run was usually only for about 250 units. A total number of about 9000 receivers were built between 1935 and 1941. 

Early HRO Serial Numbering System

At first glance, National's serial numbering system seems confusing. There was a rumor that the serial number system was designed to hide the actual production quantity of receivers being built but this seems unlikely. Several years ago, Charles Fisher performed a survey of about 70 early HRO receivers to determine the serial numbering method that National used. His article was published in the AWA OTB in 1989 (it is also currently on the web.) The Fisher article contains a wealth of information for the HRO enthusiast.

 Fisher's survey determined that National used a combination letter prefix and number suffix for their serial numbers on the early HRO line, e.g. D-12 or G-115. Each production run of receivers were given a new letter prefix and the numbers are then sequentially assigned. The first HRO run begins with prefix "D" and was probably only about 100 units. Run "E" and most subsequent runs are made up of about 250 units. Sometimes runs overlapped each other. Letters "O" and "I" may not have been used due to their similarity to numerals. By about mid-1938, the letter prefix had gotten to the end of the alphabet and the serial numbers were changed to number prefix with a letter suffix but numbers up to 500 were used, e.g. 123-F or 434-H. By the beginning of WWII, this type of serializing had reached the "L" suffix and was halted. During WWII, the numbering again changed - this time back to the letter prefix but using number suffixes higher than 250. Also, during WWII the letters "O" and "I" were used in the serial number combinations and there has been an HRO-M discovered that is serialized with a double prefix followed by three numerals,... PC-134.

With the 1946 HRO-5TA, a completely different method of numbering was used consisting of two groups of numbers with no letters. The format is usually three numbers, a space followed by four numbers, e.g. 184 1054. This serial numbering format was used from the HRO-5TA up through the HRO-60. It appears that the first three digits identify a production run and the four digits the particular receiver in that run. Certainly more examples of these later serial numbers and the receivers to which they are assigned would help to reveal what the numbers actually indicate.

Total production quantity up to about the beginning of 1941 was around 8000 to 9000 receivers. Around 5500 to 6000 receivers comprise the letter prefix serial numbers with the remaining 3000 or so using the letter suffix serial numbers.

Dating Early HRO Receivers by Serial Numbers

Charles Fisher compiled a list of serial numbers from 70 early HRO receivers during his survey for the AWA's OTB article. By using the serial numbers compared to the evolution of the example it was possible to extrapolate a logical sequence of serial numbers. To the right is Fisher's table of serial number prefixes and suffixes tied to probable dates of manufacture. Since the survey was only concerned with early HRO receivers the cut-off for entry into the survey was that the receiver had to use the plug-in crystal on top of the crystal filter unit. The dates only go up to the beginning of production of the HRO-M receiver which had crystal installed inside the crystal filter unit.

As more HRO serial numbers are reported and entered into our log it may become possible to also include the later HRO receivers in this table.

National HRO Serial Numbers Needed   

    I have started a serial number log here for all tube-type National HRO receivers. The log will be divided up for each model of HRO and will include all tube-type models from the first HRO receivers up to the HRO-60. This log will eventually be an on-line source for dating any of the HRO receivers. The more serial numbers sent in, the more accurate the data will be. If you have several HRO receivers, be sure to designate which serial number goes with which model receiver.

Send HRO serial numbers:  Western Historic Radio Museum

             D, F                            January-March 1935                                          

             F, G                           April-July 1935                     

             H                              August-September 1935

             J                              October-November 1935

             K                           December 1935-January 1936

              L                              February-March 1936

             M                                  April-May 1936

              N                                     June-July 1936

           P, Q, R                       August 1936 - February 1937

           S, T, U                         March-September 1937

           V, W, X                       October 1937-April 1938

             Y, Z                              May -October 1938

       Letter Suffix  A                     Nov-Dec 1938

       Letter Suffix  B - L           Jan 1939 - early 1941

 HRO Serial Number Log (Pre-war and Wartime)

HRO Senior: D-38(R), G-113, H-103, K-83(R), 463-K, 434-L(R)

HRO Junior: L-58(R)

HRO M:

HRO-5:

Navy RAS, RBJ,etc:           

(R) = Rack Mount, otherwise receiver is a table version

HRO Serial Number Log (Post-war)

HRO-5TA, TA-1: 184 1054(TA1),

HRO-6:

HRO-7:

HRO-50, HRO-50-1:

HRO-60: 345 0165(R), 393 0255(R)

(R) = Rack Mount, otherwise receiver is a table version

Differences Between Early HROs and Later HROs

Though there were several minor changes to the HRO throughout production, there are a couple that are quite obvious and used by collectors to identify whether a particular HRO is of early production or is from a later period. Here are two of the very noticeable changes shown below, the chassis paint color and the cabinet style. The early HRO chassis is painted black. This was the same color that the SW-3 and FB-7 chassis were painted and the HRO followed National's manufacturing practice at the time. In 1936, the HRO chassis paint was changed to gray. This was probably to keep with the chassis color of the new NC-100 series and eliminate the need for two different paints for National chassis. Other distinguishing features between the early and late HROs are,...early chassis have round IF transformer shields and round BFO coil shield while later gray chassis usually have square IF transformer shields and square BFO coil shield. Sometimes collectors will refer to an HRO as a "black chassis" or "round IF cans" and "gray chassis" or "square IF cans" for the later HROs. For more differences, see the "Production and Engineering Changes" section further below.

Early Style Cabinet  vs  Later Style Cabinet

When first introduced, the HRO receiver had minimal ventilation provided in the metal table top cabinet. Thirty, .25" diameter holes were provided on the back of the cabinet but no louvers were used. Beginning in 1936, the cabinet design was modified to have sixteen, 1" diameter holes in the rear and, shortly thereafter, the louvers were added to each side. Shown in the photos below are the early style cabinet on the left and the later style on the right.

The 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 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 actually inquired if they could order the HRO - minus specific circuits they believed they wouldn't need - at a reduced price. 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 income to get an HRO. National announced the budget HRO in February 1936, dubbed "HRO Junior." From that point on, the standard HRO became known as the "HRO Senior."

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 push-pull switch and adjustment pot. Additionally, the coil sets for the "Junior" did not bandspread and were identified with a "J" proceeding the regular coil set letter designator. 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 40 meters was a CW only band. National even offered to upgrade the budget receiver at a later date (perhaps when the owner had saved up the money) for a very reasonable price.

The HRO Junior wasn't very popular and today it is quite a rare set in its original configuration. Perhaps some were sent back to National for conversion to an HRO Senior though there doesn't seem to be much evidence that happened too often. 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 they had little use for an S-meter, Crystal Filter or Bandspread coil sets.

The HRO Senior

From February 1936 on - coinciding with the HRO Junior introduction - the standard HRO was referred to as the "HRO Senior." From this point up to just before the WWII, the HRO went through several minor physical changes. Most noticeable was the change from the nickel-plated micrometer dial to a black lacquer dial. The black paint color on the micrometer dials varies from dial to dial and sometimes appears blackish-olive drab and other times dark gray. The inner dial readout also changed from nickel to black with white numerals. Other changes were to the chassis paint, probably to conform with the paint used on the new gray paint being used on the 1936 NC-100 receivers. Also, at this time 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, this was probably to conform with the NC-100 series. The next change (in 1937) was to the S-meter - going from a non-illuminated white scale unit to an illuminated amber scaled meter built by Marion Electric Instrument Company. Finally, an identification tag was added to the upper right corner of the front panel in 1938. This completed the physical evolution of the HRO from 1936 to 1938. From then up to WWII, the HRO looked the same except that some very late HRO Seniors will have a bar knob for the selectivity adjustment. Most National enthusiasts will refer to the 1935 receiver as  "HRO" - the 1936 to 1941 receiver is called "HRO Senior" - though many collectors just use "round IF cans" or "black chassis" to ID the particular characteristics of the receiver they are describing.                                                          

 photo: HRO Senior sn: 463-K was built in 1940 and has most of the later characteristics.

photo: Rear coil is an "E" coil showing the logging chart. The front coil is a standard bandspread C coil with two graphs. Both coil sets have the early style 3/16" thick aluminum panels.

HRO Coil Assemblies

The individual HRO coils are identified by a letter and number stamped into the insulator block. The letter will designate the frequency range and the number identifies the function of the particular coil. The following shows the system used:

Frequency Range:    A = 14-30mc   B = 7-14mc   C = 3.5-7mc    D =1.7-4.0mc   E =.9-2.0mc   F =.5-1.0mc

Coil Function:   1 = RF1     2 = RF2    3 = Mixer   4 = Oscillator

Additionally, coil assemblies have graphs on the front which also identify what the frequency range is. The left side graph is for general coverage and the right graph is for bandspread. Switching this function is accomplished by moving a 4-40FH screw from threaded holes in each of the insulator blocks. The screw position should correspond with the position of the proper graph to select either general coverage or bandspread, e.g., screw in left hole=general coverage, screw in right hole=bandspread. Only the A,B,C and D coils sets can be set to bandspread. The eight holes on the top of the coils allow access to the trimmer capacitors for general coverage or bandspread alignment. The holes in the rear of the coil shield allow access to the padding capacitors and inductance adjustments. Early coil panels are made out of 3/16" aluminum. During WWII, the panel material was changed to 1/16" thick steel. Since the coil set of four A,B,C & D were sold with the receiver, they are serialized with the same letter-number combination as the receiver. The coil serial numbers are usually stamped on the upper rail or on the top of one of the coil shields. Coil sets E and F provided a logging chart for favorite stations since a bandspread graph wasn't needed.

The Various National Power Supplies

Initially, there was only one of the smaller power packs ( the "Dog House" style) that could be used with the early 2.5vac HRO - the 5897. The 5897 was originally designed for the SW-5 receivers in 1930 but was later redesigned for the FB-7 receiver in 1933 and designated 5897AB. It had enough current carrying ability to operate the HRO. In fact, many early HRO receivers were paired with 5897AB supplies that have printed on the top ID label "Especially Designed for the FB-7." Under full load, the 5897AB will provide 2.5vac heater voltage at the receiver and +230vdc B+. All of the National power supplies have an increased tube heater voltage at the transformer to compensate for the IR drop across the power cable to the HRO (or other receiver.) There were also rack mounted power supplies, the GRSPU to operate a single receiver and the GRDPU for operating two receivers. Either rack supply could be ordered for either 2.5vac, 6.3vac (or a combination of the two) heater voltages. Be aware that there are a couple of other "dog house" power supplies that look just like the HRO types but do not have the current carrying ability to operate the receiver properly. For instance, the 5887 was another FB-7 power supply but its B+ rating is for +180vdc. The 5886 was designed to operate the National SRR and later the 1-10 receiver. Although it will seem to operate a 6.3vac HRO Senior, actual measurement of the B+ will show that the load has dropped the B+ to ~+150vdc. National did publish that the HRO would operate on B+ as low as +135vdc but added "at some sacrifice in performance."  There is also the 5880 power supply that was advertised for the SW-3  receiver, though similar in appearance it doesn't have the current capacity to power an HRO.

The later (1939) Model 697 "Dog House" was designed to operate the 6.3vac HRO Senior receivers and provides the correct voltages under load. During WWII, a heavy-duty 697 was produced, the 697W. It is has a plug-in electrolytic can capacitor and is usually fungus proofed.

The photo shows a standard Model 697 on the left and the earlier "Velvet AB" 5897AB on the right.

National Speakers

National was a little vague about speakers when the HRO finally became available in 1935, saying only that one could be supplied if desired. The rack mounted speaker was the first to be shown in an advertising photograph. It looks very much like the rack speaker that was supplied with the AGS receivers. The standard National black box speaker with the cloisonné "NC diamond" emblem was available very early but photos were not used in National advertising until much later. The standard National box contains an eight inch Jensen PM speaker with audio output transformer mounted on the speaker. National also built large "NC" black box speaker using a 10" diameter Rola speaker, however these normally have a field coil speaker installed. They were designed for the NC-100, NC-101X family of receivers with push-pull audio and provision to excite the field coil of the speaker. The HRO doesn't have that capability, (I suppose one could replace the original Rola speaker with a 10" PM speaker and use a Triad S-53X audio output transformer - if you really wanted to use the large "NC" speaker box with an HRO.) After WWII, the speaker cabinets were available from National as NDC-8 and NDC-10, so it is possible to find an original 10" cabinet with a PM speaker installed. Shown in the photo are the 10" speaker for the NC-100 series on the left and the 8" speaker for the HRO on the right. NOTE: There was also a wooden speaker enclosure sold by National, probably for the NC-100 family. Few survive in good condition.

HRO Coil Boxes

National offered at least two types of wooden coil boxes for storage of unused coils. The smaller coil box allowed for storage of three coils and assumed that HRO owner had the four coils that came standard with each receiver. One coil would be installed in the receiver and the remaining three would be in the small coil box. Also offered was the large coil box that would allow storage of five coils. It was for the HRO owner who had opted for the .5-1.0MC coil (F) and the .9-2.0MC coil (E.) These two coils provided coverage of the Standard AM BC band and also allowed the user to bandspread coil D (80M) and still cover 160M with coil E.

The wooden boxes are assembled with finger joints and the rear strips are mounted with nails as are the small blocks that hold and space the coils. The wood used is boxwood or poplar and the finish is just a light stain. Very simple construction - but the boxes really made storage of the unused coils convenient.

National's Receiver Upgrades and Services

More than any other radio manufacturer, National advertised their willingness and ability to do rework, upgrades and general service to their receivers. It was well known that National would upgrade their HRO Junior to an HRO Senior for about $20 - probably the cost of the parts. However, National would also upgrade earlier receivers to the latest specs (if practical.) Shown in the photos below is the HRO receiver originally owned by F.E. Handy, W1BMI, the well known Communications Manager for the ARRL in the twenties, thirties and forties. Copious notes in Handy's original HRO manual reveal that he sent the receiver to National in March of 1946 for a new S-meter and alignment. In November 1946, he sent it in for upgrades that included a Noise Limiter circuit, polystyrene antenna terminals, new style IF transformers and changing the cabinet to the later style with improved ventilation. The workmanship is first class, including the small chassis that was added to house the noise limiter circuitry. Serial Number is G-113. The F.E.Handy HRO is owned by: Dale Sanford

The F.E.Handy HRO Receiver - Handy was Communications Manager for the ARRL in the 1920s,30s&40s

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. In fact, some catalogs do mention that any 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, the Navy HROs will usually have an "anchor" ink stamped somewhere around the chassis or chassis mounted parts.

Perhaps the HRO's most famous use was in England where banks of HROs were set-up as intercept receivers. Bletchley Park still has many original HRO receivers on display at their museum. At first, Britain couldn't buy the HRO 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. After Lend-Lease though, the British were able to have a steady supply of HROs direct from National.

The HRO Senior was given a major engineering upgrade just before the end of 1941. This was the modification of the crystal filter to use an internally mounted crystal, thus eliminating the "easy to lose" crystal that plugged into the top of the filter assembly. The design also changed the operation of the crystal filter to a switched selectivity with variable phasing adjustment. After that change, National referred to the new HRO as the HRO-M. The Navy wanted a simple to use receiver and National supplied HRO Juniors in fairly large numbers as the RBJ, RAW and the RAS. The RAS had a 175KC IF frequency to allow the receiver to cover the 400KC to 500KC range without interruption. The RAS models also have special coils for 175KC IF operation. The Navy receivers were rack mounted and usually were supplied with anywhere from five to nine coils that were housed in a combination speaker and coil storage rack unit. Actually, the Navy bought a lot more NC-100A (RAO series) variations than they did HRO variations.

Near the end of WWII, the HRO was upgraded to all octal metal tubes (except the 6V6GT audio tube) and most of the components became JAN standard values. This receiver was dubbed the HRO-5. The round ID tags on the toggle switches were installed on the last of the WWII HRO-5 receivers. The HRO-5 was sometimes powered by the 697W heavy duty power supply. Sometimes receivers were fungus proofed depending on the service location.

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 models with general coverage coils and a aluminum silk-screened panel with graphs and ID mounted to the coil assembly. Some of these coils sets will be ID'd as JA, JB, etc., to indicate they are general coverage only. Updates came fairly fast and National quickly upgraded the A,B,C and D coils to have the bandspread function, designating this receiver the HRO-5A. By early 1946, a new Noise Limiter circuit was incorporated, adding two tubes to the receiver bringing the total to 12 (counting the PS rectifier.) There was no ideal place to add the noise limiter control so National mounted it about the only place there was room for it - to the left of the micrometer dial. The noise limiter potentiometer somewhat blocks easy accessibility to the 1RF adjustments when performing an alignment. Because of the new noise limiter circuit, the receiver designation was changed to HRO-5A1, although usually a "T" or "R" is added to indicate table or rack mount, e.g., HRO-5TA1. Still early in 1946, the round S-meter was changed to a square housing made by Marion Elec. Inst. Co. All of the remaining 1946 production of the model HRO-5A1 receivers had the square S-meter installed (this was probably until about June'46.) In mid 1946 a short lived HRO-6 was produced with an improved Noise Limiter but its appearance is identical to the HRO-5TA1. For the 1947 model year a complete "make-over" was given to the old HRO. Introduced in October '46, the HRO-7 had a new rounded corner cabinet, smooth light-gray paint job, levers for easy coil removal, S-meter mounting behind the panel, different knobs....however, the circuit remained basically the same. The local oscillator was changed to a miniature tube and a voltage regulator was added, but the HRO-7 was really not much more than a cosmetic upgrade. It was the end of the rectangular black wrinkle finish box though - the HRO that had remained basically unchanged for the past 12 years. The next upgrade was going to change almost everything about the old HRO receiver.     

photo: The 1946 HRO-5TA1 sn: 184 1054

The HRO-50, HRO-50T1 & the HRO-60

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 to go. The new HRO power supply was built-in, though on a separate chassis that was thermally insulated from the receiver chassis and bolted behind the main chassis. Relying on the micrometer dial versus graphs was also gone. Now a 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. Sockets were provided for the optional 1MC/100KC crystal calibrator or NBFM adaptor. Voltage regulator, push-pull audio - almost everything necessary to update the old HRO. The plug-in coils had to remain along with the micrometer dial - otherwise it wouldn't even be 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 each IF stage. With 12 tuned IF transformers, the receiver was very selective. The HRO-50-1 was produced in 1951 through 1952. The finial evolution of tube type HRO receivers was introduced in 1953 - the HRO-60. With 18 tubes, double conversion (above 7MC) and a current regulator on the oscillators, it was the final evolution of the tube-type HRO. The HRO-60 was produced from 1953 up to about 1964. At the end of production, the selling price had escalated to an unimaginable $745.00! 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 HRO-60 and the HRO-50-1 were trying to cope with the crowded band conditions of the fifties and sixties and successful communication was the goal - provided by steep skirt narrow bandwidth resulting in fabulous selectivity. Operating a rebuilt and correctly aligned HRO-60 is a pleasure - QRM is rarely (if ever) a problem and it is still a competitive performer at any frequency.

photo left: 1956 HRO-60R  sn: 393 0255, rack mount HRO-60 receiver in MRR-2 table rack with SC-2  8"speaker & coil storage unit

1935 HRO Receiver Restoration

To the right is a photo of HRO sn H-103 taken when the receiver was first obtained. It is obvious that the cosmetics were in very good condition. The yellow "R-014-8" designation may have been an identification for commercial use but nothing was documented as to its meaning or of the receiver's former use. Most of the problems with H-103 were under the chassis. Foremost was the 1.25" hole that had been punched in the front left section of the chassis. This was going to be a challenge to repair. Also, the receiver had been totally re-capped using modern-style yellow plastic capacitors. Additionally, there were some non-original resistors, a few modifications with some extra non-original parts installed.

The goal in the restoration was to bring H-103 back to full operation using the original design circuit and using parts that would have the appearance of the originals but were rebuilt new parts. The most time consuming part of this kind of restoration is restuffing the paper-wax capacitors. Since I didn't have the original caps, I ended up robbing a complete set of HRO "Sprague" paper-wax capacitors from an old HRO parts set. These caps were restuffed using modern "yellow jacket" caps installed into the original paper-wax shells. The shell ends were then sealed with colored wax. Proper orientation was maintained though this was for aesthetics only. While this is sometimes tedious work, the end results are a receiver that looks totally original and performs as new. Photo right: HRO SN H-103 before restoration

Below is a "before" photo of the underside of the chassis showing all of the non-original type capacitors along with other non-original parts and circuit modifications.

Below is an "after" photo of the completed restoration of the underside of the chassis showing how restuffing caps preserves the original appearance.

This photo shows the aluminum hole plug installed but not painted. Also note that the plastic cap is missing from the push-type "ANT" post.

After the plug was fit and the interface filled, the paint matching was attempted. Since this was a rather large area, artist's acrylic would be too flat of a finish to match. The photo to the right shows one of the "black" paints used that was a fair match. Several "black" paints were tried but the best match came from Testor's Jet Black, plastic model paint that comes in the little glass jars at hobby stores. See the chassis photo below for the final appearance. Though not perfect by any comparisons, I think the fix is better than the gaping hole.

All of the tube sockets should be cleaned. I usually spray De-Oxit on the pins of the tube before inserting it into the socket a few times. I also clean the grid caps as they are always oxidized. On the HRO, you also have to clean the flex finger contacts for the coil sets. These are usually very dirty and if not clean will compromise the all-important contact with each coil in the coil set. 

The next step was alignment of the IF, which is straight forward although the IF frequency is determined by the actual crystal frequency - in this case 456.5KC. After the IF is aligned, each coil set has to be aligned for proper tracking. Most HRO receivers don't have all of their original coil sets anymore. H-103 was no exception, having only the 40M coil set with it. We usually have to rely on "orphan" coils to make up a complete set of coils for each HRO receiver. These "orphan" coils must first be disassembled, cleaned and inspected. Many times dirt, dust, bugs and spider webs, along with broken wires or bent condenser plates will be found in each of the coil boxes. In fact, the 40M coil that came with H-103 had a broken connection inside the 1st RF coil that had to be repaired. After reassembly, the coil set needs to be aligned to the specific receiver that is going to be used with. There is a significant difference between each HRO receiver, especially between the 6.3vac tube type and the earlier 2.5vac tube type. While any coil set will function okay in almost any HRO, the alignment will assure greatest sensitivity, correct tracking and best performance of the receiver. When aligning HRO coil sets, the general coverage must be aligned first and then the bandspread. Switching to bandspread requires relocating four screws on the coil contact insulator blocks. The eight adjustment trimmers are accessed from the top of the chassis through holes just in front of the tuning condenser. It is easier to access these holes if the front-top piece of the cabinet is removed. It is held by the two top screws in each corner and the dial pointer "diamond." If the lower end of the tracking needs to be adjusted, the padders can be accessed through the holes in the back of the coil bay from under the chassis. Once the coil set is aligned, the final adjustment is to readjust the 1st RF coil trimmer to the station antenna. I happened to also have correct vintage E and F coils for this HRO. The E coil provides tuning of the 160M band along with the upper part of the AM BC band. Coil F covers 480kc to 950kc for the lower part of the AM BC band. These coils do not bandspread. They also have a padder adjustment on the top of the coil set so when aligning, both high-end and low-end tracking can be adjusted from the top access holes. 

The photo above is the finished result. Like all fully restored HRO receivers, H-103 is very sensitive and the bandspread capabilities make the amateur bands seem to tune on and on - 400 divisions in fact, or the equivalent of about nine and a half feet of linear dial. The feel of the micrometer dial is smooth and the accuracy of any reset is precise. Audio is very pleasing even though it is just a single-ended 2A5. I am using an early National speaker box with the cloisonné "NC" emblem. Since the HRO receiver originally has no internally mounted audio output transformer, National installed the output transformer in the speaker box.

1940 HRO Senior Receiver Restoration 

The following is an article I wrote in 2006 for use in  "Assembling an Authentic Pre-war Ham Station" that detailed the restoration of my 1940 HRO Senior. Temporarily deleted - but not lost. Here it is again....

I have had HRO 463-K for 20 years having purchased it from an old friend for $40 back in the mid-1980s. I never did much with it except acquire a complete set of coils (it did have the matching 40M coil with it,) replace some missing tube shields and general clean-up. I thought back then that the performance wasn't as good as other HRO receivers that I had (and had sold) but I never investigated further. So, the HRO became just part of the pre-war equipment that was on display but not used. 20 years later, a rekindled interest in early HRO receivers prompted me to take another look at this old relic of mine. With a little coaxing the HRO did seem to function. An alignment also seemed to help but generally the receiver didn't have the sensitivity expected and also gain across the band was not constant.

A complete re-cap job was performed on the HRO but rather than just replace capacitors with no regard for under chassis appearance, I decided to re-stuff all of the paper wax caps to preserve the original looks of the assembly. Performing this type of detailed rework turned up something unusual - a factory mis-wire that was not caught during test and alignment at National. The error was that C9, the bypass cap for the 2nd IF transformer secondary, was connected to the wrong side of R-8 which placed C9 connected directly to the AVC line. Additionally, C9 was only wrapped to the terminal strip - not soldered. Obviously, this was a "transparent" type of problem - one that would not be caught in test because the error would not have affected performance enough to be noticed. Then one also has to take into account that the receiver was used for many years and this problem never noticed - certainly not a major concern but interesting how these sort of errors are found so many years later. When finished with the re-stuffing of the capacitors all resistors were checked for correct value. I usually allow about 25% tolerance but all of the resistors were much better than that. The photo to the right shows the finished result.

As for the top side of the HRO chassis, a thorough cleaning showed that there were a couple of minor rust areas. These were touched up using artist's acrylic that I matched to the exact color of the particular chassis or cabinet. Since every painted piece of an old receiver has aged somewhat differently the colors always require custom matching. Fortunately the areas involved on the HRO were small and touch up not noticeable. The tuning condenser had to be removed for the mixer finger contact assembly repair so the entire assembly was cleaned and lubed while off the chassis. When the gearbox cover was removed for cleaning and new lubrication it was found that one of the anti-backlash springs had come off of its mounting peg. Just another example of minor problems that turn up while doing a detailed restoration. The S-meter scale was severely discolored which is typical for plastics of this time period. I happened to have a very nice original Marion Elec. Inst. Co. S-meter that had a nicely colored scale. Although the red was somewhat faded, the overall appearance was a vast improvement over the original meter scale. The cabinet and front panel were in nice shape so only minor touch up was necessary to have everything ready for reassembly.

After installing all of the tested tubes, shields, the front panel and knobs we were ready for alignment. The HRO has an IF frequency of 456kc +/- whatever the exact crystal frequency actually is. This is determined by sweeping the frequency around 456kc using a signal generator with the crystal filter on and listening for the peak that occurs at the exact crystal frequency. In my case, the crystal frequency was 456.3kc and the IF was aligned to that frequency using the S-meter as an output indicator.