Collins Aerospace Museum

150A Transmitter Restoration Project

Friend of the Museum, Larry Pinkston has volunteered to restore an artifact that has been collecting dust in our Museum's back room.

It is a vintage 150A HF Transmitter, built circa 1934, consisting of several rack-mounted components.

We have reason to beleve that this unit was still in use in the late 60's.

1. 10A Crystal-controlled Exciter.
2. 3A RF Power Amplifier.
3. 500AX Power Supply.
4. Three panels with meters.
5. 19-Inch wide floor rack.

Task Completed

Below, we will upload notes from Larry on his progress...

Here is an example in this May 1932 Advertisment.

—February 5, 2019

I thought I'd bring you up to date regarding my current effort on the restoration project. I'm doing the power supply first, as it appears to need the most work and will make a good "Proving Ground" for the remaining effort. I actually fired up the old unit at a lower voltage using a variac and solid state rectifiers with some success. The filter capacitors actually still charge and I was able to get ratios of all the output voltages for future reference, although I don't know what voltage drops would occur across the rectifier tubes in operation or if everything will hold at full line voltage. I've inserted before and after photos below. The chassis took a lot of grinding, as much of the rust pitting went very deep. The finish is Krylon chrome aluminium spray and I think that it probably would mimic a nickel plate fairly well, although is more subject to receiving scratches. You can see some of the test patches I tried on the lower right of the "before" photo. The unit is not wired yet and would still be fairly easy to re-finish with metal plate at an auto shop, etc. if you think that is the direction we should go.

The 5Z3 rectifier tube is easy to find, but the 2Z2 / G84 (3 required) maybe not so much. Would the museum have access to them? Just for looks, 5Z3's could be plugged into all of the tube sockets with solid state rectifiers below the chassis. One interesting thing, the left hand filter capacitor would have about 450 volts DC on the case, above the chassis since they operate in series. I wonder if Collins though about that in the design.

Let me know your thoughts,



—February 21, 2019

This is to keep you up to date on current progress of the effort.

First of all, I thought I'd better state my goals, for agreement, so that we are on the same track. I assume that these follow normal museum practice.
a. Maintain the original design and appearance throughout.
b. Maintain use of all existing resource, including hardware, unless it becomes impossible to do so.
c. Clean, sand, polish, re-surface, or paint only to a level that maintains what would be perceived as the original design appearance.
d. Aim toward a functional unit using only modifications that maintain the original appearance or can be backed out to maintain the original configuration.

I've been performing the following tasks to date:
1. Re-surface, paint and assemble the equipment rack.
2. Remove, clean, test, repair, check calibration, and paint (only as needed), all of the panel meters.
3. Clean, re-surface, fill where necessary, and polish all of the composite panels.
4. Draw schematics and take notes on all of the design layouts.
5. Perform a complete tear down and re-build of the power supply. This required considerable sanding and re-painting. Original wiring was able to be re-used.
6. Perform a partial tear down and re-build of the 10A Exciter. This incorporates an aluminum chassis which I only sanded and rubbed until any further effort began to alter it's appearance. I maintained all of the original circuit configuration. All resistors are sand-coat type wire-wound, and all are open with no values indicated, which can easily be dealt with later. The final amp RF choke appears to have been shorted and caught on fire in a previous life.
7. Perform a complete tear down of the 3A Power Amplifier. This unit was on top and experienced a lot of dust and general corrosion. This is the current effort being undertaken.
8. Photograph as many physical details as possible during the effort.

General notes:
• Except for the 5Z3 rectifier tube in the power supply, (which was a no-brainer) none of the tube types are identified. Rod, you indicated that there may be archived schematics. None of the tubes appear to have been plate-cap types. I've found evidence of non-plate cap types for all of the tubes, including the final. These include type 82 for the HV / Bias rectifiers, types 45 and 46 for the Exciter, and "Fig. 43" from the early ARRL handbooks for the PA final, which does not have a type identification listed there.
• Coils are needed. Only one exists, see photo below, identified as the oscillator tank circuit. I've found similar types on ebay, but that can be discussed later.
• A rack wiring and R.F. network plan will be required. I have found that black test probe wire may have a 1930's appearance, and would also provide good function. I have a lot of it should that serve the need.

A few more photos...

150A/B 19-inch Rack

10A Chassis before

10A Chassis after

10A's Variable Capacitors

Meter before

Meter after

One of the 10A's Coils

—March 9, 2019

I think that I'm close to being able to return the unit to the museum.
I have the three chassis restored and mounted in the rack and am waiting on some plugs and hardware for the wiring harness plan.
It's fairly certain that the 30B modulator and 1200B power supply would be required to complete the functions of this rack assembly. If these are available in storage, that would be helpful.
The power supply provided is identified as Design 500AX on drawing #406 and only supports the 10A Exciter. I've been in contact with Gary Halverson and he has given me some lead. I have sent him more of the details to work on. I'm currently working on a wiring plan for the rack, based on clues in the schematics provided.
I have attached a word document that contains notes taken as I have progressed, and have provided some pictures below for reference.

Final chassis wiring - 500AX Power Supply

Final Chassis Wiring - 10A Crystal Controlled Exciter

10A and 3A installation in equipment rack

Detail of RF coupling to 3A Amplifier

Detail of slightly modified Antenna Coupling circuit

Populated rack as it currently appears

Notes taken during restoration, re: Collins 150B prototype.

1. Indications that this was a prototype or early development effort are apparent in the general workmanship, soldering quality, and chassis layouts. Each of the shelf brackets supporting chassis behind the rack have somewhat close, but not matching, screw hole centers. Some degree of shuffling and re-drilling was necessary for a good fit in the re-assembly.

2. There is general evidence of a lack of parasitic control and RF bypass, as well as lack of shielding. This is, however, assessed by today’s standards. A review of other date specific Collins designs (1930’s/1940’s) is required for any reasonable conclusion with regard to the E/M design of that era.

3. In these designs, hazardous voltages do appear at some unexpected areas, such as the uninsulated can of one of the filter capacitors on the power supply (approx. +400 volts) and on shafts of the tuning capacitors of the 10A Exciter and 3A amplifier (+ 750 volts, +1000 volts and various high energy RF). The insulated knobs provide operator protection, however this dependency is generally avoided in modern designs.

4. All grounding and circuit returns appear to depend upon the rack mounting. Since the 19 inch panels are non-conducting composite material, this leaves the metal shelf rails to provide these returns unless an unidentified grounding kit was present but not supplied. It is important that mounting hardware be clean, present, and tight.

5. In photographs of field installed and later production type 150A/B units, meters required for tuning of RF circuits are all placed in a row (5 by count) on the third panel from the top. This appears to be for easy visual access during tune-up operations. The prototype unit places these meters on three separate panels mounted at the third (two of the meters) and fifth or sixth (three meters) positions. In a simulated tune-up operation with the unit sitting on the floor, it was necessary to read the lower meters from a kneeling position for accuracy. This is not something that a radio operator would find desirable. This also leads to the conclusion of an early design unit.

6. The power supply provided (identified as drawing 406) is fully compatible with the 10A Exciter, but would not be compatible with the 3A power amplifier. It does not provide a 10 volt AC filament for the amplifier tube (203A, etc.) or 1000 volt DC for the plate. With some modification, series windings etc., it could be used in this capacity but probably not desirable. The 1200B power supply unit, per the schematics found in Collins Museum on-line drawings appears to fully match the 3A requirement. This power supply was not provided. Note: A power potentiometer is included in the lower panel, assumed for line voltage adjustment.

7. The rack rails do not contain 10:32 tap threaded holes, so nuts would have to be placed behind each hole for panel mounting. This was found to be unnecessarily difficult, especially when units had to be removed for further work or shifted in location. Therefore, inconspicuous 10:32 expanding threaded inserts were swaged into each hole, making the effort much easier without altering the dated appearance of the work.

8. Two additional 4 pin plug-in coils, similar to the one supplied, are needed for the 10A Exciter (Crystal Control Unit) for the buffer and amplifier tuning.

9. A plug-in coil assembly for the 3A amplifier plate tuning/antenna coupling is needed. A mock-up coil unit was constructed for visual museum effect and also appears to work functionally for tuning, but using non-period design and construction.

10. Most of the vacuum tubes complement, including types 10, 46, 47, 82, 203 - and type 45 (for CW application) is needed for accurate restoration.

11. Schematics with part values are needed, in that all of the wire-wound resistors in the 10A unit are open and values are unknown due to metallic corrosion. An alternate design schematic was supplied (10J version, drawing #1146) of the crystal oscillator, also using a type 47 tube, so similar resistor values were substituted as a starting point for this restoration.

12. There is no AM modulator chassis supplied, although a modulator plate current meter is provided on one of the meter panels. The modulator would be required for the 150B version.

13. In CW mode, a type 45 tube in the power supply provides both bias rectification and current limiting, probably to allow bias cutoff for keying and possible “key click” suppression. This probably can be replaced with a type 82 or similar in the 150B (AM) version where bias voltage remains a constant.

14. Two switches appear on the same panel as the power potentiometer. One switch appears to be the master on/off switch and the other appears to be a standby switch which removes cathode return to the 10A Exciter while leaving filaments and B+ active.

15. Accurately matched drawings (schematics) for the 3 provided chassis were found on the Collins Aerospace Museum website. These drawings, however, lacked any numerical part values.

16. It is determined that a Collins type 30B modulator and type 1200B power supply would be required for complete restoration of this rack unit.

17. It is interesting to note that all of the available drawings for the 150 transmitter units indicate that they were checked by A. A. Collins.

—L.W. Pinkston, March 5, 2019

—April 8, 2019

I am happy to report that the 500AX and the 10A sections of the 150(B) prototype restoration, using the resources available plus some substitutions, appear to work.

It is very unstable in the tuning, however, with everything appearing to interact. The E/M design, as previously observed, is poor by today's standards. Many more bypass capacitors are needed, and the composite rack panels don't provide any form of ground plane for the chassis. Hand capacitance through the front panel appears that it may even have an effect on critical tuning.

Nonetheless, I think that I am ready to call this phase of the effort finished. With the correct tubes, the unit should display well at the museum.