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