Hi TomG,
My computer died before completing this. Most of these radios are of a rather standard design. The schematics have only minor differences. Use the schematic for the Truetone D3500A if the tubes are the same. Most Truetones were made by Detrola. Don't use the other schematic pictured, as the Battery / AC - DC Radios usually have the filaments (heaters) in series, while the battery only sets have the filaments in parallel. The schematics are a little small for my eyes but it seems that the D3120 Schematic has series filaments and a 7.5V A Battery
RETMA stands for Radio, Electronics and Television Manufacturers Association (Formerly Radio Manufacturers Association). I don't know if RCA, GE, Westinghouse or Zenith were members, but many lesser known companies were members. See the following Wikipedia Link:
https://en.wikipedia.org/wiki/RETMA_tube_designation
Hinners Galanek was one of many small radio manufacturers that was based in Long Island City, NY, just across the Queensboro Bridge from Manhattan. Detrola was based in Detroit, MI
This looks like a 4 Tube Superhet. The circuit is fairly standard with a 1R5 Converter, 1U4 IF, 1S5 or 1U5 Detector /1st Audio and a 1S4 or 3V4 Audio Output Tube.
Your unit looks quite clean. These were the last of the "Battery Tube" portables and were rapidly replaced by transistor radios. Unfortunately, these are not rare, and many of these in great condition still exist. The rare ones are the ones with subminiature tubes. Although the first transistor radios were much more expensive than these 4 tube portables, the cost of the batteries caught up fast, making the transistor radio more economical in the (not so) long run. I bought my first transistor radio in the late 1960s with $3.99 +tax of my paperboy money. This was much less than the cost of the "B" Battery alone for your radio.
The A Battery of your radio is probably the diameter of a C cell but the length of a D Cell or slightly longer. You are correct in using a 1.5V C or D Cell with some sort of spacer arrangement and 7 (63V) or 8 972V) 9V Batteries.
In most cases, it is a good idea to change all capacitors, but in this radio, almost no heat is generated. In addition, the "caps" may be ceramic disks which are usually reliable. However, you will find a 10 uF electrolytic cap from B+ to chassis ground (C11 on the D3500 Schematic). This capacitor should be replaced or it may kill your 9V Batteries or heat up the bias resistor (R7 on the same schematic).
The only other issue that you may have is "Silver Mica Disease". This affects capacitors that are part of 2 IF Transformers. Cross (or fall off) that bridge only if the path leads to it.
Good Luck with this family heirloom.
Best Regards,
John, MrFixr55
