02-28-2023, 08:24 PM
I am wondering if you can't just replace the chokes with just plain resistors. RCA used these a lot in earlier biased "power detectors" but have not used them in later radios, especially those using diode detectors. These RFCs are in the plate and coupling circuits from the 1st AF Amp to the interstage transformer. Iam pretty sure that the output stage is Class A Push Pull. If the secondary of the antenna transformer is good, just connect a long wire antenna to the grid cap of the 1st RF Amp, at least to test the radio. I won't bet my life on it but will bet lunch off the (non-existent) McD's $1.00 menu that the radio will work in that manner if nothing else is wrong.
Below is a section of the R75A schematic showing differences in the detector and audio section.
[attachment=28025]
Note that L12 is not in this circuit. The detector circuit has been rearranged from diode biased 1st AF Amp to a much more standard AF Circuit with a more standard volume control. This will also allow you to input an external audio signal. You will likely need to keep the original volume control circuit as it works by adjusting the bias on several circuits, in conjunction with AVC. in later radios with single end AF Output and RC Coupling between the 1st AF and Output, there is no RF Chocke in the audio circuit. This is my rationale for suggesting replacing L11 and L12 with resistors and seeing what happens. The fact that the output tubes are 2A5s and not 47s does not really have a bearing on this issue. In fact, 47s can be replaced by 2A5s without much modification other than the tube socket, wiring in the cathode at the center tap and possibly the cathode bias resistor.
Below is a section of the R75A schematic showing differences in the detector and audio section.
[attachment=28025]
Note that L12 is not in this circuit. The detector circuit has been rearranged from diode biased 1st AF Amp to a much more standard AF Circuit with a more standard volume control. This will also allow you to input an external audio signal. You will likely need to keep the original volume control circuit as it works by adjusting the bias on several circuits, in conjunction with AVC. in later radios with single end AF Output and RC Coupling between the 1st AF and Output, there is no RF Chocke in the audio circuit. This is my rationale for suggesting replacing L11 and L12 with resistors and seeing what happens. The fact that the output tubes are 2A5s and not 47s does not really have a bearing on this issue. In fact, 47s can be replaced by 2A5s without much modification other than the tube socket, wiring in the cathode at the center tap and possibly the cathode bias resistor.
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
Best Regards,
MrFixr55