This particular radio was very well cared for. The case is in nice shape and it was relatively clean inside. The first thing that was done was to check the tubes. All the tubes were in good condition. The Audio Input preamp 12BA6 was the only one that was beyond what I consider "weak" and was consequently replaced.
The gain control was removed and cleaned. The carbon track was filthy and required cleaning for smooth uninterrupted operation. However, this was not the problem. The 5uF capacitor that goes between the wiper and ground C1613 was shorted. This was easily replaced with a modern capacitor.
The PA Load adjust knob did not turn the capacitor. The coupling was a piece of 1/4" fuel hose. That hose had disintegrated. The easiest fix was another piece of fuel hose, but this time instead of trying to glue it to the shafts, I made some crimp clamps out of aluminum tubing.
There had apparently been some arcing on the PA Tune capacitor. This caused the plates to scrape against one another as the knob was turned. This was rectified by filing off the arced metal and bending the capacitor plates back into alignment.
The lack of audio modulation was another matter. One of the wires was off inside the microphone, so that was repaired properly with shrink tube rather than duct tape. Figuring that this was all the issues solved, we plugged it in and tried it out.
Success was not yet obtained. There was still no modulation. So the microphone was opened up again and investigated. A small LM358 audio amp that I keep in the desk drawer is perfect for use as a signal tracer. The crystal microphone itself ended up being the problem. It wouldn't produce any output, and it was open-circuit.
Nothing could be done to make it any worse, so the foil part of the element was opened like a tin-can exposing the real issue. The mic must have been dropped at some point, which broke the wires from the element. These flat "wires" are a strange material that resists solder, so they had to be crimped in place. Some small scraps of resistor leads were used to crimp around the delicate flat wires out of the crystal element, and then soldered to the output leads. This procedure was not at all easy, but finally successful in that the mic then produced output into the test amplifier. The foil was straightened back out and hot-glued back in place along the edge. The repair looked worse than it really was and actually sounded fine. The SDR was set up and modulation was finally obtained. A quick adjustment of the PA Bias was required before putting the case back on. Even though the output wasn't a full 260 watts, it could probably be attributed to the age of the final tube.
