[RGK20m3]

Estate sale find, was told it was “restored “, but obviously not, all there with cabinet in fair original finish. Pulled the chassis and found that the filter caps had been replaced (extent of the restoration) probably in the 70-80s (Sprauge orange and blue TVA label). Also the power transformer had been replaced with a Philco part that has the rectifier on the end bell. Brought up slowly on a variac, had sound at about 90v, but at very limited volume even when brought up to line voltage.
Tested tubes on my emissions tester, all ok. Measured antenna coils and both had primary and secondary continuity; oscillator coils seem ok as dial tracking is good. Replaced all the paper and wax caps, and restuffed some period electrolytic cans for the power supply. Out of spec resistors replaced, most were within 10% so left alone. Speaker field coil measures right on spec, and output trans and voice coil measure good.
Fired up and has good volume and selectivity initially; after a minute or two the volume drops in half. Sometimes probing the b+ will bring volume back for another minute or two. The b+ and bias voltages do not change appreciably when this occurs. The AVC voltage does not seem to change, either when going from a station to off station, or when the volume drop occurs. The screen voltages do not change, either.  I tried a new 2nd detector tube, along with a known good IF tube.
Volume increases when grid cap of the detector/oscillator is touched, both when first turned on and after the volume drop. I have another 6A8 coming to try….
Any ideas? I did recheck the antenna coils and they are ok.