|:trying to help out an old s38c, basically an aa5 with converters, I swapped out all the old paper and electrolytic caps. I also had to remove the if cans to lubricate the stuck slugs. Now I get an oscillation in the audio which gets much worse when the volume is increased. This occurs with switch in am or cw position. I checked to make sure the replaced cap in the cathode circuit of 50l6 output tube was ok. It is. Could be the one of the IF amps, but I don't know where to start there. Any ideas?
Make sure the 12SA7, 12SG7, and 12SQ7 are metal tubes, not GT. Make sure the IF is the specified 12SG7 not a 12SK7. Make sure pin 1 of tubes is grounded as on schematic. Make sure wiring to speaker is routed away from front end wiring and 12SQ7. Check AM / CW switch. Also, all electrolytic capacitors are not the same; the new types may be inductively wound - the original were not. Try bridging C14A, C14C, and C14D with .1 mfd, 250 volt film capacitors. Hopefully you have replaced 'paper' capacitors with 'film' capacitors - not ceramic. C5 should be a high quality film capacitor. Try connecting capacitor from 50L6-GT plate to grid 2 - not to cathode as on schematic; leaving this connection, disconnect C14A, the 50L6-GT cathode bypass resistor. Unfortunately the Halicrafters S38C did have some stability problems - they changed the BFO from the S38A to cut costs, and the new circuit is funky. I have seen in service literature that certain tubes could cause problems - I think certain date codes of Hytron tubes were suspect. Hope this help. EdM