I completed resoldering all joints on the preamp today. I set all voltages and they remained stable for the 30 minutes that I was monitoring it. I plugged it in to an amplifier, and it sounds EXCELLENT. I only got to listen for about 10 minutes before I had to go to work, so I don't yet know if it's reliably stable.
So far, this is the work I have done:
- Re-solder all solder joints, including flush mounted power transistors, and cleaned all flux from the circuit boards
- Cleaned the volume, bass, and treble potentiometers
- Replaced filter capacitors (original capacitors measured lower than specified capacitance, although ESR measured OK)
- Replaced all other electrolytic capacitors
- Replaced pair of 2SA913 (Q) / 2SC1913 (Q) on tone control board with 2SA1930 (Q,M) / 2SC5171 (Q,M) - So far they work, but I haven't yet monitored it for very long
Work yet to be done:
- Inspect and replace cracked resistors on the tone control board (as suggested by Clinic-Audio)
Adam