I've got a VT-22 / V4 amp running 7027's on the bench right now.
1st channel has Vol/Distortion, 2nd channel has Vol/Bass/Mid/Treb/Reverb. Two white rockers controlling power/standby. There is no polarity switch on this amp. The schematic
lists this version as 1976, this version is probably late '76 at the earliest.
Testing the amp/known issues:
The amp is a bit finicky, hiss and hum typical of an amp with original power section. The only real malfunctioning component is the distortion control. When you turn it up you can hear that signal is indeed breaking up, but the volume gets quieter as you approach the end of the pot's swing. Any ideas?
Grid bias -32vdc (-615vdc on standby)
I've got the electrolytics on order from Fliptops, I'm going to rebuild the power section, bias supply, all diodes and plate resistors.
As for the other power supply resistors - R218 (10W), 219 (20W) & 220(10W) all seem to be OK - they are of the hollow-tube style and they all measure within spec and don't look cooked. Is it okay to leave them be?
To make matters more frustrating, the aforementioned resistors are mounted on the amp chassis by the PT and filter caps...but I see those same resistor numbers printed on the power/bias supply circuit board. So i'm peeking around and see that some of the resistor numbers match up to the schematic, but others don't. I suppose I'll just have to replace the parts as they appear on the circuit board.
I think i'll be okay for most of this rebuild, I'd love to hear some ideas about the distortion issue, as well as adding an adjustable bias voltage. Would it be best to add two trim pots (one for each pair of output tubes), and if so, where in the schematic to insert them? I was thinking after point F - replace R211 and R212 (100K) with a lower value (47K?) and add a 100k trim pot in parallel with each resistor.
Also, which coupling capacitors are important to replace? Anything I've missed?
Thanks for reading! I'll keep this posted as the work commences.