I would highly advise against the extra resistor mod from between the diode and the potentiometer to the base of Q1. What it will do, is to put a resistor in parallel to the potentiometer, which decreases the resistance value of the extra resistor and the potentiometer from the diode side. That might kill the amp just as much as a failed potentiometer.
Instead of a 2sa539, I used two bc637 which are closer to the original longtail pair transistors with an hFE of about 40-160.
On my Nikko TRM-600, Q1 was totally dead. Someone already worked on the right, dead channel, but didn't get it to work right, because of the dead Q1. They also replaced the power transistors with BD317 and BD318. They have a higher voltage drop between their base and emitter, so the idle current adjustment wouldn't work right. The highest idle current i got out was 3mA. Instead of the original bias adjust diode, i used two 1n4005 in series to get the voltage drop of them higher than the main transistors voltage drop. That worked nicely.
Now i'm the proud owner of a working timebomb. Yay!
Instead of a 2sa539, I used two bc637 which are closer to the original longtail pair transistors with an hFE of about 40-160.
On my Nikko TRM-600, Q1 was totally dead. Someone already worked on the right, dead channel, but didn't get it to work right, because of the dead Q1. They also replaced the power transistors with BD317 and BD318. They have a higher voltage drop between their base and emitter, so the idle current adjustment wouldn't work right. The highest idle current i got out was 3mA. Instead of the original bias adjust diode, i used two 1n4005 in series to get the voltage drop of them higher than the main transistors voltage drop. That worked nicely.
Now i'm the proud owner of a working timebomb. Yay!
Very important observation. Thanks!
For posterity... This is a Nikko STA-8080 receiver schematics. I have one on the bench right now and if you drop in a pair of BC556's for the differentials w/o upgrading C661/C662 to 100pF (like I've initially tried), it will definitely oscillate. If you're not on a DBT is will also ruin your day!