Just purchased a ST80. As I discussed in the other thread, it had issues with the bias adjust display, which turned out to be a burned PC trace. This was on the right channel. I repaired the damage to that, as well as some collateral damage to bias adjust board. Noticed last night that it had some distortion I couldn't put a finger on.
Let the unit burn-in today. Noticed that the left channel (not associated with my repairs), is distorting when increasing volume. Did the usual to try to isolate: swapped power tubes and input tubes. Problem stays with the left channel. Looking at the LEDs for the bias while it playing, the two LEDs for the left channel bias, start to flicker red as the distortion increases, indicating that the voltage (current) on those tubes are getting driven high.
Unfortunately, there are no test points for the bias, so I can't readily track the bias voltage. (For that matter, running this thing without the bottom cover is going to be a real issue, so trying to get ideas up front).
Can I get a 'reality check' on my trouble shooting plan? Going to get a set of tube test adapters so I can probe the tube pins while it's operating. I suspect first thing to do would be to check the high voltage supplies. Each driver tube has it's own dedicated solid state HV supply (+475V) and a dedicated +480V B+ for the outputs.
As the issue is only on one channel, if this is power supply related, I'd assume (looking at the schematic), that the .68uf / 630V caps are fine as they are common to all supplies.
Now where I need a little guidance is where / how to check the amplification, if the power supplies check. As I said, tubes were swapped channel to channel with no change, so it's not a tube issue. I'm thinking if I can put my hands on a scope, check the output of the first stage of the 12AT7 going into the 6DJ8. I guess I should be looking at pin 2 and 7 of the 6DJ8? I suppose I can compare results with the right channel.
Assuming inputs are distorted there, I would guess I need to check the passives (R7, R8, R9 R10,R11) If those check then C1 and C2 would be suspect? If there's no distortion there, then I guess look at pin1 and pin 8 of the 6DJ8? If that's distorted,thoughts? I'd assume I start looking at C3 and C4. If it looks good there, then I guess C5A / C5B?
Having not done this before, can check the driver stage without the power tubes in place? Or do I need them to load everything down?
Sorry to be wordy, but it helps if I talk out my logic, and I'm trying to see from the collective group if I'm proceeding reasonably? (of course this all assumes I can get my hands on a scope).
I've attached the schematics in case anyone wants to play along.