Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Hello all. I'm new to the forum and SS guitar amp work in general, but I built my first tube amp in the 90s as a teen, cut my teeth working at Rockford Fosgate/Hafler around the turn of the century, and have been doing tube amp and guitar effects repair on the side off and on for a while.

I'm looking for some sort of primer for how to diagnose a vintage SS Sunn Studio Lead amp so I don't have to pull ALL the transistors to test them.
 
The amp came to me with the complaint of "it only hums." I found that the caps were all original and the output cap had puked it's guts out. Against my own better judgement, I shotgunned all the electrolytics as the originals are all older than I am, and as one might expect, it did NOT fix the issue. The output transistors test good on my Peak DCA55. Voltages in the power supply seem to be more or less in the ballpark with the power transistors removed. I have a crappy USB Hantek O-scope, but it's been uncooperative since I moved recently.

If anyone could point me in the direction of some sort of primer on how to proceed without simply pulling all the transistors to test OOC, I'd greatly appreciate it.

T.I.A.,
Tim C in AZ

Well how about some voltage checks on the transistor legs? Personally I never pull a transistor unless it's dead or very questionable. I do chuck old dried up electrolytics in the garbage. Voltage checks and tracing signals are your best bet. A good transistor may never work if you have a cracked trace in the circuit board. What part of AZ are you? I'm in Scottsdale 👍

Since it was not blowing fuses, you can put the output transistors back in.
Like DrGonz said, voltage checks are the best way to proceed.

Since it has an output cap, it will be AC causing the hum, not DC on output.
You can disconnect the speaker if the hum is too annoying, and measure the AC volts at the output.

Schematic available here:  https://www.ssguitar.com/index.php?msg=16694