New forum member; looking for a primer for SS amp work.

[carlsoti]

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

[DrGonz78]

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 👍

[g1]

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

[carlsoti]

   Thanks for the replies, folks. Sorry for the incredibly delayed return to this post. This amp got shelved for way too long and now I'm stumbling my way through it again. I shoulda cut bait early on. In short, this old Sunn Studio Lead amp just hums and no signal passes to the speakers. The output transistors test good on my PEAk teater and all electrolytics are new(ish.) I'm generally vehemently against shotgunning parts, but the electros were all older than I am, and I've been getting AARP ads for a few years now.
  I think I've discovered one of the reasons that tube geeks hate working on SS amps. Having to scan between the layout and schematic for the appropriate places to probe, in addition to needing to have a half dozen different datasheets open to ensure the proper leads are being probed is a p.i.t.a. when I'm used to knowing what's going on around a tube socket by memory and rarely need to verify the PCB trace paths with vinage hollow-state gear. In addition to that, I really don't understand the function of all the transformer sections of T2.
  With that said, it seems I've got excessive voltage at the second node. Around 47V at the first and 45V at the second. IIRC, around 25V on the third PS node. The first and third seem to jive with the increased mains voltage over the years, but I would've guessed that the second node should be around 40V. This would lead me to believe something around that node is not passing current. (Also of note, I have something like 17V AC ripple at the first node. That seems like a LOT for such a low voltage power supply. Could it be that a bad bridge is clouding the process?)
    I suspect I'll have to probe around Q104-Q106. Without reference voltages, how do I make the determination between an active component failure and the T2 transformer being hosed, which I read here is a "common" problem, despite a lack of relavent info regarding the Sunn Studio/Stage Lead amps?
    I really don't understand why this is flummoxing me as much as it is. I can debug oddball vintage EHX and mutron pedals without nearly this much anguish. It could be an unreasonable fear of an avalanche style failure, or maybe the fact that I can't probe all the transistors IN the amp, without removing the PCB or cross-referencing all the parts to find a suitable test point. The stupid little "suports" prevent me from probing the transistor legs directly. A nudge in the correct direction would be greatly appreciated.
 P.S. I'm in Tempe, Dr. Gonz.

[Kaz Kylheku]

I suspect I'll have to probe around Q104-Q106.

The problem is no signal through the amp, right? Q104 is a good place to start a binary search; it's the start of the power amp.

Use your USB scope to see whether the input signal is reaching the base of Q104.

Ooh, this is a quasi complementary output stage (both output devices are NPN). They are using the T1 as a phase inverter, because for the output stage to swing low, Q1 must be turned on by a positive swing, not a negative one as would be the case with a complementary stage with a PNP on the bottom.

In some vintage solid state amps, you also see an even more tube-amp-like approach where the phase inversion is done by a transistor circuit, but then a center-tapped output transformer is used after the two NPN transistors to combine their output, just like in a tube amp.

It seems the designers here wanted the tone to be shaped by an output transformer, but they decided to make it a smaller one and put it before the output stage, and when you do that, you can kill the phase inversion problem with the same stone.

If the base of Q104 has signal, check that the T1 transformer isn't blown: the schematic gives the resistances that the windings should read.