No problem thanks for these details g1. But where exactly is sector A4? I'm on page 2 of the schematic and  can't see it...thanks.

Also looking for a decent pre-amp schematic of for the SWR220--the ones I've uncovered so far are blurry when enlarged and hard to see otherwise. You think SWR would have a decent archive of their schematics!

Does anyone know where I can find a schematic for the power amp section of an SWR studio 220 bass amplifier?

Thanks Roly and Enzo. Good info moving forward. Caps were definitely bent to fit into chassis--they can't be stood up  as they would stick about 1.5 " above chassis. Not sure what the orginal caps were but I will find out to satisfy my curiosity.

Questions: The graphic equalizer faders on this amp are in rough shape, full of static, seeming to lose their smoothness and "sticking" ability once pushed downward to their lowest setting (they suddenly speed up as if freed). There are four and one of the far left cuts out the signal when pushed down to the bottom. There are de-greasers, de-oxit products on the market, but they are not cheap! What are the best products in your opinions when it comes to a good FaderLube and contact cleaner. And what would you do, in terms of a fix, for these tired old faders (aside from trying to buy new ones which SWR no longer provides. Didn't have much luck so far trying to locate any).
I live in Canada.
Thanks again!

Thanks Roly. I have the same data sheet and what you say makes total sense. I think what I'm finding confusing is how the TDA relates back to CON1 and all its 15 pins. How is it that pin 10 of the TDA goes to the  +power rail (12,14,15 of Con 1)? When I look at the TDA block diagram pin 10 is considered to be part of the "mute" and I find it hard to determine if that should be part of the -rail or + rail. Thanks.

I've included two images. Amp belongs to a friend. There are three leads from the two caps on the board and only two are being used. To me this doesn't look like a dual cap, so I don't understand why there is an extra lead. It's not used in the amp and the amp works just fine. Thanks!

Great stuff thanks. When you pointed out W13 and 15 as outputs the penny dropped. :tu:
The only thing that doesn't makes sense is when you say that  7 and 8 are +/- supplies. According to the TDA7293 Data Sheet pin 13 and 15 are the supplies and 7 and 8 are +/- signal which, on the schematic on page 3, go from 9 on Con1 to pin 14(out) of the TDA7293. To me that would make more sense to have, but I may be missing something. Thanks again!

Great, that really opens things up for my understanding! I see what you are talking about.

Still confused about power amp schem. Shows both having the speaker outputs at W12,13,14,15. On Con 3 we have W13 attached to Pin 9 and W12 attached to Pin 1 (similar for Con 4) On the last page of the schematic, pin 9 from Con 3 appears to go to pin 14 of the TDA7293 which is the ouput but Pin 1 goes to the inverting input of the TDA7293. In my mind these speaker outs should come off pin 7 and 8 as they are the neg, pos signal. Is this an AB amplifiers? Shouldn't we have two opposite signals at the same time? Guess I'm showing my ignorance here. How exactly does the signal go to and come out of the TDA7293? Thanks!

Sorry, I was referring to the schematic.

Last question for the night At the guitar input there is a line that goes up and all around the page and ends up at Pain R. The line also goes to the Headphone Out and ends in a circle and then on the other side of the Headphone Out the line goes to the circle on Pain L and then Pain Left goes to Pain R. Then on the far side of the page this is either fed to Con 2 or vica versa.  I understand the idea of PreAmp and Power Amp and older schematics make it much clearer. When I look at this it seems backwards to me. Looks like the master volumes are feeding pin 1 and 7 of the outputs of the opamps??? What does Connector One Feed??? Thanks again!

It is thanks! It sounds like it should and no crazy radio signals and horrible sounding start-up noise. It's my mule so I'm sure I'll muck something else up but that's how I've been learning. I was hoping that I could somehow have the board turned upside down so I could read voltages at certain points in the circuit board that I can't access from the component side. But obviously that would screw up the grounding scheme and would be asking for trouble. Guess it's a matter of trying to pinpoint the issue, then turn over the board (without power and of course draining caps first) and measure the components to try and find the fault. I suppose you could try and measure the PT secondary voltages by somehow scooping your test leads under the board but that sounds dicey

Second Question: Re: the DSP board on this amp. It's about 2X2" and has three chips on it and SMD components. My guess is that this is for the Digital FX processing. I can't see how this could be repaired as everything is really, really small. Would it be a case of chucking out the old one and ordering one from Marshall? It works, just wondering what others have to say about that and how they've dealt with it. Thanks!

So I should be embarassed here! I believe my problem was a huge grounding issue!! When I put the board back in I didn't put on my nuts/washers and tighten them because I intended to take out the board and study some more components etc...so now radio issue gone, amp sounds great, no nasty noise at start up

When I plug my guitar in and touch the volume pot the radio issue is non-existant.

Clear enough. So that nasty sound occurs sometimes but not always. Plugged a jack into the FX Send but couldn't hear the nasty sound when I turned the amp back on. I have the bare pot shafts exposed so I turn them without the knobs. On the Overdrive Channel when I touch the Volume shaft I get a local AM radio station coming through loud and clear!  Wondering if this has anything to do with it... Just turned it on now and that nasty sound came back--noticed that the gain was maybe half way, turned it down and re-tried turning the amp on a few more times but no problem...problem is not consistant...I've tried different volume settings and couldn't get the nasty sound to reproduce....

I'm wondering if this problem has to do with the Metallized Polypropolene  100nK 275 Volts cap on the line side...