SWR Workingmans 12 - amp blows fuse

[AudioDoc]

This amp has an oscillation that causes one of the two output xsistors to get hot (bringing it up on a variac). If full 110v is applied, it blows the fuse.

All the output, driver and other transistors/diodes all check fine. +/- supply rails seem fine.

No cold/cracked solder connections noticed...

any ideas?

Thanks!
Dan

[gbono]

Workingman's 12-15-2004 Power and Pre Amp Rev 1[1].pdf

What devices are failing or getting hot Q3/Q4 or Q9/Q10? Did you check the network (zobel?) on the output (Li,R3,etc)?

Do you see osc on the output waveform - how do you know output device is osc?

Is there a load connected to the output?

[Enzo]

Same idea here, has R44 gone open? Over at the output bus.

And turn the bias to the coldest setting.

[AudioDoc]

gbono,

Thanks for the quick reply and questions. Q10 is getting hot (interestingly, not the other output) with or without load. I replaced Q10 just to eliminate it as a problem - same symptom. The supply rails seem to be coming up symmetrically (only getting to about 20-30 VAC before too much current draw.

As I'm novice at these amps (my experience is in hi-fi/stereo repair for many years), I'm wondering if one of the op amps could be causing the problem? (TL072CP)... All the xsistors and diodes test fine...

I'll check R44 to see if it's open per Enzo's post.

thanks again!
-Dan

[AudioDoc]

Enzo,

Thanks...I think I checked, but will double check R44 didn't open. Do you have any suggestions where I can find a service manual/schematic? Or, do you have any suggestions on an on-line service which provides the 'real deal' schematics/service manuals for guitar/musicical inst amps?

I have literally 1,000's of service manuals from my days as a service mgr at Pacific Stereo, (stereo gear) if you ever need a copy. My sets are on fiche, but can make copys.

I'll let you know if R44 was open.

THanks again!
-Dan

[Enzo]

In post #2, gbono posted the schematic. Does that not match your unit?

You are new here, notice the different appearance of the first sentence there? That sentence is a link, click on it.

We have a schematic request section in the forum, you can popst your needs there. Ther is no one repository for documentation, but a good first step is to check the manufacturers. SWR is a Fender brand, and contacting them will usually get what you need. ANd do a search of the forum here for a model, just enter a model name, it may already have been posted somewhere around here. MANY MANY schematics are posted on these boards.

If you have been in audio service, R44 is the resistor in your zobel network, or stability network at the output of the amp. If it burns open, then the amp will turn into a giant RF generator, cranking hi freq oscillation as hard as it can into your load.

Q10 is driven by Q9 and there are a couple 100 ohm resistors to check - R40,41

There are no op amps in the power amp.

[AudioDoc]

I am indeed 'new' here and appreciate your collective input/info. Yes, the schematic looks like the right one. I double checked R40, 41 and 44... all fine. I noticed the pcb is mis-marked for R36 and 39 (swapped)...and still have the same problem.

Now that I have the schematic, I'll dive back in one more time. I'll let you know what I find...

Again, thanks for your help!