Ampeg SVT 350H: Troubleshooting/repairs

[spsimmons]

When the owner dropped this amp off, he said that the fan wasn't working consistently. He said that the fan would cut out, then the amp would overheat and shut down.

When I tested the amp, it had power, but no output. (The fan doesn't run either.)

I opened it up and found several burnt components in the output section.

R24 off of Q13 looks like it exploded. Q13 "looks" fine.

R35 and Q8 both appear to be burnt/heat damaged.

I'm still pretty green at this amp repair thing. I've successfully done 4 or 5 simple repairs on SS amps and built a couple of simple tube amps. I can test. I can solder. I'm not charging for the repair, just doing it for the experience. (Well, I do owe him, he gave me a sweet music stand once.)

I can replace the components that are obviously and/or probably damaged, but I'm wondering if there is something deeper going on.

The basic 350h/B-2 circuit makes sense to me, but I don't really understand what these damaged parts do.

Are these symptoms indicative of a bigger problem? Any other parts I should replace while I have the soldering iron hot (and the parts shipping paid)?

THANKS for ANY advice.

-Shannon

[spsimmons]

OOPS! Here's the schematic http://music-electronics-forum.com/t13334/

[Jazz P Bass]

Seeing that there are issues with Q13 & Q8 circuits, I would look real close at the output mosfets Q10, 11 & 12.
One or more of them may be shorted.

[MarkusBass]

You may also check ballast resistors - at least R37 - whether they are not open. With such a failure it happens that that more components fail.

Mark

[spsimmons]

Thanks for the guidance, guys. I haven't had a chance to dig into it yet, but intend to this weekend.

[spsimmons]

I had to set this amp aside for a while because I had too many irons in the fire (and my wife complained that she was being neglected).

I pulled the obviously damaged components (Q8, R24, R35). I then pulled the whole bank of MOSFETs from that side (Q10, Q11, Q12).

I figured that I would test and probably replace all three. When I test them, they appear to be shorted in every direction.

I tried to follow the testing procedures in these videos: https://www.youtube.com/watch?v=gloikp9t2dA https://www.youtube.com/watch?v=RBJGOOTEwfU

I'm still unsure if I'm testing them correctly. To get a comparison to a good MOSFET, I decided to pull Q16 and test it. I get the same results.

Either I'm not testing these components properly, or they are all fried. Is it likely that both sides of the power amp have failed and all 6 MOSFETs need to be replaced?

I'm seriously questioning my test procedure. If all six are bad, what could cause such a massive failure?

[J M Fahey]

Yes, they can all br fried.

Typical cause for such a catastrophic damage is, after a more normal failure (probably involving 1 to 3 transistors) owner tries to repair it with a roll of aluminum paper or a 20/30A fuse he pulled from his car.

[Jazz P Bass]

Yup.
When one side goes, it usually takes out the other side.

[spsimmons]

I just sat down with the amp and checked through the output section pretty thoroughly. Boy, this thing is a mess!

Here is a list of parts I found that were bad: Q8, Q10, Q11, Q12, Q14, Q15, Q16, R24, R35, R36, R37, R38, R39, R40, R41, R47, R48, R50.

I went back and checked the fuse. It's a Bussman 10A 250v ceramic. Seems like the right fuse and it's not blown.

I would have thought the fuse would blow before the whole power amp would burn up.

Any ideas of other components to check before I place a parts order?

Any ideas on a possible cause?

Thanks, Shannon

[Jazz P Bass]

I would replace all of the 47 ohm gate resistors on the outputs. (flameproof)

If Q8 took a hit, I would also replace Q3.

Double check the diodes 15-18, 25 & 28 as they had to be under a whole lot of stress.

[g1]

Sometimes users will remove the tin-foiled fuse/crowbar and install a new one before dropping off the unit for repair .