Ampeg SVT-3 Pro Repair help, R27 catches fire on power up

[Barnaby Jones]

Hello,
So I am attempting to repair an SVT-3 pro. I think the input jack was broken and just wanted to check it to see if the connection was intermittent.
Firstly I made a rather large mistake, from the speaker output I plugged into my fender amp 'power amp in' thinking it would pass through to the 8 ohm speaker. Turned it on and 1 second later R27 caught fire and R33 heated up and exploded. I turned it off and yeah realized how dumb that was, the amp says Effects loop over top the jack and power amp in underneath the jack. My assumption is that that impedance was too low or outputting a signal to the output stage of the svt-3. ???

So, I replaced R27, R33, Q5, and Q6 because I think they were faulty. I turned the amp on again with nothing hooked up (input or output). And sure enough R27 went up in flames instantly. R33 survived but was warm to the touch, no other components felt hot that I could tell.
Also I think C10 may have failed Open, is that possible? how important would that be to the circuit's functionality, is it for decoupling/noise? I don't have an immediate replacement for it.
Other notes: D9, D8, D28, D29 did a diode check and they showed consistent voltages across each. (none failed short)
Besides me being not so savvy with the music equipment would someone be able to give me some advice with finding the root cause? Maybe a next step?

Thank you so much for any input. Everything helps.

[guitician]

So you were trying to run the Ampegs pre-out into the Fender amp? But, instead plugged the speaker out into the Fender? What signal was running the output of the Ampeg at the time?
I'm guessing that you shorted the output devices and burned up those resistors in the process. You need to check the SoildState devices too.

[Barnaby Jones]

Ampegs speaker outputs into the fender effects loop, power amp in.
So you think I shorted the mosfets and any connected transistors? Is there a way to check the mosfets without taking them off of the board?
SCM ELECTRONICS SVT3PRO SCH Service Manual free download, schematics, eeprom, repair info for electronics

Thank you

[guitician]

The ground, or sleeve of the 1/4" plug doesn't seem to have a problem being shorted, so you maybe OK with the MOSfet's. but Q6 may have blown.

[Barnaby Jones]

Ok, I'll replace that again.
Do you know functionality of C10 in that part of the circuit?

Thank you for your help.

[Jazz P Bass]

My honest opinion is that you will not succeed in repairing this amp.

You pulled a bone head move by plugging a speaker output, which can deliver amps of current, into a small signal power amp in jack.
By the way, have you checked the Fender? (what model is it anyway?)

You blew up a $1000 amp.
As it is, it is most probably repairable by a very competent tech.
You are only going to make it worse by messing with it.

I am not trying to be a hardnose but this is not a simple amplifier.
It will take a good understanding of each circuit function and a rack of test gear to repair this amp.

[Barnaby Jones]

Yeah, I realize I made a huge mistake and am very disappointed with myself. Yeah, my logic was flawed and saw what I wanted to, I saw power amp in and thought well I have a power amp, impedance of the speaker is good, and disregarded effects loop label above it.
I see it is possible to purchase the main circuit board for around $400.
Could that possibly fix the problem or is it possible that it is damaged in the pre-amp stage as well?
I do realize that this is not a simple amplifier, I would like to learn the functionality of each circuit, and attempt to find/fix the problem(s).

I appreciate your honesty, I honestly want to fix this thing though not so much for the money but for the experience too.
The Fender model is m-80 and works fine of course, much rather have blown that.
Thank you for your input.

[guitician]

Pull the power amp board and see if just the pre-amp works by itself. You should have tested all the parts after the first thing blew, I don't think that you know enough to do a component level troubleshooting/testing to the power amp section. There may still need to be a setup/biasing procedure for the new power amp board which could require test equipment too.

[Barnaby Jones]

So if I separate the boards I would have to input the proper voltage to pins on J5/J6 to power up that board, correct me if I am wrong.. I'll have to look at the schematic to see what pins/voltages.
As for testing all the parts after the first thing blew, I checked what I thought was blown Q6 was toast and so were the resistors mentioned. Based on the part of the circuit that was pulling current I thought maybe replacing those would fix the problem. By test all the parts, you mean every single component on the board?

If I had a working SVT-3, would it be safe to disconnect the jumper harnesses from the power amp board to the preamp board and then turn it on?

Test equipment such as...

Thank you

[Jazz P Bass]

Where did you locate a power amp replacement board?

Sometimes they arrive LESS the power transistors.

Edit: Not in this case though.
They are already mounted to the heat sink.

http://www.fullcompass.com/product/4...FY4XHwodeHgAgg

[Barnaby Jones]

Same exact one you found. I should have linked that, sorry. I was a bit off on the price.

Do you think it would be safe to swap in a new board like this? And what adjustments, if any, need to be made?

Thank you

[Jazz P Bass]

There are no 'safe' conditions until tested 'safe'.

At the very least, the power supply voltages should be confirmed good.

The new board may or may not be set for a proper low idle bias.

I would hope that it was 'tested' at the factory.

[Barnaby Jones]

So are you suggesting I take this route, and do some pre-functional checks like checking power supply? It sounds like I could do more, what else would I check? As far as biasing I have biased my tube amp before, would I be biasing the current flow through the tubes and/or power output stage?

Looks like the driver tubes are not included, should I just put the old ones in?

[52 Bill]

I guess that you could replace the board and that may fix the amp. Or you could install it and the new board could blow up because of a problem with the power supply. Or ...

This approach to amp repair bothers me. It's like saying , "My car won't start. I know, I'll replace the engine. That'll fix it."

You can and should do whatever you want to do, as it is your amp, but it just goes against my basic thinking. Best of luck.

[Jazz P Bass]

Way too many assumptions.

My suggestion is to take the amp in it's present state to a qualified tech.