Yeah I think I might have screwed the pooch there...   xP
I thought the file I had loaded was right but just had the wrong named file....

I think this file is correct and maybe the OP can confirm this schematic is correct or post a copy?

Edit: Enzo is right on with these amps having a grounding screw on one of the DSP board standoffs. That needs to be looked at too.

Good news on the power amp side testing good!  :tu:

Assuming you measured the correct location on Q204... That measurement really caught my eye looking it over... Everything looks good as Roly said, but still if we are looking for -25V and only measure -.693V. Well then yes Q204 could be a problem. Personally I would measure that again and make double sure. Possibly you can measure Q202 for the +25V and Q204 to show the -24V. Or measure R208 and R201 respectively, or what are the voltage readings at pins 4 and 7 on A201??

To have an amp putting out 160-180mv on the output at start up is not a huge concern really. However, the surge and steady downfall in DC on the output, with hum for about the same time, does make me question the caps too. Solder joints maybe suspects too. Do we have a schematic for this amp? Would want to test some voltage readings on the amp to see if they are healthy.   

Test the speaker leads for DC on output. Test it as you turn on the amp to see if there is DC voltage on the output. Lets see if there is a surge of DC as you start the amp and then see if it goes away.

Well posting a schematic may prove to be over your head, but to some here it will help them "help" you better. Plus you can read it a bit to understand the basics of the circuit. How handy are you with a soldering iron? Put the amp on the overdrive channel and give the amp a nice hit with your fist.  :trouble Now we are wanting to hear if the signals returns or responds suggesting a loose connection. Do the same on the High Gain channel too. A lot of times cold/cracked solder joints or dirty jacks can cause all sorts of havoc in amps shunting the signal. So, we need to do this basic test to see if it responds to getting whacked. :duh

Look on page one of the schematic that I have attached, quadrants (B,3) bottom left. It says something about the DSP PC Board (pins soldered to preamp). Notice that it says switch lines... DSP pin14= clean, DSP pin16= solo/od. I would be making sure that all the solder joint on all the pins connected there are good.

Basically, that is a simple way to start thinking about troubleshooting the amp. Give us some feedback as to what you might find and describe all symptoms with as much detail as you can. Good Luck.

So yeah Obama met yesterday and I was disappointed to see a green tie when I was expecting yellow. The funniest amp I ever bought was a Peavey Backstage Plus... I called it "The Package" not quite sure just why either?? I find if you want to have broken/dirt filled pots the Peavey amp needs to date to at least the 1980's.  :tu:

Edit: Notice the smaller Reverb knob?? That pot was missing a knob and it could not be turned. Still put a Peavey knob on it but it was from one of those small porta-amps from the 80's.

Personally I would want to measure TP14 and TP15 for the (+/-40vDC). I would measure this once immediately after turning on the amp and then play the amp until it starts to cut out completely. Then I would test the voltage on the power side of the amp. To me it would seem something is heating up and likely could be a bad solder joint/component. I mean the power amp is lost already to some degree (half of or quarter the volume) and then after it heats up, it cuts out completely. If you are doing your tests right we can't blame the preamp for this occurrence so far.

What I think we all would like to see here all the TP9-TP13 voltages measured, recorded and reported. For another I would like to you to check all the IC(1-5) chips pin 4 and 8 for the -/+16v to see what those are measuring. Describe how you are measuring these voltages as much as what numbers you actually measured. Got to know you are doing this correct, since reading this I am a bit confused as to where we are going at this stage of troubleshooting. Take is slow and don't rush, as I don't mean to come off as rushing. Let's get down to numbers and measuring them the right way.

For example, I would test all the TP9-TP15 at the starting of the amp... Write those numbers down, and then play mp3 signal into the amp till it cuts out... Then test TP9-TP15 and lets see what we get... Are they the same or different?? My gut tells me it is something very simple and the voltages might show us better where to look for a bad solder joint or component. Personally, after doing this I would solder the entire power amp portion again. Then try the same test again.

P.S. When you run the iPod into the FX return for while does the amp eventually cut out too?? All of these questions need answers. Good luck  :tu:

P.P.S. Also here is a site that you can create a free login to get 1kHz sine wave sample that is 30 seconds long... Just create mp3 and loop that track on your iPod. That will suffice on testing the power amp circuit.
http://www.freesound.org/people/klangfabrik/sounds/28636/#

Yeah well here is the 87 version of the amp if that proves to be any help at all...

Well the problem is probably not the caps. I would suggest checking the amp further to figure out what is the real problem. Check all jacks that might shunt the audio output stage. Low output just does not make sense with filter caps being shot. Hum would make more sense here if it was the caps.

Does the schematic that I posted resemble the amp that you have??

Edit: Simulpost!! lol Sorry Enzo!

One thing to think about in a situation like this is other components that may have been damaged. Do you have a light bulb limiter? It is the best tool you can build for about $10 cash, so why not build one. Even though the fuse has not blown out this time it will be useful as you do repair work on shorted amplifier circuits. So, are you sure the replacement parts were good?? I mean they blew up and all... But maybe they were not specified for the voltage or amperage on this amp??? That is the first question that needs to be answered. Send a link to the datasheet that shows the output transistor that you purchased for the amp. I mean to say that the casing types may differ but maybe heat transfer to a heat sink caused the trouble. But it sounded as if these output transistors blew up rather quickly, which does not sound like a over heating of the output chips. I would be looking for bad resistors or other shorted components as I inspected the related circuits.

Maybe my day was just crazy and my brain is mush.... Maybe that is everyday for me... Sounds like your referencing a schematic here and it would be nice to follow along in better detail. Can you send us copies through messages/email or post a copy on the thread? It will help greatly to follow along with you, thanks.

Edit: Sorry I thought was looking a schematic that might be the right one but was wrong. However, I think this was the right one?? Let us know if this schematic is the same as the one you are referencing...?

Double Helix Edit: Yeah that looks like the right to me... Look at the link to MEF website for the exact problem you described.
http://music-electronics-forum.com/t13806/
Read the last post for a possible fix.

First thing first unplug the speaker before turning on the amp at this point. Nothing on the preamp side of things should be blowing fuses. My concern is first for the speaker and then being able to run the amp without blowing fuses. So it would be nice to build/use a light bulb limiter which there is a sticky thread and many threads about building one located on this site. Once you have the amp through a limiter then see if there is any DC voltage on the speaker leads so use a DMM to check. Got to figure where the short is on the amp and at this point a light bulb limiter will hopefully keep the amp from blowing fuses. Sure there could be bad solder joints and things going on, and an inspection of power amp area of the board is very important.

Got to make sure we didn't fry the speaker and using 9 volt battery touched on speaker leads listen for a pop.

@Phatt>>> If I understand correctly "Plug pack" could be an AC to DC adapter for foot pedals? Perhaps that adapter had a reversed negative type adapter??? To me that might do something later down the line on ground line for the power strip...>>>

Try one trick to get rid of hum... Guaranteed not to blow up your amp too... Try clipping a ground wire from the input jacks ground to the chassis. I know this violates star grounding schemes etc etc etc... But it won't cause your amp any harm at all I promise. Let us know if connecting the input jack ground to the chassis makes any noticeable difference, as I am curious on this front.

I would not replace those main filter caps on first check... Instead I would remove the solder and re-solder the existing caps to start. While I was in there I would solder up the Bridge or rectifier diodes, while also hitting solder joints on all the wire wound resistors to make sure those were solid. Also, hit the input jack solder joints as that might be the culprit right there to start. Let's see how many parts really need replacing and how many parts just need to be soldered w/ real leaded solder...

Edit: BTW what make model is this amp and what year do you think it was made?

Also, to note is that the board you have is a bit different than the board w/ the under over wiring. It is like someone modified the board by use of a drill right through the main ground trace on the board. On your board that same trace appears to be not as wide and I would not recommend ever drilling through a trace in the first place, especially on your board revision.

@G1 >>> I like the idea of adding glue at the end to ensure that the wires stay in for a long time.

@Roly >>> I don't think it is solid core wires... I remember the wiring on these was stranded but just lifeless copper due to age. The wiring was not the best quality IMHO. Also, great idea about getting good hook up wire from dead computer power supplies!! I got about three that I will gut soon. I will have to revisit a post that you had too about building a dummy load using a power supply enclosure. That was a great idea too!