Peavey XR600F no output troubleshooting help please

[clinkous]

I would like some advice on troubleshooting an XR600F p.a. head that was given to me.

I'm told it was last used off of a generator with suspected low voltage. It quit working and was shut off and not turned on again until I got it. I started by turning it on and running a guitar into the input and the peak light is working and I get sound running the main line out into another amp. I'm assuming the pre-amp is working ok. I'm getting signal through it anyway.

I get no sound when plugging in to the pwr amp in jack so I'm assuming I have a power amp problem.

After being turned on for a minute or so a burning smell started coming from the fan ducts in the back. I shut the unit down and removed the power amp board and couldn't find any evidence at all of burned parts or traces. After re-assembly and cool down I tried it again and still have the pre-amp signal but no output and after a bit the burning smell returns.

I'm not sure what to do now. Do I let it get hotter until I can find out what's heating up? That was my next step so I decided this was a good point to ask for help!

I believe I've attached the schematics.

Any advice would be appreciated.

Thank you,
Chris

[Enzo]

You are going to have to zip the 400SC file for it to open here.

[clinkous]

Let me try again with the schematic.

[clinkous]

Sorry, here is a zip. Hope this works.

[Enzo]

That worked. Note: you were not doing anything wrong, normally a simple post works. But we had a server crash here a while back, and certain files that had been posted before are now "poison" in the system, so we have to post them somehow different. A zip form is different enough.

[clinkous]

Makes sense. I've worked on and kit built tube amps and pedals and am trying to learn to fix my own solid state stuff as well. I'm in over my head but that's how I usually learn something new.

So, any advice will help tremendously and I'll tackle each unfamiliar thing as they come. Mainly wondering at this point if I should try to find what's getting by running this amp or if there are things I can check first.

[Enzo]

This is a stereo amp, or at least it has two power amps. The mixer is mono. So do BOTH power amps refuse to power speakers? Plugging into either power amp in jack makes nothing come out either speaker? Turn both master volumes up midway. And then switch the amp assign over from MAIN/MAIN to MON/MAIN. Now the main master works only one side, and the monitor signal feeds the other. Make sure to have monitor feeds up.

You are not trying to use the bridge output are you?

If neither power amp works in either mode, then look for something they have in common. They share the +/-60vDC supplies, are those present? Do the op amps in the power amp input stages have good power on their power pins? Note the op amps on the PA have their own supply, it is not the supply for the front panel op amps.

The amps could be stuck in mute. near the inputs on the left are Q105, Q205. They are JFET mute transistors. They must have a voltage on their gates to turn them OFF. Is that voltage present? Negative something, I have no idea what, several volts at least.

Also possible, the ribbon cable from the mixer back to the 400SC could be failing.

[clinkous]

Thanks Enzo, I will check these things when I return from work this evening.

[clinkous]

Sorry, I had to put the amp down for the holidays but have just gotten back to it. I tried to power it up to check what Enzo suggested but continued to have active burning smells present.

I disassembled again and found the following burned parts:

R256
R257

R255
R252

R230
R204
R203

Q203

All of these were located under the fan box. I've been trying to trace the schematic and layout to figure out what these have in common. Does anyone see something that likely failed ahead of these parts that could cause these to fail? Or should I look after these burned parts for the problem. I'm a bit overwhelmed by the schematic and am having trouble understanding the entire path and the functions of some of the failed parts.

Any help or explanations would be greatly appreciated.

Thanks,
Chris

[Jazz P Bass]

Q203 & Q204 are limiters for the output section.

If the output signal goes too high, Q3 & Q4 will turn on and 'steal' some of the signal being fed to the 'driver' transistors. (Q200 & Q213).

The fact that Q 203 is confirmed bad, along with all of the associated resistors, tells me that the output transistors themselves are probably shorted also. (Q201, Q202 / Q206, Q207).
If they are, then the ballast resistors are most probably open. (R211, 212/ R221, 222)

What I find often helps is to print out that section that failed.
Go through & highlight all of the failed components.
This will give you an idea of the current path at failure.
While it is nearly impossible to know exactly which component started the cascading failure, knowing the path that the current took can help in identifying additional stressed or failed components.

[clinkous]

Thanks Jazz P Bass. I will check the other possible affected parts and print out that section as well. Thanks for the explanation of Q203 and Q204.