Peavey XR600B no sound

[dentsinger]

Greetings.

My XR600b powers up, but then the compression light comes on. I followed some troubleshooting steps from another thread and all leads to a power amp problem. I removed the limiter IC, but now the compression light is somewhat intermittent, meaning mostly on but occasionally coming off. Regardless, no sound whatsoever. What should I check next?

thanks!

[dentsinger]

Also...

I was also wondering what the possibility would be of turning this pa head into a solid state guitar head. Aside from all tube snobbery (I'm one) I'm just curious. Say removing the mixer section and rewiring the channels to act as gain stages or just using the power amp section as such in a different project.

Just trying to keep this thing from getting the boot. Any help out there?

Thanks guys!
mark

[Enzo]

You can get any schematics from PV by request.

What power amp board is in that unit, 400BH? 400B/G? something else? If it is not in large letters on the rear panel, it will be on the foil side of the main power amp board.

COmpare all the DC voltages on the board to the schematic references.

First isolate the problem. PLug the signal source into the POWER AMP IN jack. Anything out the speaker now? That is the entrance to the power amp. Likewise, connect MAIN OUT jack to the inoput of some other amp. This lets you hear what - if anything - is coming out of the preamp. Preamp, mixer, same thing.

If the PA is bad, check the power supplies. The main power for the PA is +/-50VDC more or less. Is it present and both sides more or less the same? Then there are TWO sets of +/-15VDC. The three terminal regulators provide one set to the connector for eth preamp. There is also a set regulated by zeners for the PA itself. Are all those present? If not, find out why.

Still nothing? Try a new TL074 IC.

Also verify continuity from the output bus on the board to the tip of the output jack. A easy spot to check from is that large inductor in the center of the board. SHould be zero ohms from it to the jack tip contact.

[dentsinger]

Thanks Enzo. I do have the schematics and the power amp is a 400BH. I tried the main out to the power amp in. There are a few things I still need to check and will get back to the forum asap. I'm still a little new to my nice multimeter and don't want to fry it or I. Therefore I need to read up on testing the voltage when the unit is on.

[Enzo]

The point I made with the main out and power in was that they be tested separately. Oh yes, connect them together first just to see if there is a jack problem, but once we know it is not that, we want to input some signal - a guitar even - into the power amp in jack to test the PA alone. Then we want to send the main out to SOME OTHER AMP top listen to what if anything comes out of the preamp.

[dentsinger]

I'm able to get sound out of the MAIN OUT with a guitar to the AUX IN, but only to an external source and not through the PA.

[Enzo]

Do you follow the significance of that? I suspect the guitar into one of the input channels might also come out the MAIN out. This tells us that the preamp part seems to be working. So the problem appears to be localized to the power amp.

So on to the power amp. Did you try a new TL074?

The power amp is the rear panel. There is a cable from the corner of that board that runs to the front. That cable caries powr and signal for the preamp. The power amp will operate on its own. I usually disconnect the preamp and run the power amp flat on the benchtop. The pins on that corner connector are ground, +15VDC, -15VDC, and signal to the power amp in. When it is powered and a speaker connected, touching the connector pin for the input should result in hum out the speaker. Once you have it making sound, we can worry about fidelity.

[dentsinger]

I did try the channels and they didn't seem to work earlier today, but now just like the AUX worked, all the channels send audio to the main out. Stranger still is I'll actually get an intermittent second of audio from the PA to the cab sometimes hit or miss. That sounds like a capacitor to me. I thought about ordering a TL074 today but thought I'd not waste an order for a .39 part if there were a few other things I should have ordered thereby saving shipping charges and such.

Testing some resistors, it appears that R20 (a 2k/5w) is open if I'm looking at the schematics correctly. It's the big red resistor next to a heat sink.

[dentsinger]

I'm getting 14.8VDC at those points and no hum, just an occasional scratch sound.

[Enzo]

Well, if it is open, replace it. That would certainly shut down the amp. I bet you don't have that +26V at the end of R20.

I bet that restores the power amp.

But if you still need to try a new TL074, the preamp voards have a bunch on them, and I believ they are in sockets, so one could be stolen from there for testing.

If all your channels are not working, you could also have a bad 074 in the preamp. But cross that bridge when you come to it.

[dentsinger]

I'm assuming I can buy any 2k/5watt resistor to replace it? Sorry, many gaps in my knowledge.

[Enzo]

Yes, it is completely generic.

[dentsinger]

I swapped the TL074 from one of the channels. I seem to get more sound but it's still sporadic and distorted. When I turn the power on, my speakers pop loudly. When it goes off it sounds like a fading zip. Still need to find a 2k/5W resistor to try in R20.

[Enzo]

REplace the open resistor first, nothing else will help much with that still open.

I was thinking AFTER the resistor is repaired, then sub the TL074 if you need to.

[dentsinger]

Well, I thought the resistor was open, but I took it off the board and it tested fine. Aside was taking every component off the board and testing where should I go? One thing I noticed is I'm going by an XR600C schematic rather than a XR600B schematic. They both include the 400BH schematics, but there does appear a few differences. Are there differences? I'm awaiting PV to send the 600B schemes.

Actually, looking on the trace side of the master drive board it say's 400 B/G while on the back of the amp it says 400BH. What gives!