Peavey Mark III - 260C: Getting 40 VDC out to the speaker

[fieroturbo]

Hey fellas!

Looked through this topic since the same amp with the same problem was covered: http://music-electronics-forum.com/t18559/

Alas, trying everything mentioned didn't help... every semiconductor tests good (except for U2)

So I'm getting a wonderful 40 VDC out to the speaker... which is BAD!

Pulling U2 out of the circuit gets rid of the 40 VDC to the speaker, but only +15 VDC goes to U2 in the schematic... there isn't any 40 VDC that's supposed to go to U2 from what I can see in the schematic.

Also, I've found that turning off the Compressor/Limiter switch kills the 40 VDC.

Here's the catch: I get rid of the 40 VDC going to the speaker, and it gets replaced with 19 VAC... and there's still an annoying hum. I know people have mentioned the two large filter caps, but that was if there was only a hum... not a hum and 40 VDC with it.

It's definitely NOT the preamp; No harsh voltages coming out of it, disconnecting the preamp does not fix the problem, and I can plug the preamp output into another power amp, and everything sounds fine, so that is ruled out.

Any ideas?

THANKS!!!

[Enzo]

Oh, I get it. Just because U2 doesn;t connect to the 40v rail, that doesn't mean it can;t control things that do. Besides, U2 doesn;t connect to the output line anyway. So we wouldn;t expect the 40v back there at the IC. All U2 has to do is tell the input pin of U1 to swing its output all the way over, and the output of the amp will follow.

I know people have mentioned the two large filter caps, but that was if there was only a hum... not a hum and 40 VDC with it.

NO! Never think up reasons not to test something. Bad filter caps can cause hum, and having DC on the amp output doesn;t prevent the cap from being bad.

No circuit works with bad power. YOu have +/-42vDC rails. Check BOTH. Are both up to more or less 42v? Is one quite low? Now flip the meter to AC volts and measure the same two spots. Is there more than a volt of AC on either? Like maybe 19VAC?

U2 is a compressor chip, used to prevent clipping. It is not necessary for the basic function of this amp. Remove it from its socket and leave it out until we have the amp working perfect without it. If I read you, we have a large hum that way. Yes? Once we have made sure both power supplies are smooth, there are not a lot of places large AC signals can come from.

Just to isolate the problem, unplug the cable to the preamp if you have not already done so. Just to eliminate any grounding issues. The 260C can operate all by itself. That exposes the molex pins there in the corner. The pin closest to the corner is the input. I often just touch it with my finger to see if amplification occurs. You can clip a test signal to that pin and ground.

You may find that solving the AC on the output problem takes care of the DC from the compressor IC.

[fieroturbo]

Anyone know of a source for radial, non-screw contact, 5,000uF, 55V or higher, electrolytic capacitors? Cause no one seems to make them anymore.

At least eBay carries the CA3094 to replace U2.

[Enzo]

That cap has not been made in many years. Use current production 4700uf/63v caps. I use snap-in types, and usually I have to drill a new hole to handle the different lead spacing. If you order a 5000/55v cap from PV, a 4700/63 is what they will send. May I suggest changing out a bad filter cap before changing U2, just to see if U2 is OK when the rest of the amp works.

Further suggestion, replace both caps.