Peavey XXX 40EFX - almost no output

[gui_tarzan]

Well, very little output to be more concise. When either channel is turned all the way up you can barely hear anything. Running the head into different speakers doesn't make a difference.

I checked for bad solder joints, cables loose, etc. and found nothing out of sorts. I checked voltages on the tubes, at the power supply connectors, everything looks fine. I've swapped all tubes, checked every resistor and diode on the power tube board and everything checks out normal.

I've run the pre-amp into another amp which gives me normal sound, plugged the guitar into the effects return which results in no sound. I jumpered the send/receive and still nothing. When I put a 1k tone through the input jack I get no grid voltage changes on the power tubes.

Peavey suggested I check Q9 and R125 on the send/receive board. I get 0-14.9-0 on the leads of Q9 when the amp is on. R125 is a 49.9k resistor to ground from the transistor and that reads 0v when the amp is on which is what it's supposed to read. I get 62 ohms between the CT and each side of the OT primary.

I'm stumped. I have no idea what to check next, but I'll be making another call to Peavey tomorrow to see what they say. The only thing I question is the lack of a cable between the send/receive board pins P900 & P901 to the pre-amp board pins P1000 & P1001, which you can see on the right side of the schematic. I don't know if this is needed or not or if it's missing or not. It just stood out to me when I was looking at it.

[DonMoose]

The cable or jumper between P900/901 and P1000/1001 is not in the schematic, but is it in the amplifier?

Q9 looks like part of a timed mute for channel switching. If you have a scope, you might watch for pulses at the gate when you switch channels - the timer may be stuck in Mute mode.

Otherwise, it sounds like Q9 is toast - what happens if you lift the Drain or Source (either one will do)?

Hope this helps!

[Stevem]

pump sound thru it some how even from a I pod or whatever and use your voltmeter set for ac volts to follow thru the circuit and see where it fails to continue on!

Anything that can generate .150 volts of signal will be plenty to drive that amp to its full output!

[gui_tarzan]

The cable or jumper between P900/901 and P1000/1001 is not in the schematic, but is it in the amplifier?

Q9 looks like part of a timed mute for channel switching. If you have a scope, you might watch for pulses at the gate when you switch channels - the timer may be stuck in Mute mode.

Otherwise, it sounds like Q9 is toast - what happens if you lift the Drain or Source (either one will do)?

Hope this helps!

No, it's not in the amp which makes me think it's not supposed to be there but there are marks on the pins as if it has been connected at some point.

Peavey said Q9 is indeed a switch so I lifted the ground lead (source) and checked the voltage and it read 14.9v so it was working.

[pops]

jumper across any jacks that are in the circuit like line send and return. They are shorting jacks and if not used can oxidize and not pass signal. Sometimes plugging something in 20 times will clean the contacts and it will work if that is the problem.

[DonMoose]

The cable or jumper between P900/901 and P1000/1001 is not in the schematic, but is it in the amplifier?

No, it's not in the amp which makes me think it's not supposed to be there but there are marks on the pins as if it has been connected at some point.

So does your ohm meter show continuity from 900 to 901 and from 1000 to 1001? If no - there's your problem.

[gui_tarzan]

Well, part of the problem is solved, I jumpered P900 & P901 to P1000 & P1001 and I have some sound. Not a lot, but a lot more than I had before. I asked the tech at Peavey about the cable and he couldn't tell me because it's not on the schematic, and he didn't have one there to look at but that tells me there's a cable missing.

Damn I hate circuit boards!!! lol!

[DonMoose]

Maybe a few photos? - especially of the connectors and jumpers.

[gui_tarzan]

Got it. I think. With Q9 out of the send/receive board and the aforementioned jumpered I have lots of volume. Huh. Now I just need the part number for the cable that's supposed to be there, I can't find it on the parts list.