Peavey Butcher Repair

[BenSC]

Hello,
i have a mid 80's Peavey Butcher which suddenly stopped passing signal.
the amp still turns on, and when the post gain knob is turned up the speaker cab gives off a higher volume hum. but the pre-gain knob gets no response and there is absolutely no guitar signal being passed through.
i figured the problem might be in the preamp, so i searched for bad connections or problematic parts, and despite some shoddy looking repairs done previous to me owning it, everything seems to be alright.
i've checked for continuity throughout just about the entire thing (seems like several times), but haven't found the problem.
i've also checked the obvious things: input jacks, pots, preamp tubes.

anyone have any suggestions or ideas?

thanks for any help,
Ben

[JazzGuitarGimp]

If the amp has an effects loop or preamp out / power amp in jacks, you can use these jacks to isolate the amp into two major chunks. At least you will be able to tell if the problem is in the preamp or the power amp (or both).

[Cameron]

If the amp has an effects loop or preamp out / power amp in jacks, you can use these jacks to isolate the amp into two major chunks. At least you will be able to tell if the problem is in the preamp or the power amp (or both).

And clean the jacks of the loop....it's where the problem may be.

[martin manning]

And clean the jacks of the loop....it's where the problem may be.

Good point Cameron. A quick way to see if there is an issue there is to put a patch cable across the loop jacks, jumping send to return.

[BenSC]

thanks for the advice.

there isn't a typical "send/return" loop.
there is a "booster output" on the front of the amp. i cleaned that and tested it, but it gets the same output signal as regular output jacks: just a mild hum which gets slightly louder when the post-gain knob is turned up, with no guitar signal going through.

any other suggestions?

[martin manning]

With no signal at the "booster output," there is no signal getting out of the preamp, or the PI is not functioning. Have you tried substituting a known good tube for each of the preamp tubes?

[BenSC]

yeah, i've tried a whole different set of pre-amp tubes.

good idea about about the PI, though. i'll check out the PCB again and look for components around the PI.

any other suggestions appreciated

[Reeltarded]

Pull the PI and turn it on. Is the output still hummin a tune?

What the heck is a booster output?

[BenSC]

Pull the PI and turn it on. Is the output still hummin a tune?

What the heck is a booster output?

Booster output goes out to another power amp to use as a slave.

pulled the PI. it still has a faint hum, but it was no longer affected by the post-gain pot. but i'm not sure what this tells me...?

[martin manning]

Do you have a multimeter to get some measurements, and are you comfortable poking around with several hundreds of volts present? If so get voltages to ground from the preamp tube pins 1, 2, 3, 6, 7, 8, going clockwise from the pin 1-9 gap, looking from the bottom of the tube. These will be DC voltages. Also check AC voltage from pins 4, 5, and 9 to ground, just to be sure that the filament circuit is working properly.

[Reeltarded]

Sorry. Was just a step to decouple the power from the preamp. It needs filter caps and then it just wont pass signal but it will not hum, probably.

The signal problem is a little more involved but I am scared to tell you what to do until we've both had coffee and you swear you understand how to probe in a live amp because with SC behind your name you either like old 911s or you live just north of me in a place that claims to be south and if I kill you I don't want your cousins driving down here to kick my ass and finding out how much nicer it is here because there is way too much traffic already and the property valies are artificially high enough without rich people from Walterboro taking over.

Whew.

With all that out of the way, are you comfortable working in a running amp?

The idea is to take a detachable probe off your meter or use a well insulated tool like a small screwdriver or slim leather punch to test for ticks and pops starting from your input and working the signal path all the way to the output.

You touch the joints with the volume barely up and listen for a positive change in the tick sound. When you find strong signal it tells you the problem is in the circuit just before the place where the probing got louder.

Do you have the schematic?

[martin manning]

This:

[Reeltarded]

Holy cow I never worked on a left handed upside down amp before.

[JerryFJA]

No loop on the Butcher. The most common problem with those is the ribbon cable connecting the power tube board to the main board burns up. Look for burn spots on the cable connectors. Do the power tubes heat up? If the cable is bad they usually don't. Check all the fuses on the power supply board too.
Jerry

[cbass]

Holy cow I never worked on a left handed upside down amp before.

You've never worked with an architect then.It's all backwards and upside down