No Pre-Amp output on early 80s Peavey 300 Combo

[BillyS]

I recently purcased a Peavey 300 Combo and discovered it has no pre-amp output. After searching and then joing this forum, I've emailed Peavey and received both sets of schematics for the amp. After reading what I could find on the subject, I checked that the speaker pops when the amp is turned on and the red Power LED is lit. The amp functions flawlessly other the pre-amp out issue. I'm fairly skilled at electronics repair and have everthing but a capacitor tester. Other than something unplugged, what should I be looking for when I pull it apart?
Thanks in advance,
Billy

[glebert]

Can you post the schematic here? Quick look doesn't show a working schematic here currently. So the preamp is working (since the amp works) just the preamp output signal isn't, correct? I know on some Peavey amps there is a separate op amp that drives the preamp output signal. Seems like the problem has to be right there if the amp works OK otherwise.

[olddawg]

The first thing I would check is the jack itself and the connections to the jack. Is it board mounted?

[Enzo]

Hi, do you mean Combo 300?

Isolate the problem. If it thumps, the power amp probably works, but easy to check. Plug a guitar or something into the POWER AMP IN jack. Get sound? That jack can be a problem, so plug a spare cord from PREAMP OUT to POWER AMP IN. Does the amp work now?

Do you mean the amp as a whole works and passes signal and all, but just the PREAMP OUT jack doesn;t work? Or are you saying the preamp makes no output?

The preamp and power amp have separate boards. There is a multiwire cable connecting the two. The corner of the power amp board opposite end from the fuses. If you pull that connector, disconnecting the preamp, you see four pins sticking up. The end pin away from the corner is the input from the preamp.. Touch that pin, and a working amp hums.

[BillyS]

Hi, do you mean Combo 300?

Isolate the problem. If it thumps, the power amp probably works, but easy to check. Plug a guitar or something into the POWER AMP IN jack. Get sound? That jack can be a problem, so plug a spare cord from PREAMP OUT to POWER AMP IN. Does the amp work now?

Do you mean the amp as a whole works and passes signal and all, but just the PREAMP OUT jack doesn;t work? Or are you saying the preamp makes no output?

The preamp and power amp have separate boards. There is a multiwire cable connecting the two. The corner of the power amp board opposite end from the fuses. If you pull that connector, disconnecting the preamp, you see four pins sticking up. The end pin away from the corner is the input from the preamp.. Touch that pin, and a working amp hums.

Guys,
Thanks for the quick replies. I know it's a Combo 300, but I think my fingers forgot as they were typing;-). I'll upload both schematics Peavey sent me shortly.
Enzo,
I think the issue is in the PRE AMP section of the amp. The amp passes signal and has good clean output except when I connect the PREAMP OUT to my studio board with a good cable, then I have no output to the board. The amp has good output from the speaker with or without the PREAMP OUT and the PWR AMP IN looped. I'll pull it apart and check the jack and connectors/cables and the pin you suggested and report back.
Thanks again,
Billy

[BillyS]

After quickly scanning the provided schematics, it appears that the Combo 300 and the 400 BH share circuitry. Is that common?

[Enzo]

400BH is the power amp board. It is used in many models. Combo 300 is the whole thing. The preamp board is what is unique to the model. 400BH only applies to the power amp board itself.

OK, so the amp works by itself, you can play it like normal. Only problem is the PREAMP OUT jack itself puts out no signal? That almost HAS to be the PREAMP OUT jack itself. Either the jack is bent up inside, or the solder to the jack has failed. Conceivably the circuit board under that jack could be cracked. Look at the schematic, there is no circuitry as signal passes by that jack. Your layout drawing also shows the trace work under that jack.

[BillyS]

I'm assuming it's the jack/pcb. With the amp on and an instrument plugged in, I can play a note and plug a cable into the AMP IN jack and the sound goes away. If I plug the cable into the PRE AMP jack, the sound does not go away(it should, right?). Are there vendors that support this forum that are a good source for the jack on the pcb? I went through all of my jacks and I don't have one with that foot print. What do you suggest?
Billy

[glebert]

If I plug the cable into the PRE AMP jack, the sound does not go away(it should, right?).
Billy

The preamp out jack always let the signal pass with or without a plug inserted. That is why there is a dot on the wire that goes up towards the power amp input jack. At the power amp input jack there is an arrow there, which indicates that connection is broken when a plug is inserted.

But if you are not getting a signal out of preamp out, and the amp is working otherwise, the problem has to be with that jack or your cable. There is no op amp driver or anything there to fail.

[Enzo]

Preamp out jack does NOT cut out the signal to the power amp. The preamp out jack normally has the same signal as is sent to power amp, so you can send it to some other amp as well.

[BillyS]

Copy. Can you guys suggest a source for the wide foot print jack? A Google search shows a few amplifier specific parts houses. Where would you guys go?
Billy

[Jazz P Bass]

"suggest a source for the wide foot print jack"

How about Peavey?

[Enzo]

Says right on the schematic 66032. That is their part number. I THINK the complete number is 71466032, maybe superceded by 31466032. In any case, 66032 should be enough for the parts people there.

Not sure what you mean by wide footprint jack. They appear to be common Switchcraft 112APC. Little square black guys perpendicular to the board. Mouser sells them:

https://www.mouser.com/ProductDetail...xEypeHDa9Uw%3d

But the ones from PV have an extra tip contact for greater reliability. Either works.

[BillyS]

I wondered about the number on the sheet. I'm used to those being engineering numbers in the Automotive world. If you take those kind of numbers to the parts guy at the dealer he politely informs you that is not a part number Mouser has what looks like the right one. I guess it is just bigger than any of the ones in my spare parts selection. I'll order a few and let you know if it fixes the problem.
Thanks again for all of your help!!!!!
Billy

[Enzo]

Really, you can just call Peavey and get the exact things. Note Mouser uses a generic picture of the jacks that shows solder lugs, but the actual part with the PC suffix will have pins for PC board.