Help fixing a very vintage Peavey

[52 Bill]

Yes, that would seem to show a bad transistor.

I have not yet done the power off tests of each resistor or caps, but will do that next, although some caps are above my meters ability.

The higher value caps are probably more for power supply filtering and decoupling. The ones that you really want to know about are the signal path coupling caps, which are typically smaller values.

What Enzo said is correct about swapping drain and source pins, I was more concerned about having the different pinouts of the two transistors mixing up the elements.

[fastmerc]

Yes, that would seem to show a bad transistor.

The higher value caps are probably more for power supply filtering and decoupling. The ones that you really want to know about are the signal path coupling caps, which are typically smaller values.

What Enzo said is correct about swapping drain and source pins, I was more concerned about having the different pinouts of the two transistors mixing up the elements.

Some success! It appears one of the 2mfd caps was bad along with the mpf102...switched the two working parts over and I now have signal!! It does seem to be louder than the same volume setting on the clean side but that may be my imagination at this point. Next is to test the reverb. Edit: Reverb functions except it won't turn off, I assumed if I put an rca jack in the footswitch port and shorted it that it would act like a switch to turn it off which it doesn't. Any ideas here?

[SpareRibs]

Hello,
Did you try going to the T60 Mafia forum ? Some of those guys may have the same amp, and may have gone thru the same situation you are going through now.

[fastmerc]

Hello,
Did you try going to the T60 Mafia forum ? Some of those guys may have the same amp, and may have gone thru the same situation you are going through now.

Just signed up...Thank you for the recco.

[52 Bill]

Some success! It appears one of the 2mfd caps was bad along with the mpf102...switched the two working parts over and I now have signal!!

When you order parts, you might want to order enough 2.2uF caps to replace them all on the preamp boards.

I would assume that the footswitch would work as you have described. If it doesn't kill the effect, follow the wire that connects to the footswitch jack and see that it is still connected to the circuit. You might also find that it connects to the circuit through a cap or some other part that may have gone open circuit.

[fastmerc]

When you order parts, you might want to order enough 2.2uF caps to replace them all on the preamp boards.

I would assume that the footswitch would work as you have described. If it doesn't kill the effect, follow the wire that connects to the footswitch jack and see that it is still connected to the circuit. You might also find that it connects to the circuit through a cap or some other part that may have gone open circuit.

Got the switch working now....Putting this aside till the parts show up.

Thanks for everyones help so far!

[ewd]

Peavey tech support has always been really good to me. Seems like the trick is to call them first thing in the morning before they get busy.

[fastmerc]

Finally got the transistor and replacement caps in and both channels are working well! So excited for this to finally be near 100%. Last thing to tackle is the tremolo part. Would this part work on the photo resistor side? https://www.radioshack.com/collectio...ant=5717568901

[52 Bill]

Glad to hear that it's working. Can't tell if the RS part will work or not, as they don't give any description as to what the part is.

The circuit just sends the audio signal to ground through the photo cell, so it needs to be a light dependent resistor. The photo looks more like a photo diode.

[tedmich]

supposedly you can cut an excellent one out of a VTL5C1 Perkin Elmer Vactrol,
Digikey also sells CdS photo resistors, usually they are mated with a neon lamp

[J M Fahey]

FWIW I buy LDRs`from people who sell color TV parts, which use them for an automatic contrast regulator.

Just to suggest another possible supplier.

[Leo_Gnardo]

Would this part work on the photo resistor side? https://www.radioshack.com/collectio...ant=5717568901

Take another look at post 16 ^^^^^^^^^^.

Never been much impressed with Radio Shack parts except in an extreme pinch.

[fastmerc]

Ok...got the tremolo working now also. So all in all the amp is functioning now. The last thing I might need help with is this. When you fist turn the amp on there is a fairly loud pop, then some crackling coming from the speakers for 20 or 30 seconds. After that it quiets down for the most part, except the crackling comes back every now and then. What might this be?

[52 Bill]

Never been much impressed with Radio Shack parts except in an extreme pinch.

If I can still find a store that is still open, they only seem to sell Sprint phones now. In recent years the only time that I would go there was to buy a fuse that was some odd value or that I ran out of.

Her's what I found on their site.
https://www.radioshack.com/products/...ant=5717598661

These are cds photocells and may work for you. I believe that these are surplus parts, so they don't publish specs. If it hasn't been mentioned before, the photocell that you need should have a fast response time, a fast recovery time and a good high to low resistance ratio.

You will probably find that almost any photocell will work in the circuit, but only some will work well and give you nice deep tremolo.

Edit:
Never mind the cds cell info, as you got it working. The noise could be a bad solder joint, a failing resistor or cap. Try and isolate the noise.

When it makes the noise, do any of the front controls change or alter the sound of the noise? Does physical vibration or mechanical disturbance of any of the boards or parts cause the noise to start or stop?