The pot worked but I think I'll put a much smaller value one in and mount it on the back. After about 10k of resistance there isn't any change in feel. My other amp a PV Special has a discrete transistor output driver stage. Looking at the schematic I think raising R79 would increase the current feedback. R96 is the current sensing resistor? There dosen't seem to be any voltage feedback?  Does that look right to you guys? 

Peavey service dept. is pretty good at emailing schematics of their older products. It will have a board layout too. It's nice to have the print if you discover other problems. Just send them your amp info with S/N. The other guys have pointed you in the right direction for that cap.

I finally got back to that amp. We lost power here for a week.  I put 1 meg audio taper pot in series with R109, routing the wiring through the low input jack. I'll try it out at my next rehearsal and report back. I have another amp with a different topology I want to try this with next so I may have to pick your brains some more. Thanks for the help, J M

I picked up a another cheap old PV a month ago ( it's an addiction for me ) and had all sorts of strange buzz's and noises but worked otherwise. I took the chassis out of the cab, pulled the pc board and cleaned everything, I mean everything, pots, jacks even the board it's self. I paid special attention to the switching contacts on the jacks. Some jacks may ground to the chassis, make sure that area is cleaned. Contact cleaner, scotchbrite, and q tips. Check the solder connections at the input jacks, I'd reflow them while it's apart anyway. Be careful with the Tech 21 board I've heard that they are muti layer, I've never seen one.

Thanks for the explanation    I'm used to tube amp power stages. I didn't know current amps behave in the opposite. I think I'll add some switches and see what happens..

I looking at adding a dampening factor adjustment to one of my SS amps. I'm looking at loosening up the feel a bit and before I put it under the soldering iron I thought I would ask around here.. My first impression would be to add more series resistance in the neg feedback loop at R108 any other thoughts?

Quote from: Bernaner on January 08, 2011, 02:53:14 PM
Hey Guys thanks for the suggestions!

Circuit board is pristine, nothing is burnt.  All the power supplies are up and running strong.

In my experience the output device (of whatever kind) goes short and blows up the supporting circuitry unless it is protected very well.  This one does not appear that way which is why I was going to pull out the scope and start scoping the signal path.  I'm not used to working on amps this small so maybe these guys don't "blow" up they just quit working.  I'm used to seeing TO3 devices going short

The reason I initially asked on here for the schematics is because I've seen reference to them in other posts.

I think it's a chip amp too. All the supporting circuitry is pretty much on board a chip amp. So no driver transistors as in a discrete amp ala to-3's.

Found These... Hope they help.

Newguy, The preamp will be on the circuit board mounted inside the chassis. Some other posters have commented on the build quality of the board being fragile. I guess we assume you have some electronics experience. If you haven't like replaced caps on a computer motherboard or modded some stopboxes before practicing on your working amp may not be the best idea.
Why don't you  try and borrow a cabinet from somebody and make up a cord to plug it in to try some different speakers out 1st if that is what your gravitating to. A couple of insulated spade crimp connectors to plug into the wiring that goes to your speakers now. You could use on old speaker cord for the 1/4" plug and wire. Tape up the unused pair Different speakers will make the amp sound "different", better is kinda subjective.
If your confident with your soldering/desoldering skills get the schematic from the fender website and go at it. Sockets help a lot. A TL072 opamp starts the preamp out and is pretty much the entire clean channel. Duncan's amp tools will help with the tonestack. There's a ss amp book on this site that's a good read too.

I know I've been a member for a while... I try not to post on old threads (although I've seen a quite few I like)  and be a good lurker... Thanks Guys

I've had to play through one of these before... The clean channel isn't that bad but
the speakers are really upper midrange spikey. I've had to keep the treble at 2. I'm not a fan of those fender or PV Eminence speakers myself. You could for really cheap money put a low pass filter before the speakers to tame some of that harsh high end. I've done it before to SS amps, it will work. Figure a cutoff frequency of 3k or so. This article is from a recording mag and it's for an 8ohm load, you'll have to calculate for a 4ohm load:

THE FILTER FACTOR
Guitar amp purists have their speaker faves for good reason: They can drastically affect the amp tone. Some are bright while others are warm and mellow. This next pair of mods will tame any harsh-sounding guitar amp simply by applying readily available crossover components. Do you see where I'm going with this? Not much fidelity is needed from a "full-range" 10-inch woofer, so I installed a passive 6dB/octave filter — half of a two-way crossover network. I made note of the values needed to cover a frequency range between 1.8 and 4 kHz, and then found the nearest standard inductor at www.mcminone.com (part number 50-3019), a "choke" in the form of a 0.5-milliHenry (mH) air-core bobbin inductor.
There's a handy online crossover calculator at www.the12volt.com/caraudio/crosscalc.asp#cc, a car audio Website. After some ciphering, I determined the roll-off would begin around 2.5 kHz, so that 5 kHz would be down 6 dB. You'd think this was too low, but it was very effective and made the mic choice much less critical.

The schematic is available on the Fender website. You could mod the preamp, change the slope and response of the tone stack, roll some different opamps in. I like BurrBrown opa2607 in my PV SS amps. It's all up to what you want to do and how much time you want to put into it.  Fender is cheap they don't want to pay to fix them, doesn't mean they can't be fixed.