Welcome to Solid State Guitar Amp Forum | DIY Guitar Amplifiers. Please login or sign up.

March 28, 2024, 07:27:47 AM

Login with username, password and session length

Recent Posts

 

Peavey Studio pro 40 has squeel

Started by mikeskory, November 27, 2017, 01:37:38 PM

Previous topic - Next topic

mikeskory

#30
I found your c26. In curcuit, there is no resitance between the 2 grounds. i measured across the cap. Is that normal?
Pulling IC1 makes no difference. The Bass and mid have no effect but the treble sure does. It also has to be at 5 to trigger the problem. So the POST, the REVERB and the TREBLE have to be at 5 or more to trigger the problem. i hadnt noticed that. 

Enzo

C26 is shorted across by the chassis connections of the jacks, when the board is installed.  If there is a bad jack/chassis contact, that cap is the only thing keeping the grounds together.  SO it was to be checked.

So U1 is out of the feedback loop.


ALL three of those controls have to be up for osc?

I can easily be wrong, but this is still screaming ground at me.  I hope I am not deluding us.

Plug a FS into the jack, does clicking the reverb stomp off and on turn the osc off and on?

mikeskory

#32
Yes the POST, the REVERB and the TREBLE have to be at 5. Bass, mid and volume no effect. The suturation has a minimul effect. Could C26 be shorted?? C24 and C25 which are the same value, show no continuity in cuircuit but the C26 shows dead short. 

Yes I do think that its a ground problem. So far the only thing I can do to make it worse is touch the outside of the RCA at the IN jack of the reverb tank.

mikeskory


Enzo

C26 is being shorted by the chassis, as the jacks connected to the two grounds are all chassis-connected too.  C26 is fine.

IN jack of the reverb?  OK, look at the drive circuit, note the drive input on the pan is not grounded, the circuit has a 22 ohm resistor to ground from there, R39.  Is that open? 

I don't know that they are involved in the problem, Mike, but the amp is 35 years old, and Peavey used a ton of those little 2.2uf/35v caps.  I have found that those do not age well, and usually need to be replaced.  I see six of them here, two in the reverb return circuit.

phatt

#35
This might sound dumb but I've seen it happen. :duh
The in and out thing has confused a few muso folks I've known over many years so just to clarify.
Reverb Out from Amp (Driver) goes to Tank Input and tank Output goes back to Amp input (pickup).
I've met some folks who think that it's *In to In and Out to Out* which of course will short out the 22 Ohm resistor (R39) on the driver and hence cause havoc.

Also you mentioned that you resoldered the connector on the PCB,, is it possible you inverted the wiring? That would cause the very same problem.
As I said before just swap the RCA leads to test that.
Most reverb circuits will produce reverb if leads are swapped but wise to check.
Hey it only takes a moment to check. ;)
The right way is usually the highest reverb output.

If that checks out then back to what Enzo says, He knows these circuits far better that I ever will. ;)
Yes with the reverb tank all plugged in then you should read close to 22 Ohms between pin6 of U3B and chassis and same between pin6 of U3B and reverb tank case. If open then R22 is likely stuffed, if shorted then you will have to back track the ground path of the reverb drive circuit and find out why it's shorting out.
Maybe take a close up picture of tank internals and reverb drive output section as it might help to clarify what's going on?
Phil.

phatt

Quote from: mikeskory on January 05, 2018, 01:08:40 PM
FYI Here's an old thread from 2011 on the exact same topic.  https://www.ssguitar.com/index.php?topic=2279.0

Re that link; note the comment about the tank bag,,
Yes the tank must be mechanically isolated from the cab, usually a soft bag is all that is needed.
A reverb tank that is hard mounted directly to cab can cause things to squeal at high volume.
Phil.

mikeskory

Sadley I had to give up on this repair. The customer wanted the amp back. Here's a recap of what I did that did not help.
1)Replaced IC1,2,3
Replaced input jack
3)Replaced reverb tank w new
4)hardwired new RCAs for reverb
5)replaced R66
7)replaced 6 2.2 small elecytrlytic
8)Jumped 4 .1 electrolytics
9)Replaced out transistors

See yiu all next time and thank you!
Mike

Enzo

Darn, I figured we'd set that one straight when we come visit.

Enzo

I went over to Mike's shop today to visit, and between the two of us, we got this fixed.  Problem was in fact the reverb cable wiring, the shield and core leads were reversed at the chassis connector.  This is how it had been wired when it came in, but the shield was acting as an antenna now, which caused feedback.

it was a fun project, and a lot easier to handle in person than by remote here.

And I always enjoy visiting other folks' shops.

We also waded through an OLD Fender 600 (A Champ variant), and ancient Bandmaster, and a PV Classic 30.  Plus a Buttstomper 1000 or something like that.

It was a fun day.

Plus Mike sprang for lunch.


galaxiex

Cool!  8)
Glad to read you guys got it fixed.  :)
If it ain't broke I'll fix it until it is.