Peavey Studio pro 40 has squeel

[mikeskory]

https://www.ssguitar.com/index.php?topic=2279.0Peavey Studio Pro 40 with a bad squeel. It seems to be hinged to the reverb section. With everything in it's place, the amp will squeel when the reverb is past 1/2 and the "Post Gain" is past 1/2. Input signal or not.  I was courius if this old post was ever solved but I decided to start a new post.

[Enzo]

Hey.

Turn the reverb down, but scope the output of the reverb IC, U3 pin 1.  Is the oscillation still there?  Also scope pin 7, is it there too or either?

Does the reverb control affect the frequency at all?  Does the post control affect the frequency?

Make sure the correct pan type is used - 4EB2C1B.  It must have the input jack insulated from the pan.   And does unplugging the reverb pan stop this or not?

Pop a different 4558 into the socket at U3, to see if there is any crosstalk in the IC.

[mikeskory]

Hi Enzo!

BTW I'm still waiting for the transistors for the Pizza Party amp. Will call when all in.

Enzo sez: Turn the reverb down, but scope the output of the reverb IC, U3 pin 1.  Is the oscillation still there?  Also scope pin 7, is it there too or either?
Mike sez: OK will try that tomorrow

Enzo sez: Does the reverb control affect the frequency at all?  Does the post control affect the frequency?
Mike sez: yes for sure. If reverb is off then post works fine. If post off then the reverb control won't trigger the squeal. When reverb is unplugged no squeal.

Make sure the correct pan type is used - 4EB2C1B.  It must have the input jack insulated from the pan.
Mike sez: I'll check that tomorrow.

Enzo sez: And does unplugging the reverb pan stop this or not? \
Mike sez: Unplugging reverb stops the squeal

Enzo sez: Pop a different 4558 into the socket at U3, to see if there is any crosstalk in the IC.
Mike sez; I have already done this.

[Enzo]

That both reverb and post must be up reinforces my theory that there is some feedback loop through there.

[mikeskory]

Hey Enzo,

Yes I have popped in a new U3 4558. No change. I just checked the Pin 1 and Pin 7 and YES when the oscillation kicks in, I see it at Pin 1 & 7 immediatly.  Here's another curious thing which I thouight was irrellivent but 22 ohm 1 watt resister R66 is obviously burn from over heating. It tests ok in curcuit cold. Maybe I should run over and get one fron Fultons before we go any further...

[mikeskory]

Hey Enzo,

Yes I have popped in a new U3 4558. No change. I just checked the Pin 1 and Pin 7 and YES when the oscillation kicks in, I see it at Pin 1 & 7 immediatly.  Here's another curious thing which I thouight was irrellivent but 22 ohm 1 watt resister R66 is obviously burn from over heating. It tests ok in curcuit cold. Maybe I should run over and get one fron Fultons before we go any further...

PS the reverb pan is Type 247 and appears to be stock. I ordered a new pan#4EB2C1B. That will solve the hum or eliminate that as the cause. I need a know good pan in stock anyway.

[mikeskory]

Notes to myself: After replacing the IC3, the input jack, 4 small 2.2mf caps, tested a new reverb pan and resoldering the 4-pin reverb-connector base on the main cuircuit board...the squeel has shifted but still there at high volumes when I turn up the "saturation" pot.  I don't have a reason yet for the burnt c66 22 ohm 1 watt resister that I replaced.  Going to look at that curcuit today for an excessive current or voltage drop.

[Enzo]

R66 is part of your zobel network across the output of the PA.  It burns up when the amp goes into oscillation and cranks out RF for all it is worth.  The high freqs pass through the cap and the poor resistor becomes the load for the amp's power.

[mikeskory]

The amp sounds great (other then the squeal) Could the 2 power transistors SJ6392 be bad causing the squeal?  FYI I changed the 2 power transistors. Exactly the same squeal at the exact settings.

[mikeskory]

12/26 update. I reordered the exact reverb tank I needed. It's a little better.  This may be a clue: The channel 2 input is fine. The channel 1 input (the louder one) is where the problems comes in. If I had to describe it, I'd say its overloads and shuts the signal off. The symptom is only when the "saturation" and the "reverb" controls are past 3/4. Maybe its ok?  Any opinions.

[Katoda]

12/26 update. I reordered the exact reverb tank I needed. It's a little better.  This may be a clue: The channel 2 input is fine. The channel 1 input (the louder one) is where the problems comes in. If I had to describe it, I'd say its overloads and shuts the signal off. The symptom is only when the "saturation" and the "reverb" controls are past 3/4. Maybe its ok?  Any opinions.

Hmm, perhaps there is something wrong with transistor bias, perhaps there is DC voltage on one of the pots which would mess with that. A leaky cap maybe?

[phatt]

Something simple to try;
Get a length of wire and connect the reverb tank Case back to the chassis of the amplifier.
Some reverb return circuits squeal like a pig if case is not grounded.
Another potential spot is R32 and R33, Check that they actually connect.
R33 looks like it goes to Circuit Com while R32 goes to Chassis point.
The chassis point for R32 might be a screw tab which might have corroded. Also wise to check the continuity of the leads that connect the tank. 8|
Of course I maybe way off but my hunch is a ground point is missing somewhere.
I would be checking those long before I'd replace parts which might lead to more confusion.
Phil.

[mikeskory]

I'm getting so close I can taste it!

@Phatt The wire trick did squelch squelch the problem but it reduced the reverb effect to almost nothing. Soldered the 2 resisters. Same.

@katoda There was a slight change in voltage at the center and top of the Reverb control as the hum kicked in. Like negative .05 but very consistant. So I went to "reverb send" pin on the board. There it changed from .23v to NEG .2. Same at pin 7 of U3 4558. .23 to neg .2v as I tyrned up the reverb and the hum kicked in.

So I went to R66 which is a 22ohm 1watt resister from the speaker output. (I had replaced this R66 early in the process because it appeared badley over heated). It fluxuates from 0 to neg .68 as the reverb control is turned up and the hum kicks in.

So them I checked voltages at the speaker output yellow. It goes from .001 to Neg 3.4 volts as the problem kicks in with the reverb control

[Katoda]

DC voltage on speaker is definitely not good. I think something is causing the power transistor on the negative rail to conduct more current than the one on the positive rail.

[phatt]

Ok if the wire trick altered the squeal then something is wrong with the reverb wire up.
As already mentioned. Check the ground path of the tank and the connections.

Remove RCA plugs from tank and Establish input/output.
If Enzo's part number is correct and it often is,  then a 4EB2C1B tank should read;
Drive 58ohms / Pickup 200ohms.
(give or take a few ohms they are never exact)
If the tank connection is reversed then it's highly likely to cause the problems you are having. Just swap the RCA's and see if it solves the issue,, you won't hurt anything.

A 4EB2C1B tank has the input (Drive end) isolated from the tank Case while the pickup end has circuit common connected to case. this allows the Current drive opamp trick to work.
If the RCA's are reversed the Input (Drive transducer) would then be grounded and squeal it's head off as you approach high volume while the pickup would also be very weak due to the Z mismatch.
HTH,, Phil.