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.
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.
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.
That both reverb and post must be up reinforces my theory that there is some feedback loop through there.
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...
Quote from: mikeskory on November 29, 2017, 02:24:34 PM
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.
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.
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.
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.
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.
Quote from: mikeskory on December 26, 2017, 12:43:29 PM
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?
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.
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
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.
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.
Phatt: I wasn't clear. The "squeal" went away around the time I re-soldered the bass of the 4 pin reverb tank connector. But the remaining problem is that the signal at the speaker dies when the POST and REVERB controls are between 1/2 ans 3/4 up. At the same time the signal dies, a small negative voltage appears at the speaker output (and various other places in that output circuit. I did purchase a 8EB2C1B tank and tried it. Same results as with the stock reverb tank. The irony is that the amp sounds great and the reverb effect is good.
Have you looked at the output on a scope?
I wonder if you still have oscillation that has just moved up in frequency beyond what is audible. And once it starts it overwhelms the signal so it sounds like the amp is just cutting out. And the meter could mis-read it as DC.
Quote from: mikeskory on November 27, 2017, 06:38:23 PM
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.
If this is still the case then Reverb is suspect.
Quote from: mikeskory on November 27, 2017, 06:38:23 PM
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.
Have you verified that?
Quote from: mikeskory on November 27, 2017, 06:38:23 PM
Enzo sez: And does unplugging the reverb pan stop this or not? \
Mike sez: Unplugging reverb stops the squeal.
Ditto,,,
If this is still the case then Reverb is suspect.
Quote from: mikeskory on December 28, 2017, 02:19:58 PM
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.
Bridging the Reverb case and Amp Chassis with a wire should have ZERO effect on the sound,, but it obviously did Soooo,,, logic sugests the reverb is your problem.
I'd be checking the ground return path of the reverb drive in the amp as well as verify the Case ground path as well as check your RCA leads for continuity.
If that checks out then ,,What *Katoda* mentioned,, a leaky Cap,, obvious one is from the reverb return opamp.
Sorry if I've missed something ,,I read now you have a hum problem?
That also might be caused by a reverb circuit wired up wrong.
I'm actually right in the middle of redesigning a reverb drive circuit for a crappy Elcheapo Valve Amp and yes I did connect the wrong leads and it squealed as I turned up the high gain channel. Once I realized my mistake I got full crank and full reverb.
Trying to follow ground paths is actually quite tricky so tiss very easy to miss something.
Regards Phil.
Right now I am unable to wrap my head around anything complicated. Hey, I am old...
But one thing to for sure verify is the pan itself. The drive side MUST have the shield side isolated from ground. The output end of the pan SHOULD be grounded to the pan. If you measure the disconnected pan for resistance from the outside of one RCA to the other, you should see OPEN. If you connect the amp cables to the pan, you should see 22 ohms between RCA outer sides.
Can't work on it till Wed, but I'm not explaining it clearly. Maybe I can post a video Wed.
There WAS originally a squeal. Around the time I re-soldered the 4 pin Reverb connector it went away. Enzo recommended tank 4EB2C1B but we ordered #8EB2C1B because of the small cabinet for this amp. Both the new and original reverb tanks sound great. In fact. the amp sounds great...until I turn the reverb control past 1/2 way with the post control up 1/2 way. The position of the volume control is irrelevant to the "cutting out". This post by G1 seems logical. I'm going to post a short video on Wed so you can hear what I mean.
Posted by: g1
« on: December 31, 2017, 01:38:04 PM » Insert Quote
Have you looked at the output on a scope?
I wonder if you still have oscillation that has just moved up in frequency beyond what is audible. And once it starts it overwhelms the signal so it sounds like the amp is just cutting out. And the meter could mis-read it as DC.
Ah, throw a scope on the output to see if it is going to RF.
Here are the scope views on the Peavey. .50 mean at 50% or 5
Here's the same scope settings normal reverb and just prior to the problem. Rev and post at .50
Yep, your amp turned into a giant power RF oscillator.
Make sure the 22 ohm R66 over by the speaker output is not open.
I still worry that the reverb IC may be compromised, so do check the pan jack. A lot of them now have a configurable jack.
When it bursts into oscillation, then check the 15v rails to see if it is riding those rails.
Is the oscillation present at the preamp out jack? If so, does plugging something into the power amp jack stop it, or does the oscillation remain on the preamp?
Isolate even further: when it is singing, short across the return from the pan with something. Does that kill the osc?
Behind the gain controls is the connector to the footswitch jack, Pull that off the board, does that change anything? The lead over to that jack could be acting as an antenna - it is wired right to the reverb pot and preamp final stage IC.
OK Enzo...logic has to solve this. The R66 22ohm resister was burnt looking at the outset of this project. I replaced it. Just now I accidently touched. Burnt my finger. There is a wierd looking .1/100v electrolytic in front of it. I have similar. Should I change that?
Footswitch control. No change when unplugged. RF at pre-amp out-YES.
Enzo: Yes the RF appears at Q1 and Q2 emitters. They are tied together. The resister R49 takes these emitters to the -15v. The -15 voltage budges very slightly and NO RF when I emulate the symptom. At the Q1,Q2 side of R49 we have all the RF.
Could you clarify "NO RF when I emulate the symptom" ?
The customer complaint is that the amp squeals or otherwise cuts out when the Post control AND the reverb are both past 1/2 way up. I verified this and can make it happen like clockwork by turning up the Post AND Reverb past 1/2 way. The far left Volume seems to be irrelevant. The Saturation control has some effect but does not seem primary to the complaint. "Cuts" out is an inaccurate description. More like the signal is blanked out or covered up. If there was a way to post sound clips at this discussion I will do it.
CLip if you can, but I totally understand what you have there, Mike. When it goes into RF, it swamps all else. I would never call that cutting out, but I see why a customer would.
In cases like this I always suspect grounds. Note in classic Peavey style, there are two grounds. The power amp and power supply have one, and the preamp another. Look below the reverb drive IC and find C26. That cap ties the grounds together. On the board it is right next to Q6 output transistor. Is that cap OK? And is board totally installed into the chassis, so all jacks make good contact to chassis? Chassis also ties them together. SOmetimes I have to loosen the jack nuts, move the star washer for a fresh grip, and retighten.
But if the grounds are all good, oscillation implies a loop of some sort. SO we can isolate the problem. Pull U1, does it still osc? Post and reverb controls have to be up for it, but do the tone controls affect the oscillation in ANY way?
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.
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?
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.
FYI Here's an old thread from 2011 on the exact same topic. https://www.ssguitar.com/index.php?topic=2279.0
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.
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.
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.
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
Darn, I figured we'd set that one straight when we come visit.
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.
Very fun day!
Cool! 8)
Glad to read you guys got it fixed. :)