Mesa Studio .22 troubleshooting

[Boy Howdy]

I'm trying to find the cause of some problems with this Studio .22 Caliber thing.

SCHEMATIC:
http://www.webphix.com/schematic%20h...ogie_22cal.pdf

Symptoms:
-Loud hiss, especially in the clean mode.
-All the pots located in the preamp are very static-ey when rotated, the volume control is the worst.
-The volume control gets louder up to 3 on the dial and then gets progressively quieter as it's turned to 10.
-The tone controls do very little if anything in the clean mode.
-In the distortion mode the bass and to a lesser degree the middle control act more as volume controls that work backwards, with the most gain (massive gain) at zero.

Troubleshooting thus far:
I see no visible signs of trouble.
I plugged into the return jack on the back and the power section checks out fine.
Reverb is fine.
I subbed a new pot in place of the volume and the static continued, but to a lesser degree. That and the fact that the non-preamp pots are without issues suggests that the problem is not with the pots themselves.
I bypassed the tone stack and if I recall correctly the problems disappeared, but I'm not sure that the V2a plate resistor isn't the problem.
Problems appear in both the clean and distortion modes, so that would indicate the problem is somewhere in the vicinity of V2/tone stack and not anywhere in front of V2.

TO SUM UP: the problem seems to be in the V2/tones stack area. I've not run into a problem like this before. And I'm not proper repair tech. But my best guess is that it's either the V2a plate resistor or a cap. I have no idea how to check a cap to see whether it's bad - can someone advise on that?

I haven't removed the circuit board yet.

So there it is. Can you guys help me figure this out?

[stokes]

Check to see if there is dc voltage on the volume control.If there is,you have a leaky cap in the tone circuit.

[Amp Kat]

Check to see if there is dc voltage on the volume control.If there is,you have a leaky cap in the tone circuit.

Sounds like one of the Vactrol Isolators may be out. Those things get hot and start failing. Check signal in both states and see if they are working right. Think they are VL T5C1's.

[Boy Howdy]

Thanks, Stokes and Amp Kat. Both of those sound like likely candidates alright.

Voltage bleeding into the signal path, that sounds familiar. I may have run into that before. And come to think of it, I was checking -- or trying to check - the readings in some cases were way off -- the ohmage on the pots and I was getting sparks between the pots and the voltmeter probes. That probably shouldn't be happening, right?

How would I know which cap is the culprit, if that's what it is?

I'm not sure I would know how to evaluate those opto-switches. I'll see what I can figure out. Any quick hints?

[Jazz P Bass]

How would I know which cap is the culprit, if that's what it is?

If a tone cap is leaking (they are keeping out V2A's plate voltage) the best way to find out which of them is leaking is,first, measure what voltage is on the tone controls.
Then remove the caps one at a time.

[stokes]

The fact that you can switch between channels would rule out the switching circuit being the culprit,I think,but to be honest I really am no expert on those switching matrixes.If you are saying you get sparks when trying to measure resistance of the pot,then yeah,you've got voltage that doesnt belong there.See what Amp Kat has to say about those vactrols,if he thinks they could still be the culprit,go with whatever he has to say.If it is one of the caps in the tone circuit,I would just go with the "shotgun" method and change all 4 of them.Otherwise,you would have to disconnect the outboard side of each one and check for leaking,one by one.If only one is leaking,the voltage will show on all with them "in circuit".

[Boy Howdy]

I'm on it. Back in a few days.

[Alex R]

might be a HF oscillation problem, scope the signal path to check.

[Boy Howdy]

Success!

Yeah, it was either the mid or the bass cap. I took the treble caps out first hoping it would be one of them, but that didn't do it. So I then changed out the mid and bass (too much of a pain to do just one at a time) and that did the trick.

Thanks everybody, couldn't have done it without ya.

A couple of questions before wrapping this up though:

1) That amp has way too much gain on the lead channel. It completely squashes (compresses) every note, allowing for no dynamics whatsoever (subject to subjectivity, of course). I have an idea for getting rid of one gain stage. It occurred to me that if I just clip off pin one from the first 12ax7 tube it would disable that gain stage but would continue to work just fine, only with less gain. Any of ya'll see any reason not to try it?

2) The bass and middle control do very little to the tone of the amp. It was actually a little difficult for me to decide that they were actually working! If I changed that 150K slope resistor to something smaller would that give me more usable controls, controls that actually do something?

3) Okay, a few questions, not a couple. That amp sounds pretty small overall. Is that 47pF cap between the input and V2 pin 2 partly responsible? If I made that bigger would it help?

Just trying to learn something here. Thanks again, and in advance.

[soundguruman]

Broken ground bus wires (solid wire) soldered to controls.
This wire may appear perfectly soldered, but may not actually be soldered properly.
Resistor sitting on top of circuit track, which has burned down into the fiberglass.
You don't see the burn until you lift the resistor.

[Jazz P Bass]

Why not try a different preamp tube instead of 'snipping' out pin 1 of the 12AX7?
A 12AT7 has 80% of the voltage gain & a 12AU7 has 40%.

[stokes]

Dont cut one side of that AX7,the 2 triodes are wired in parallel,you wont get the desired effect and the bias to the triode you leave will be out of whack,they have a shared cathode.You can add a lead drive pot by replacing R292 a 470k resistor with a 500k pot.If you google "Boogie .22 lead drive mod" you should find instructions for it,I saw it someplace a long time ago,but dont remember where.

[stokes]

The Boogie Board • View topic - Lead Drive mod Studio 22+ Here it is,this one says to use a 1meg pot,you can use either,but a 500k will give you close to stock when turned up full,the 1meg may give too much drive when turned up full.Real easy mod,3 pieces of wire and a pot.

[Boy Howdy]

12au7 - of course! I are a idiot.

[stokes]

I find the AT7 and AU7's are too flat sounding in the early stages of a preamp.They work well as a PI or reverb driver,but you'll never see them used as the input.Has to do with the impedance of the tube,not well suited there.Try it,you might like it,but I think the lead drive mod will do the trick much better.I've never used it in the .22 but have seen it in a .50,which is basically the same preamp,different value for that resistor,but otherwise the same circuit.