Peavey Ultra switching pops

[johny85]

Hi, I have some troubles with "Peavey Ultra 60". It makes loud pop when switching from Clean to Crunch/Lead. I thing that something is wrong with "mute" circuits. There are two J174 (jfet switch), Q5 and Q7 on schematic, which are supposed to mute signal to ground. When I manualy bypass either of those, and switch channels, there isn't any pop noise. Can you give me some advice?

[Enzo]

So look at the circuit that controls them. When those turn on, the signal path is shunted to ground, so sure, no noise comes out. They are controlled by CLAMPC.

CLAMPA and CLAMPB are merely taps off the footswitch circuits. When you stomp a switch, onie of those is pulsed. Ultimately rthe little circuit decides if one of those will send a pulse to little triac CR102.

When CR102 fires, it conducts, grounding CLAMPC and thus the gates of Q5,Q7. Grounding the gates of those JFETs lets them turn ON, and shunting the noise signal.

The trigger pulse to CR102 comes through a cap, so it is not steady. The JFETs will stay ON until R200 has a chance to recharge C200.

SCope the gate of on of the JFETs and watch that muting pulse. It may or may not be there, and you deal accordingly. Also the JFETs themselves could be the issue, a shorted one could kill the whole clamp operation.

[Jazz P Bass]

Thanks for the circuit description Enzo.
The function of the switching circuitry has always eluded me.

[johny85]

First I want to say, that I very appreciate your reply Enzo!!

Second, here are my observations:
- there is 12V on gates of Q5 and Q7
- it only pops when switching Clean --> Lead.....not Lead --> Clean
- I don't have scope, but I put voltage from gate of Q7 via voltage divider (from 12V to 1V) into "Line in" of my sound card...I recorded waveform during switching. It seems OK, voltage drops, and than rise slowly to 12V. It takes exactly 32ms.
- Than I made another test. I put some caps, paralel to a C200. I have been increasing it's value to increase mute time. Pop was gone when mute time was way over 1second. That's useless. And i don't understand that, 'cause pop apears right with switching....not 1second after or so.

[Jazz P Bass]

I don't know if changing C200 is the way to go.
I think you should be looking at the actual Clamp A, Clamp B pulses.
I would look at the non ground side of R303 (330 ohm).
The Clamp A & Clamp B circuits enable the relays, with a flyback diode & a 22uf cap.

[johny85]

I don't know if changing C200 is the way to go.
I think you should be looking at the actual Clamp A, Clamp B pulses.
I would look at the non ground side of R303 (330 ohm).
The Clamp A & Clamp B circuits enable the relays, with a flyback diode & a 22uf cap.

I know, it was just a test, i was wondering if going from factory 30ms to 50ms (for example) will solve the problem...

Pulses from Clamp A and B are Ok, I think, 'cause there is 30ms mute on every switch press/depress.

oh, and another thing, it only pops when not playing. Loud enough to make people stare at you at show lile "WTF are you doin' man"