Peavey Ultra 112 Combo Channel weird slapback mute effect when going from Ultra to Crunch

[Delta362]

I have a weird issue that I'm chasing. When changing from the Ultra channel to the Crunch channel, every so often it will give a slapback muting effect. When it occurs, I can typically get it to occur a few times in a row before it goes back to working normally. It never occurs when going from Crunch to Ultra or from either dirty channel to Clean.

I scoped the signal and see that it is dropping out at the junction of K3A(5)/Q7/R27 node just before the tone stack on the schematic. I am suspecting the mute circuit and perhaps CLAMPC activating, or related to the CLAMPB signal. I do not experience the issue when going back and forth to the clean channel so I'm ruling out CLAMPA. I also was thinking it may be relay K3C not engaging fast enough. Edit: Maybe relay contact bounce, but it's not every time?

K2 and K3 are energized with the Crunch channel. K1 and K5 are energized with the Clean Channel (de-energized is Ultra by default).

Suggestions on where to scope/look next? 24VDC power supply dipping too low? Any caps? Thanks!

[g1]

I am suspecting the mute circuit and perhaps CLAMPC activating

Agree and suggest replacing Q5 and Q7.

[Delta362]

Agree and suggest replacing Q5 and Q7.

Thanks, g1. I'm waiting on the parts to arrive.

[tedmich]

The mute/clamp is a cool design (see here https://music-electronics-forum.com/...-design/51810-)

As Enzo said its almost always the J174, they are now only available from InterFET for $3 (Mouser) or LSI inc for $6 (DK) The ONSemi/Fairchild ones were sooo cheap

[Delta362]

I should have stocked up on them before.

Got the parts due in today/tomorrow so should get back on it soon.

[tedmich]

a reputable dealer (Win-Source) has several of these old ONSemi/Fairchild Peavey parts for fairly cheap in bulk. Their MOQs are pretty high though!
https://www.win-source.net/

[Delta362]

Good to know of another source, but I'd rather spend the $3 each for a few instead of $58 for 35 as I don't see me using up 35 of them anytime soon.

[Delta362]

Got the new J174s installed. Unfortunately I still had the same issue. I then got to thinking about perhaps it's the contacts of the footswitch pushbutton. I swapped the Ultra/Crunch with the Reverb one, and that solved it. Thanks!