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

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

    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!
    Attached Files
    Last edited by Delta362; 06-28-2024, 04:30 PM.

  • #2
    Originally posted by Delta362 View Post
    I am suspecting the mute circuit and perhaps CLAMPC activating
    Agree and suggest replacing Q5 and Q7.

    Originally posted by Enzo
    I have a sign in my shop that says, "Never think up reasons not to check something."


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    • #3
      Originally posted by g1 View Post
      Agree and suggest replacing Q5 and Q7.
      Thanks, g1. I'm waiting on the parts to arrive.

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      • #4
        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

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        • #5
          I should have stocked up on them before.

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

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          • #6
            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/

            Click image for larger version  Name:	WinSource.jpg Views:	0 Size:	45.1 KB ID:	1001249

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            • #7
              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.

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              • #8
                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!

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