Peavey 3120 Channel Switching and low volume issue

[stereomonostereo]

Hey guys,

I'm working on a Peavey 3120 that's having a switching issue. Initially, the amp was worked on by a tech when the channel switch on the front of the amp was physically broken. The channel switch was replaced, and apparently channel switching was working, but the lead channel knobs weren't doing anything. Now I'm going over it, and it's definitely got some issues. With the switch on the middle RHYTHM position or LEAD position, the LEAD channel light is on. Putting the switch on the CLEAN position takes you to RHYTHM. Also, the overall volume is very low.

The switch wiring looks to be okay. When I plug into the Effects Return I'm getting a strong input signal. I've jumpered across the Send and Return with no difference. Can anyone point me in the right direction of where the problem might be occurring? It's seeming to be a preamp problem, possibly a relay?

triplex.pdf

[Jazz P Bass]

The first thing that I would check is the 'switching' logic.

[Enzo]

Isn't that three way toggle a special switch? m And might the previous tech put in a plain old vanilla one instead?

[tedmich]

Probably Enzo is right. its an oddball SPTT, S1 SW2368 71322368 TOGGLE SWITCH
There is a 2002 (slightly newer ) XXX schematic here with only top PCB trace? Some things are clearer... nice french site:
SAINT GERMAIN MUSIQUE

[stereomonostereo]

Thanks for the schematic, it's much easier to read. The switch is the proper replacement. The problem there was that the ribbon cable coming from the switch was replaced with single wires and done upside down. That's now out of the way.

Moving on, the volume is still low. The output section seems fine. Preamp tubes appear to have proper voltage.

I've also noticed that Relay K3 sticks occasionally and cuts out the controls for the lead channel.

[stereomonostereo]

I'm getting audio signal at the relay end of R49, but nothing at the other end. Should I be seeing something there? If I inject an audio signal at C34 and R52, everything works at it should from that point on out.

[tedmich]

I think Enzo solved something like this here:
Peavey Triple XXX Fault - Peavey Forum

These transistors often go bad, also in 5150, Q7/8/10/11 are MPS6531, about $0.75 at Mouser.

[stereomonostereo]

I checked those guys out and they test out okay

[tedmich]

Qs and Rs? then it could be bad relay, rare but it happens. Oddball low volume happens in 5150 when 1st PS cap goes, I'd check all those too, WTF.

[stereomonostereo]

Yep, they all test out good. All channel switching is functioning properly, and everything from the FX Return on out through the output section is working properly. Should I be getting an audio signal at R47?

[Jazz P Bass]

Where on the schematic is 'R47"?

[stereomonostereo]

It's on the Effects/Footswitch Breakaway Board section, up at the top. It ties in to pin 3 of K5B and pin 6 of K5C.

[stereomonostereo]

The more I go over it, the more I suspect the K5 relay, though I don't really have a way of testing it. If I run a jumper wire from the relay end of R49 to the relay end of R104, the signal is loud and clear.

[tedmich]

My XXX has a Nais/Panasonic DS2Y-S-DC12V in the position, $3.21 from Mouser, whats the newer version got?

[Enzo]

You test a relay with an ohm meter while the circuit is live. Nothing special about a relay, it is nothing more than a switch. It just has a magnet coil to flip itself.

So when a relay switches from channel A output to channel B output, pick the pair of contacts that are suspicious, set the amp up so they ought to be closed, then measure resistance across them. If the contact terminals are not reachable, then probe whatever they are wired to - the end of some resistor or whatever.