Marshall Haze 40 Mute Circuit Issues

[cc2023]

Hi,

I am having problems with my Marshall Haze 40. I think I have narrowed the issue down to the mute circuit firing when it should not be. Here are the details:

• Issue: Mute circuit seems to just trigger randomly producing a mechanical relay clicking noise (not through speaker). This causes sound to cut for maybe a few hundred milliseconds or so.
• Frequency: Seemingly random. Some days it won't do it at all, some days it will be doing it multiple times in a few second window.
• Circumstances: Have not been able to narrow this down. It seems to happen on either channel. It does not seem to care if the FX loop is in use.
• Troubleshooting so far:
  • 1) Reseated all tubes and ensured they were all glowing, swapped position of preamp tubes. Inspected for any burn marks, cracks, etc. Looked fine.
  • 2) Checked circuit board for any burn marks, burnt/ cracked resistors, leaky caps, etc. Looks pretty clean.
  • 3) Prodded board with chop stick to see if moving anything caused any issues. I am not sure it is causal but one time it did have a fit when I pushed the wiring harness and started clicking like crazy.

My question is if anyone has seen anything like this before. I am pretty certain the mute circuit is the issue because it makes the same noise when switching channels (I assume to mute the channel switch pop). I understand how the mute circuit works but I don't fully understand when it gets triggered. Any common solutions or root cause of the issue?

[The Dude]

Please upload a schematic or schematic link. It's difficult to give advice without seeing the circuit.

[g1]

Schematic attached.
Looks like relay RL1 does the mute function. It grounds the PI input when given the command from IC2 in the switching circuit (via TR1).

[The Dude]

My first step, if you suspect mute circuit, would be to monitor the voltage across D6 (also the mute relay coil- on last schematic page) and make sure it is a mute circuit problem. Just because you hear a clicking sound doesn't mean for certain it's a mute circuit problem.

[cc2023]

Appreciate the replies.

The Dude, I will try to do so. Unfortunately because it is a PCB it is pretty challenging to get a multimeter on all of the traces, but I'll try.

The reason I think it is the mute circuit isn't just because it is clicking. It is definitely a relay actuating though. The reason I suspect the mute circuit is because the behavior is EXACTLY the same as when you switch channels. When you switch channels the mute circuit is momentarily activated (to prevent popping noises I would assume). There is a very loud click, followed by a very brief cut of all sound, followed by another click. The duration of the cut is always the same. The frequency of the cuts is what changes.

I made sure all of the quick connect fittings were clean and securely connected and it hasn't done it since. I don't trust that it is truly solved, but I'll keep using the amp over the coming weeks and see if maybe that was the issue. Part of me wonders if maybe the standby switch's connection to the board was a little either loose or corroded. It seems the mute circuit is also actuated during standby, since I can hear relay clicking.

I have noticed too that I have a tougher time replicating the issue with the back off of the panel, which leads me to also think it may have been a loose/corroded connection. I'll post again if I can get it to happen again.

I do not think it is the channel select relay because there is no change in sound, just a cut followed by resuming on the same channel. I do not think it is the boost relay for the same reason. FX Bypass relay is interesting... I think it's unlikely but if I can get it to happen again maybe I'll jumper the FX loop to eliminate that.

[g1]

The mute circuit has a 'ducking' function during channel switching as you said, to eliminate popping.
If the rapid clicking you heard in post #1 was also giving the signal muting, I would suspect a bad connection at a connector or a bad solder joint there or elsewhere on the board. If you end up doing the chopsticking again, run a test-tone while doing so and if the rapid clicking happens again you will know if it is the mute relay, or another.

[cc2023]

Good idea. I still have the chassis out of the cab and I have not been able to replicate it again in the last 4-5 hours of play. I think I'll play again tomorrow, if it doesn't do it again I'll throw it back in the cab. I did remove and reseat all of the removable connectors on the board so it possible that solved it.

I got it to trigger yesterday by pressing hard on the wire harness running odd the power/standby/transformers to the PCB but it was not the most repeatable thing.

Not sure if chassis vibration had something to do with it bc it does seem like it happens far less with the chassis removed. If I put it back in and it happens again I'll consider soldering the removable connectors. Pain to service but if it fixes the issue that's fine by me.