mesa triple rect 3 channel switching issue

[Mars Amp Repair]

hello folks,
I am wondering if anyone in their experience has had a dual or triple rectifier that intemittently won't switch channels either with the switch on the back panel or with the Din connected foot switch. The customer said the relays did not make clicking noises.

Unfortunately he had no reason to try to actuate the switching with the 1/4" jacks on the back. That would have been some helpful info.

The amp was in another shop for an unrelated problem, so I'm also looking for anything that might have been disturbed (it doesn't seem to be the customer....he seems credible ).

Apparently when this occurs there is no sound at all out of the amp on any channel & the channel led's don't light up at all. I haven't experienced the issue yet which is disconcerting but typical. Ahh, intermittents.

I see where the power source to the relays is the +3 & -3v supply that also feeds the preamp filiaments. Maybe once it decides to act up, I can see if the filiaments also don't get power. That seems plausible as I think even with all the relays in the de-energized mode, there should still be a signal path through the preamp.

Basically, has anyone ever experienced this intermittent problem or one like it. Is this common.

Thanx, glen

[Don Moose]

I could see a loss of the LDR supply taking all the signal with it - and it would probably take the channel switching with it.

I have a schematic file of the relevant page that's too big to post here. Where can I send it?

[mikeydee77]

It might be better to start a new thread, but Glen's description above is exactly what I have been dealing with, so makes sense to carry on for the benefit of future readers.

I am looking at a very similar fault - Customer reports, Valves light up but no sound. After a drive in the car to drop it off it works briefly and then fades away.



From the schematic, V1 has its own DC heater supply of -3 to +3 which is also shared with the channel switching. What I found was that the -3v was not present. Instead it was actually +3 volts with or without the valve in. I was suspicious of the 4 x 10000uf 10v caps could be the issue as they seemed to be a little wobbly on the board and a cap potential divider (?) failure could explain the +3 volts.

Anyway I swapped them all out and immediately got an improvement; amp seems to be working. After 15 mins it is playing up. It has an annoying channel switching flicker. Symptoms are occasional random switches, Channels 2 and 3 occasionally dim their LEDS to half brightness and I hear quite a few pops that may be a random relay click. This is with the footswitch connected.

One thing that I did notice when replacing the caps is that the circuit traces on some of them are on the top of the board (so underneath the cap) and so rely on good solder flow-through to ensure the circuit is made. So obviously I tried to ensure that (cleaned the cap legs and tried to observe flow through), but maybe my repair wasn't thourough.

I was wondering if there is any common problems with these footswitches. I understand that the grounding relies on the screen and the internal wiper on the DIN pot. so that is possibility but the fault doesn't seem connected to any movement or flexing of the cable.

Having written this post I realise that I haven't tried it thoroughly without the Footswitch, I am trying that right now. If it is then I guess I will be able rule out cable and FS and concentrate on the amp.

dual_rectifier-3 channel.pdf