Mesa Boogie Quad preamp "random" switching problem

[musicman700]

the problem is, for instance I am playing on rhythm ch. 1 or 2 and out of nowhere, from time to time, all 4 channels are switched on - all 4 leds are on...I need to turn off and on the rear switch on the rear side of quad to return it back to one channel...it is really strange cause sometimes it is not happening at all and sometimes it happens from time to time...anybody has a clue what it might be??? cause it is really annoying when you play mellow chords on clean channel and out of nowhere 4 channels with full amount of gain are turned on...thanks!

 

[daveyboogie]

Are you using FU-2? Try unplugging that first. To temporarily select channels, take a 1/4" (6.35mm) jack plug and short the tip and screen together. Use it in the channel select jacks on the back panel. If the fault goes away there's a good change the footswitch is fried (better that than a problem with the Quad, mind)

 

[crane]

The footswitch cable could be faulty. Mine failed and all channels were on. But it was easy to repair with a solder iron.

 

[Darth Bacelar]

Yep, I believe is a problem witch the footswitch, too.

Try with "push" the cover rear the footwitch, maybe put in a soft surface, like a bed or a cushion. Another option is putting a piece of paper or something like that between the metal cover and the "body" of the footswitch.

 

[musicman700]

thx guys for the replies, yes I use FU-2A, it's really a weird problem cause sometimes I am playing for instance for seven days, and nothing's wrong with it but maybe on eight day, the quad switches to all channels suddenly...but yeah, sometimes I use fcb1010 to select the channels, and now I cannot exactly rememeber whether it was happening with the fcb too, but I think no...gonna try it with it only and I will let you know, thanks!

 

[musicman700]

the problem still persists, even if I do no use the FU-2A controller, for instance when I switch with fcb 1010, it's really weird when out of nowhere all leds are on, do you think it might be wrong something inside the quad?

 

[Whoopysnorp]

Could be something to do with the LDRs. Those control the channel switching (at least they do in the Studio Preamp; I assume they do in the Quad too) and have a limited life span.

 

[daveyboogie]

Was FU2 definitely diconnected when you tried FCB1010? ....if it was:

Unlikely to be a single LDR problem. Sounds like the 6406N chip has had it. I've heard of this happening from time to time.

If you're lucky the chip will be mounted in a socket and you can just pull it out ... no more problems but you can never use the FU2 again. Use FCB1010 (or similiar) thru the rear jacks only.

http://tubefreak.com/s-quad4.gif

Not sure what else it could be ... possible power supply probs but think you would notice other faults if it was that.

Mebbe a massive short somewhere ... Take the lid off, hold upside down and shake like hell. See if anything falls out.

Could still be the connection to the FU2 socket need a rework.