Mark 4 problem...Please help a n00b

[mboogie76]

Howdy from Texas!
I am enlisting the Guru services of the BoogieBoard...

A friend called me freaked out when his '92 Mark 4 took a nap after a ride to practice. I asked him about the usual stuff: cables, seating the tubes, plugging in a diff guitar and cab (aka: testing with known good). nada. Here's the odd part: the FtSw lights up the Loop and EQ (switching them kills the LEDs on the FS), but the front of the amp is dim as Gomer Pyle. The Channel options on the FS are very dimly light and do nothing.

So I take it home and expect the power tubes (Sov 5881's) to be out of parity or one of the class A socketed tubes shot. No such luck. I preamped into the return of the FXloop and it came alive in Mesa fashion, but the Preamp is deed.

So I killed the lights in the room to get a good visual on the Pre's. all nice and glowy orange except V1 (the shielded one). I did all sorts of 12--7 tube swaps with know good tubes and no such luck.

Has anyone experienced such a weird occurence?
thanks
Corey

PS: I am a loyal M2000 user - I play the big strings:

 

[mboogie76]

Oh yeah, the amp chassis' out of the box and the other pre tubes are like little xmas trees.

 

[Platypus]

Did you just replace V1?

It's possible that there is a different preamp tube to blame. I would replace all of them with known good tubes to rule that out as it sounds like a classic case of preamp tube failure to me.

 

[mboogie76]

yeah, I ha some spare 12ax7s laying around. none light up in that position.

 

[Platypus]

yeah, I ha some spare 12ax7s laying around. none light up in that position.

Try replacing ALL of them at the same time, not just V1

 

[mboogie76]

I would have to go out and get 3 tubes, which I dont have a prob with, but would tubes address the footswitch and channel issues?

I dont want to throw money at the problem blindly.
Thanks guys (and gals!)

 

[Boogiebabies]

Other than preamp tubes, it could be blown capcitors in the switching buss power supply. When they go, it could basically leave the LDR's shorted and no signal would get through. If your handy, take the chassis out and look for two tall blue electrolytics, bulges, yellow goo and burn spots.

Also, have you tried to remove the footswitch and switch ot manually?
If the circuit in the footswitch goes bad, then the whole circuit could be fouled until you remove the footswitch. If the switch on the back works, the have fun coughing up the dough for a new FU-3.


Ed

 

[mboogie76]

There was no obvious burn or cap-mess anywhere to be found.


I did remove the FS with the same result.

here's a link to the pic of the innards. Which caps are they?
http://acapella.harmony-central.com/attachment.php?s=&postid=18619963

 

[Guitarzan]

Other than preamp tubes, it could be blown capcitors in the switching buss power supply. When they go, it could basically leave the LDR's shorted and no signal would get through. If your handy, take the chassis out and look for two tall blue electrolytics, bulges, yellow goo and burn spots.

Also, have you tried to remove the footswitch and switch ot manually?
If the circuit in the footswitch goes bad, then the whole circuit could be fouled until you remove the footswitch. If the switch on the back works, the have fun coughing up the dough for a new FU-3.


Ed

Hey Ed... Output Tx?