Mark III Red Channels not working.

[DISCOSTU82]

Mark III Red Stripe.
It was working great, then one day....

R1 and Lead channels are really super quiet. I have to turn everything up to 10 to get any sound out of the amp. It sounds like the tone stack is working bass,treble etc alter when I turn the knobs.

Now the strange thing is R2 works fine, full volume and works just like nothing is wrong.

I have replace all the tubes one at a time with no effect.

Any ideas before I have to ship it away.

 

[nickp405]

Usually when the channels stop switching the LDR or Switching relay would be my first guess with already checking tubes. If the volume is there when R1 is engaged but very low, It would be my diagnosis the tone caps and or The large orange Sprague capacitors are suffering from DC voltage leak. The original ones are rated for 400v usually. Its recommended to use 715p 600v replacements. If you have no training in electronics, you should just let a tech handle it.
Deadly voltages inside that can potentially kill. Hope this helps.

 

[DISCOSTU82]

Okay, the channels still switch fine. All the orange drops were changed out 2 years ago, I would hope they would be okay,

With R2 channel still working fine and I can alter the bass, mid and treble could it still be a tone cap?

Cheers

 

[user 21540]

Could be preamp tubes. I would suspect some of those in the first instance. From the Mark III manual (http://www.black-records.de/download/markIII.pdf):

"Now let's look at the other sockets so you can make an educated guess at which one to change first: V1, as we said, is usually the culprit for general noise problem, especially when both Rhythm and Lead Modes are affected, or if the amp is dead. V2 can also be the culprit for dead amps. If you plug your guitar into the Effects Return Jack and it produces sound - which it should - then chances are your power tubes and output driver (V5) are OK and V1 or V2 is at fault. V3 contains the Lead Mode circuitry so if the problem seems limited to the Lead, Change V3. (High settings of the Lead knobs also require a good quality V1.) V4 is the Reverb tube. Change it if you have a reverb problem. But first check that the reverb cables are plugged in securely. Grey cable goes in the socket toward the front of the amp. V5 is the socket which is set back mid-way to the output tubes. It can be the culprit if the amp is dead, has low gain or lacks boldness, warmth and/or clarity. If you are in doubt, you should merely replace the preamp tubes one-at-a-time beginning with V1. If the replacement tube fails to cure the problem, put the original tube back into its socket and go on to the next. Only the substitution method works for sure. We advise removing a power tube or two to help gain access to the central preamp tubes."

 

[DISCOSTU82]

Yeah I was hoping it was the tubes but I've swapped them out one at a time and no luck.

 

[murray sanders]

Same thing happened to my Blue stripe , check the 6mm input Chanel switching jack contacts , they get dirty

 

[DISCOSTU82]

Cheers I will clean the jackets up and check again.