Mark V 90w CH3 issue

[LesPaul70]

Now I've seen the day. My reliable woirkhorse, the Mark V 90w (over 10 years of faultless service under his belt), has developed a problem on channel 3.

Suddenly, in the middle of a song, channel 3 lost all gain and almost all volume. It sounds very feeble, weak and thin now. Like a castrated version of channel 1. The other two channels function as normal.

Before your write "tube issue!", yeah, that's what I thought too. And that's why I keep spares.

So I took a brand new 12AX7.
The manual says that the slots that affect channel 3 are V4, V5 and V6. I tried the new tube in each of those slots, in order.

No effect whatsoever.

I also tried different settings on channel 3 (pentode/triode, different wattages, fx loop hard bypass etc.). Again no effect (beyond the obvious and expected).

I am afraid I have no idea what to try next. What else could be wrong that only affects channel 3? If it is something that requires soldering, I guess I'd better send it to a professional repair shop. The Mark V circuit board is...a bit complicated. :roll:

 

[LesPaul70]

Not a lot of replies, so here's my update.

I've tried major retubing, and the issue remains.

Based on what I've learned so far, this kind of malfunction seems to suggest that the channel switching matrix could be the culprit. Since that kind of repair is beyond my modest skills, I'm sending the amp to a repair shop.

 

[LesPaul70]

All right, it's back already and in perfect working order now!

And I am awestruck, again, by the sheer arsenal of different sounds this amp packs. While I in general prefer the more organic sound and feel of last century Boogies, newer amps like Mark V and Road King II more than make up for that with their ridiculous versatility.

 

[Slash_83]

What was the problem?

 

[LesPaul70]

A "short circuit".

I'm afraid that's the only description of the issue I ever got. Yeah, I would have been interested in more details but that's all she wrote.

Nothing major apparently, though, since I got it back so quickly and the repair bill was relatively modest.