Carvin X60B help.

[frankjc]

Hi. I have a 1986 Carvin X60B, and the volume is very low on the lead drive channel. The clean channel is louder on 1 than the dirty channel on 5. It used to be that on 5 it would rock the house!
I had this problem a couple of years ago, and someone recommended I replace the 4049 chip that controls the channel switching, but it didn't help. That time the problem just went away on its own.
It reappeared a couple of months ago, but only for a few minutes, then I was able to play for hours without it happening. Yesterday I pulled the 12AX7s and swapped them around, and it worked fine for maybe a minute.
I have tried switching the eq, effects loop off and on, unplugging the foot switch, 100/50/25%, none of these made any difference. Anyone have any ideas?

[nickb]

If moving the 12AX7's around helped, then it sounds like you have a simple connection problem.

It could be the tube base contacts need cleaning and /or re-tensioning. With the lead channel selected on the master not too high, wiggle each tube and see if you can can hear and nasty crackly noises to identify the bad one(s). If that doesn't do anything it could be a bad connection underneath. A common way to narrow that down is to use a chopstick or other similarly shaped insulator and gently push at the connections inside, again listening for noises that indicate a bad joint.


Of course it could always be unrelated but at least it's a place to start.

[frankjc]

Actually, I didn't really make a difference. It was good for maybe a minute, then went bad again. I put an oven mit on and wiggled the tubes a bit, made no difference. I am beginning to think its heat related. I recently installed a bias pot, but I would think if that was the problem, it would affect both channels. I am going to clean the tube sockets next.

[lowell]

Must be before IC4 which mixes the 2 channels back together. It could be V1 or V2 preamp tubes or any related component. What is your experience repairing amps?

[frankjc]

[QUOTE=lowell;396340]Must be before IC4 which mixes the 2 channels back together. It could be V1 or V2 preamp tubes or any related component. What is your experience repairing amps?[/QUOTE
I have a little experience working on amps. Not a lot, I've done simple mods built a few pedals. Stuff like that. Are the ics socketed? I can't recall. I think they were 4558s, which I might have in my toolbox.

[lowell]

Ok. I'd check voltages on v1 and v2 first. And the bias current. I wouldn't suspect IC4 quite yet.

Pins 1-3 and 6-8. Please list DC voltages.

[frankjc]

Ok. I'd check voltages on v1 and v2 first. And the bias current. I wouldn't suspect IC4 quite yet.

Pins 1-3 and 6-8. Please list DC voltages.

Ok. I will do that as soon as I get a chance to open the amp up.

[drewl]

I've seen alot of intermittent PC board connections and tube sockets on these if it isn't the switching.

[frankjc]

I've seen alot of intermittent PC board connections and tube sockets on these if it isn't the switching.

Those 2 conditions could be heat related, right?
It still sounds the same, just quieter. Volume at 10 on that channel is barely above "bedroom" level. Clean channel functions normally.

[lowell]

Yes. And checking voltages might tell you if so.

[frankjc]

Yes. And checking voltages might tell you if so.

I am not sure if this is a dumb question or not but, does that have to be done from the under side of the pcb with the tubes still in, or should I pull the tubes out an check from the outside?

[drewl]

With tubes in to see if they are conducting and doing their job.

With them out you will only be able to measure the 6v ac filament voltage and the high voltage dc to the plates.

[frankjc]

A dirty pot was the culprit.