Jcm 900 Volume Fades Up And Down Randomly

[Mxzx]

The volume seems to drop off about 50% and will stay that way for who knows how long. I've tried tubes, effect looping, cleaning pots, deoxing tube sockets and pins, inspected both boards for broken solder joints, checked bias,etc.

Funny thing is that it is very random, sometimes will play at normal volume forever, then turn it off and is only half volume and will stay that way. The sound quality is the same, just differnt volume. Does it on both channels with and without reverb on.

When I went to standby once when it was quiet, it did swell up to normal volume before fading away?

When I tap and V1pins from underneath with a pencil, the pop seems to be very loud. It is almost like the input signal is being clipped somewhere in circuit randomly?

Any thoughts or ideas?

 

[Matt_Krush]

Pull the front board out/up and look and see if any of V1's pins are touching.

I found that on my 900 combo.

 

[Mxzx]

I'll look again.

Thanks

 

[Mxzx]

Well that wasn't it but darn close.

As I am poking the terminals on the V1 tube socket, I notice that the pin that has a red wire going through it and joining the pin next it, the solder joint was loose. Looks like someone was in there are some point, and didn't solder it up correctly.

Plugged it in and sounds normal so far!

Thanks

 

[Goldmember]

I love happy endings. Time for a shmoke and a pancake!

 

[fat_lenny]

If that doesn't fix the problem, try sticking a patch cable in the effects loop. My jacks went a bit funny at one point and a cable in the loop solved the volume fading in and out.

 

[myersbw]

There's one more issue to keep in mind if disassembling and even controls get a little crackly. One client of mine uses a pair of 900's. An elusive thing to find with a 900 is pot shear. This happens when you transport without a case and the pots end up getting knocked.

Well, as they age, the pot's phenol material gets a bit brittle. A bump from the front of the controls can cause the pot to shear at the point the solder connections enter the pot. Those controls are held by a long metal frame attached to the PCB. If that get's flexed from a bump, it flexes, but the control's solder point wants to stay put. The result is a shear or break that's hard or impossible to see until you actually loosen the pot.

Often, when you loosen the pot...it just falls loose since the nut was the only thing left supporting it and the break is revealed. But, that's such a hard angle to spot, you won't know it until you remove the controls from the front, etc. and loosen the board. It's worth doing at least once to make sure you don't have one causing you nightmares later.

It happened to him twice...then he bought a nicely padded cover (sort of a case? lol) for it. Never seen it back for that issue again.

One way to check without a lot of disassembly is to take a plastic probe that's a bit stiff...popsicle stick, whatever...and put a little pressure to each pot's solder points up near the case from behind. If sheared, it might reveal it. I take a plastic/nylon adjustment tool and tap in those areas.

Hope you did find the only issue though! 900's are a pain replacing pots on compared to the old 60's-70's amps.

BradM