Amp volume gradually lowers when playing??

Nerine

Not entirely sure how to explain this one. 

Marshall JCM 800. 2203x 

Happens sometimes. Not all the time. 

I’ll be playing and the volume just drops off to nearly nothing. 

Turn off and turn back on again and it’s fine again. 

It happened yesterday once (it has done it before now) and today not at all. 

Sounds fine at all times. It’s weird. It’s like someone’s just turning the volume control down to whisper quiet. 

Probably takes about 10-20s to ramp down. 

Again, standby the amp for 5-10 seconds and it comes back at normal volume and doesn’t change. 

As I say, didn’t happen at all today in about an hours worth of playing. Volume is at about 2.5. 

I’m assuming power amp valve related? 

Paging ICBM, I guess... 

 

ICBM

Definitely not a power valve. Unlikely to be a preamp valve.

It sounds like a bad solder joint or possibly a faulty resistor in the high voltage (B+) supply chain. If it drops to silent over that sort of time, and putting it on standby and back on fixes it, it's most likely to be a voltage drop-off caused by an interruption of the supply, so the cap(s) for the preamp slowly discharge. The spike caused by switching the standby back to on will jump the (microscopic) gap and make it work again.

Metering the B+ chain when it's in 'fault' mode - catching it like that is the tricky bit! - will find it immediately.

ecc83

Do the lights stay on?

Dave.

Nerine

ICBM said:

Definitely not a power valve. Unlikely to be a preamp valve.

It sounds like a bad solder joint or possibly a faulty resistor in the high voltage (B+) supply chain. If it drops to silent over that sort of time, and putting it on standby and back on fixes it, it's most likely to be a voltage drop-off caused by an interruption of the supply, so the cap(s) for the preamp slowly discharge. The spike caused by switching the standby back to on will jump the (microscopic) gap and make it work again.

Metering the B+ chain when it's in 'fault' mode - catching it like that is the tricky bit! - will find it immediately.

Hmm. That sounds a bit beyond me to be fair. 

I had my suspicions that something else might be afoot. 

Assuming I know nothing about this kind of thing, is there anything I can check in the chassis that might give me a bit of a visual clue at least? I’m assuming that might be difficult. 

Nerine

ecc83 said:

Do the lights stay on?

Dave.

Yep. All seems to be functioning pretty much as normal. To add some more detail, the volume drops right down, but then if you turn the volume control up, it does slightly increase the volume of the (now really quiet) sound. 

WiresDreamDisasters

Contrary what what ICBM has said, I'd say power valve or rectifier valve. I've had almost exactly the same kind of thing before with my VHT SigX. I replaced the rectifier tube, and it stopped happening. I also had it with my old JVM at one point. Replaced the power tubes, it went away.

Here's what I think:

If it's a very crackly jump across the volume spectrum that is super erratic and unpredictable, it's likely to be something like a resistor or other non-tube component. That's been my experience with some dodgy Laney bass amps in the past. But if it's a smooth fade down and fade up, then I think it's likely to be a tube.

Finally, it could also be a dodgy FX loop pot. I've had that cause volume loss in the past too.

IMO, YMMV, etc etc etc.

ICBM

WiresDreamDisasters said:

Contrary what what ICBM has said, I'd say power valve or rectifier valve.

A 2203 doesn’t have a valve rectifier, and it can’t be a power valve since it would need to be all four at the same time .

WiresDreamDisasters said:

But if it's a smooth fade down and fade up, then I think it's likely to be a tube.

A smooth fade-down could be a valve filament problem, but coming instantly back to full volume after engaging the standby rules that out.

WiresDreamDisasters said:

Finally, it could also be a dodgy FX loop pot. I've had that cause volume loss in the past too.

Not the pot since there isn’t one, but if the FX loop is engaged it *could* be the voltage supply to it.

Nerine

I did do the old “penetrate the fx loop sockets several times fiercely with a cable” and activated and bypassed the fx loop with the switch. I’ve known the fx loop to play havoc with some Marshall’s, but buggering about with this one doesn’t *seem* to yield any improvement. If it happens again, I’ll see if I can video it. 

It’s quite strange because it’s fairly sporadic. It doesn’t occur every time I use it. Just when it seems to feel like it. It doesn’t seem to be time dependent either. The amp could have been on for a while and used daily with no issue, or it could have not been used for a few months and it does it nearly right away. 

I suppose the real question is, is using it in this unknown state going to cause more long term/serious damage? 

***** 
One other thing that springs to mind, and it’s always done this, ever since I got it (used about 8-9 years ago I reckon. Originally from 2007, although I would have to double check) is that when powering off, after a couple of seconds it makes a high pitched zapping sound like a “beeeeyuuuuuu” space laser sci-fi kind of noise. I’ve always assumed it is something discharging etc. This is after both switches are off. I’ve never given it much thought to be honest. 

I’ve never had any other amp I’ve ever owned do this and I just wonder if that might be a clue as to what might be causing the volume fluctuations. Whether it’s related in some way? 

ICBM

Nerine said:

It’s quite strange because it’s fairly sporadic. It doesn’t occur every time I use it. Just when it seems to feel like it. It doesn’t seem to be time dependent either. The amp could have been on for a while and used daily with no issue, or it could have not been used for a few months and it does it nearly right away. 

I suppose the real question is, is using it in this unknown state going to cause more long term/serious damage?

If it's what I think it is, no. It will be the most microscopic crack in something, either a solder joint or a resistor, and it will be extremely sensitive to temperature, vibration or just pure randomness as to whether it opens up just far enough to break the circuit. It could potentially get worse if it arcs at the trouble spot and burns away a little more of the metal, but that would simply leave the amp not working and make it extremely easy to find the fault.

Nerine said:

One other thing that springs to mind, and it’s always done this, ever since I got it (used about 8-9 years ago I reckon. Originally from 2007, although I would have to double check) is that when powering off, after a couple of seconds it makes a high pitched zapping sound like a “beeeeyuuuuuu” space laser sci-fi kind of noise. I’ve always assumed it is something discharging etc. This is after both switches are off. I’ve never given it much thought to be honest. 

I’ve never had any other amp I’ve ever owned do this and I just wonder if that might be a clue as to what might be causing the volume fluctuations. Whether it’s related in some way? 

Maybe, if that sound is caused by arcing at the trouble spot as the caps discharge - the main power supply ones actually discharge first because the current draw from the power valves is higher, so for a short time the current flows backwards up the B+ chain from the preamp caps which still hold more charge.

Wazmeister

ICBM said:

Definitely not a power valve. Unlikely to be a preamp valve.

It sounds like a bad solder joint or possibly a faulty resistor in the high voltage (B+) supply chain. If it drops to silent over that sort of time, and putting it on standby and back on fixes it, it's most likely to be a voltage drop-off caused by an interruption of the supply, so the cap(s) for the preamp slowly discharge. The spike caused by switching the standby back to on will jump the (microscopic) gap and make it work again.

Metering the B+ chain when it's in 'fault' mode - catching it like that is the tricky bit! - will find it immediately.

I had an amp that did this live and at rehearsal intermittently... it was a bad solder joint, as ICBM suggested:)

Nerine

Cool. I’ll perhaps pull the chassis this weekend and take a look to see if anything is noticeable. 

I believe I have some solder and an iron, so I’m wondering whether I might be able to possibly patch this up. 

Is there any where specific I should be looking? Near the OT? Near the valve sockets? Near the caps? Etc. 

I think the Marshall ST1 board is pretty simple so I might be able to find what I’m looking for. 

russpm

Nerine said:

Cool. I’ll perhaps pull the chassis this weekend and take a look to see if anything is noticeable. 

I believe I have some solder and an iron, so I’m wondering whether I might be able to possibly patch this up. 

Is there any where specific I should be looking? Near the OT? Near the valve sockets? Near the caps? Etc. 

I think the Marshall ST1 board is pretty simple so I might be able to find what I’m looking for. 

You could look for a bad or dry joint which might be clearly visible. The solder might look dull and there might be a visible hole through or the component leg or wire might be loose. 

ICBM

The problem if it’s a bad joint is that it’s almost certainly on the underside of the PCB.

You may be able to find it by very carefully moving the large resistors on the board with the amp running and see if you can get it to cut out - these are all in the B+ chain and it’s most likely the fault is at one end of one of these. Be very careful though, as these are at the full HT voltage. If you’re very lucky, one of them may feel physically loose.

If you can identify it, the next problem is getting at it... you’ll need to lift the board. You can do this by undoing all the pots and jacks on the front panel and tilting it up - but it can sometimes be difficult to get enough clearance before the wires to the filter cap come tight.

Modulus_Amps

Sounds like a bad solder joint to me too I am afraid.

You might be able to find it by prodding all the components on the board with a chop stick.

Run the amp with the gain up and volume up so you can hear the hiss, when the his stops you know the problem is there and you can go look for it.

Nerine

Damn.

This is sounding like it might be pretty crappy to sort. Lifting the board feels like it might be a bit beyond me. I also do not want to fry myself. I'll have a look about and see what I can find, but I'm thinking this may be a bigger repair job than anticipated. I suppose I could always contact Marshall. They do actually seem to be pretty good when it comes to this kind of thing.

Very few amp techs near me, too. Absolute pain in the balls to get stuff like this sorted. 

WiresDreamDisasters

Where are you Nerine?

Tannin

Our amps used to do the opposite, back in the day. As teenagers at band practice in the living room, our mother would tell us to turn it down, which we would, and then, over the next 15 minutes or so, the master volume controls would magically, all by themselves, it wasn't us, honest! move back up to where they'd been in the first place. Then we'd have to turn down again. And so on.

I dunno how she put up with us to be honest. 

Kalimna

Modulus_Amps said:

Sounds like a bad solder joint to me too I am afraid.

You might be able to find it by prodding all the components on the board with a chop stick.

Run the amp with the gain up and volume up so you can hear the hiss, when the his stops you know the problem is there and you can go look for it.

Whilst the chopstick suggestion is a good one, be sure that they aren't of the metal variety! The chopsticks I have at home would act as a perfect conductor

Adam

merlin

@WiresDreamDisasters gets today points for challenging @ICBM ;

Gets the points but loses..... 

ICBM

merlin said:

@WiresDreamDisasters gets today points for challenging @ICBM ;

Gets the points but loses..... 

Wait for the final result before assuming that . I could be wrong, and other suggestions sometimes help to eliminate logical possibilities...

ecc83

ICBM said:

merlin said:

@WiresDreamDisasters gets today points for challenging @ICBM ;

Gets the points but loses..... 

Wait for the final result before assuming that . I could be wrong, and other suggestions sometimes help to eliminate logical possibilities...

Yeah verilly. Faults, no matter how weird or confusing ALWAYS come down to a logical solution that can be understood (assuming it does not clear itself) As The Man wrote "The answer, when you have elimintated the impossible................"

Dave.