Volume Spikes

[ido1957]

Marshall JCM900 Clean channel

I haven't played through this in a few months. I fired it up last week to do some playing in my basement.
Getting volume spikes. They last about 3 or 4 seconds (sometimes more) then back to normal volume
First day - 6 or so big volume spikes over 2 hours.
Second day - disconnect the pedal and went straight into the input with my strat (2 or 3 spikes )
Third day - cleaned the knobs on the front. Only the master volume was scratchy (enough to hear it) - one spike that time which was much lower in volume

What worries me is that I've brought his head into the tech a few times over the years and he couldn't recreate it. He just fiddled and changed tubes and said he can't recreate it and just tried a new tube/rebias blah blah blah. The tech is a good guy so it's as if this goes away until you let it sit for a while (like months).

Googling = Powere tubes/clean the volume knobs (did that) / a problem between the channel switching thingies where it jumps to the high gain (I will try turning down the high gain master next to see if the volume cuts out). I will also be trying to switch down to 50 watts (power tubes) and maybe the pencil tapping thing.

This amp is over 30 years old and has had a few tweaks/tubes but nothing major. Not sure if anyone here has experienced this but it's not a new thing with this amp, and seems to come back intermittently.

[Armistice]

I wouldn't rule out demonic possession...

Sorry, ido, that's of no use whatsoever.

[Greg_L]

Marshall JCM900 Clean channel

I haven't played through this in a few months. I fired it up last week to do some playing in my basement.
Getting volume spikes. They last about 3 or 4 seconds (sometimes more) then back to normal volume
First day - 6 or so big volume spikes over 2 hours.
Second day - disconnect the pedal and went straight into the input with my strat (2 or 3 spikes )
Third day - cleaned the knobs on the front. Only the master volume was scratchy (enough to hear it) - one spike that time which was much lower in volume

What worries me is that I've brought his head into the tech a few times over the years and he couldn't recreate it. He just fiddled and changed tubes and said he can't recreate it and just tried a new tube/rebias blah blah blah. The tech is a good guy so it's as if this goes away until you let it sit for a while (like months).

Googling = Powere tubes/clean the volume knobs (did that) / a problem between the channel switching thingies where it jumps to the high gain (I will try turning down the high gain master next to see if the volume cuts out). I will also be trying to switch down to 50 watts (power tubes) and maybe the pencil tapping thing.

This amp is over 30 years old and has had a few tweaks/tubes but nothing major. Not sure if anyone here has experienced this but it's not a new thing with this amp, and seems to come back intermittently.

So how much louder does it get? Does it go from like bedroom volume to cranked?

[ido1957]

Yeah it's very loud when it spikes - like the master is on 10.
Well, I better qualify that a bit as mentioned above. The first day it was super loud. It got progressively less loud as the days went by and I tried different things. Yesterday it got say 50% as loud as the first day.

[Greg_L]

Might be the pot still. It might need to be replaced.

[Farview]

What is the master volume set at?

When you have the reverse problem (the amp gets quieter every once in a while), it is sometimes the switching jack in the effects loop getting dirty.

[ido1957]

Master volume on both channels - and both set to about 10 or 11 o'clock so not very loud.
I will probably have to bring the head in for service.
My beef is that I never gig anymore and these issues happen (every few years) without any significant playing time.
I really don't want to buy a new head, when this one works it does what I need. And it works "most of the time"
Blah.....

[Greg_L]

Master volume on both channels - and both set to about 10 or 11 o'clock so not very loud.
I will probably have to bring the head in for service.
My beef is that I never gig anymore and these issues happen (every few years) without any significant playing time.
I really don't want to buy a new head, when this one works it does what I need. And it works "most of the time"
Blah.....

Can you tell which volume is acting up?

[ido1957]

Master volume on both channels - and both set to about 10 or 11 o'clock so not very loud.
I will probably have to bring the head in for service.
My beef is that I never gig anymore and these issues happen (every few years) without any significant playing time.
I really don't want to buy a new head, when this one works it does what I need. And it works "most of the time"
Blah.....

Can you tell which volume is acting up?

I only use the clean channel nowadays, so I guess it could be either the gain or master on that channel.
But that adds another test I will do - use the high gain channel just to see if it too gets that volume spike.

[Greg_L]

I only use the clean channel nowadays, so I guess it could be either the gain or master on that channel.
But that adds another test I will do - use the high gain channel just to see if it too gets that volume spike.

So each channel has it's own volume? Does it have a global master volume?

[Greg_L]

Never mind I'm looking at one now...

So yeah, try the lead channel and see if it jumps.

[ido1957]

Took it into the shop - the guy says usual cause is oxidation, or possible ground issue on the volume pot(s).
So Greg you pretty much nailed it from the sounds of it.
He'll clean the crap out of it and see if it fixes it and check it all over (tubes/sockets/shorts/jacks/etc etc).

[Greg_L]

Took it into the shop - the guy says usual cause is oxidation, or possible ground issue on the volume pot(s).
So Greg you pretty much nailed it from the sounds of it.
He'll clean the crap out of it and see if it fixes it and check it all over (tubes/sockets/shorts/jacks/etc etc).

Besides the inconvenience, a bad/dirty pot is a pretty minor problem, so be glad about that. The amp getting louder intermittently is better than losing power. That could be a whole big pile of very bad things.

[Farview]

Instead.of cleaning it, why not replace it? Pots aren't that expensive.

[Greg_L]

Instead.of cleaning it, why not replace it? Pots aren't that expensive.

It might be a board mounted pot on a JCM 900. I'm almost certain it is. Those are more of a bitch to change.

[Lt. Bob]

Instead.of cleaning it, why not replace it? Pots aren't that expensive.

It might be a board mounted pot on a JCM 900. I'm almost certain it is. Those are more of a bitch to change.

yeah ..... you don't want to try that unless you have good soldering skills

[ido1957]

Yeah he mentioned something about the pot being on a PC board, with little clamps and soldering being a bugger, so I suspect he'll clean and hope it fixes it.

[Greg_L]

Yeah he mentioned something about the pot being on a PC board, with little clamps and soldering being a bugger, so I suspect he'll clean and hope it fixes it.

Changing a PCB mounted pot is not NASA work, but it's more complicated than typical chassis mounted pots with flying leads. Any competent tech do it in minutes though.

[ido1957]

No word on the amp yet however the weather has been brutally cold and I wouldn't be surprised if buddy stayed home. Although he does have to pay rent on his shop so you would think he would risk it lol...
Watch - he'll phone me as I'm clicking submit (I can always hope)...

[ido1957]

JESUS! I paid the guy 183 dollars today after he did a bunch of stuff to my amp - pot cleaning, a little solder fixing, cleaned the oxidization off a bunch of sockets and pots. 2.5 hours of work (and took 3 weeks to get it back). AND THE FRIGGEN VOLUME STILL SPIKES!!!!!!!!
Like within a minute of me playing through it when I got home.

Ok so this time I got smart and quickly fiddled with the guitar knobs, and all the amp knobs and switches while it was happening ( it lasts several seconds when it first happens, then as it warms up it happens less often and less severely).

AHA! Found something (finally)
- I'm set to clean channel A, and turning the gain knob on channel A does nothing
- turning the channel B gain up and down and presto! it controls the volume
- the channel switching button on the front appears to be tripping by itself somehow (internally).
- I pushed this switch in and out and the spike continues regardless, and the little light doesn't go on which it should when high gain is engaged.
- eventually the switch does work properly and the light comes on and things work ok for a while
- then I turned the gain for channel B completely off - then it starts going silent intermittently (so proves the gain/channel is switching by itself) and volume is shut off momentarily.
- this is a 10$ switch available on the net.

So the guy says he tested it with some 1Khz wave or something, but this is intermittent so he's got to test this when it's cold and using a friggen guitar. Anyway he was closed by the time I called him - I'll bring it in on Monday. And I think I'll look around for a different shop - this guys has let me down a couple of times over the years.