Re: Identify cap problem in amplifier help

well I ordered 2 new filter caps this morning since I will eventually need them in the future and they don't make the LCR caps I need anymore so I might as well buy them before they disappear........... after doing some more reading and talking to people I believe the problem also might be the tube sockets. they might need to be tightened around the pins a bit. I will try doing this with a small eye-glass screw driver later. If it doesn't work I will start replacing screen grid reisistors.

I hope I don't have to replace the...

Re: Identify cap problem in amplifier help

I just looked at the bias caps you mentioned and there is no visible damage to them that I notice.

I just played the amp for a bit and left it on for about an hour with no problems what-so-ever which makes the problem really annoying. It will be fine for days or weeks then out of nowhere it will spuratically go intermittent on me. This is no good because if that happened at a gig, I'd be in trouble.

I will order two filter caps tomorrow just to have them on hand.

Re: Identify cap problem in amplifier help



- no, I've never heard of doing anything like that, what does it do?

- wouldn't keeping it on for an hour or so only be a temporary solution?...

Re: Identify cap problem in amplifier help

what would the sound differences be between bad bypass caps vs. bad main filter caps?


also, where would the bypass be on this schematic?
[URL=http://www.drtube.com/schematics/marshall/jmp50c2.gif]Schematic[/URL]

*if they aren't on here in can provide other schematics.


ThanksRe: Identify cap problem in amplifier help

what would the sound differences be between bad bypass caps vs. bad main filter caps?


also, where would the bypass be

Re: Identify cap problem in amplifier help

It has been serviced. As metioned, the tubes are brand new with proper bias adjustment, I remelted questionable solder joints, cleaned all sockets, pots, and switches, and all of the components on the PCB still look good (no burnt out resistors or anything). I just want to know if I should focus my energy on the caps.



Here is a pic of the two caps I got from online (its not my personal amp)...

Hi, I suspect that my caps are going bad (already are?) in my amplifier. It is a 1990 Marshall JCM900 4100, so the caps are at least 15 years old. The symptoms are intermittent fuzziness/static accompanied by random drops in volume along with an oscillating hum. There is no visible leaking in any of the caps. For example, I will let a chord ring out for a few seconds and I can hear the oscillation go up and down, this is followed by a static and intermittent drops in volume.

My question is; are these signs of failing caps? and, what are the audible signs of failing caps?
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