Volume Fluctuation in Marshall JCM 2555

[hixsix]

Hello, I have a Marshall head (model #2555) whose volume started to go down gradually and then slowly come back up. Eventually the head lost sound completely. Someone advised me to change all the tubes, so I did. Now, it works fine for a few minutes, but at times the volume seems to cut in & out. But, it's not dropping & raising gradually like it was before. I changed the power tubes & preamp tubes. Does anybody have any ideas what the problem could be? Thanks.

[PRNDL]

Most likely the power supply electrolytics are failing and need to be replaced (i.e. a cap job).

[Enzo]

Wouldn't that cause hum rather than a complete loss of signal?

[hixsix]

Yes, there was a slight hum...I thought maybe it was TV interference. By the way, what is a cap job? And what are power supply electrolytics?

[52 Bill]

A cap job is replacing all of the power supply capacitors (and sometimes others) in the amp.

The power supply electrolytics are the large value capacitors that filter out the hum in the power supply of your amp.

As Enzo points out, a cap job will probably not cure the problem with your amp.

If your amp has an effects loop, try plugging a spare guitar cable into the jacks, and see if the problem goes away. Another test is to try pounding on the amp with your fist, to see if you can make the problem either show up, or go away.

[hixsix]

So, should I try replacing the capacitors? If so, which ones? And where are they? What do they look like? Any idea how much these parts cost? Are they easy to replace?

Also, today, before I did anything major, I decided that I would clean the connections with an electronic contact cleaner. After doing this, it's no longer fluctuating, but now I'm not getting any volume. I can hear it, but it's not loud at all, even at full blast. So, now what?

Also, I did notice that on my high/low switch, when I set it to low, it hums.

[pdf64]

Try patching a guitar lead between the effect send and receive sockets - Peter