Marshall 1987s hum.

[Rich_C]

Hi,
I installed new F&T filter caps into my amp and its developed a hum. No hum in standby, but it's there when no guitar is plugged in. I can't decide if it's a 60 cycle hum of 120 cycle hum. I'm leaning towards a 120 cycle by the pitch. I also put new tunes throughout, to rule out a bad tube I put the old tubes back in...noise remained. Has anyone ever purchased a new filter cap and gotten a bad one? I was poking around yesterday and discovered one of the pins on a preamp socket makes noise when poked. But i think this is an unrelated problem.

[Leevi]

Do you mean that the hum increased after cap replacement?
/Leevi

[Latole]

Something wrong with your soldering or you put filter cap backward ?
Check your work.
I never had a bad filter filter cap from a store.

In Stanby, there is no B+voltage. So you can't hear a sound or hum or hiss in the speaker.

[Rich_C]

Leevi ~ Yes, increased after caps were change.

[Latole]

Leevi ~ Yes, increased after caps were change.

Check you work .

[Rich_C]

Latole ~ I thought it could be a bad solder job. They seems tight. But who knows. I was very careful to hook them up correctly. I suspect if it is a bad joint...its probably the one under the PCB. That wasn't easy to get at. Marshall was a bit skimpy with wire back in 1988.

[Latole]

I don't know if it is a bad solder or something else.

Every time amp is worse after I work on, it was my job, my fault.
And I read the same about other forum members

[Rich_C]

Latole ~ I hear ya. Wouldn't be the first time I made things worse. I'm going to go in and take another poke around. Only other thing running through my mind is the Bias caps. I didn't change them. With my lack of soldering skills, I wanted to keep away from the PCB. Anyhow, I'll see how it goes.

[Leevi]

The problem can be somewhere else, the new filter caps just made it louder. If you have an oscilloscope check for instance if there is ripple on the negative bias voltage.
/Leevi

[Rich_C]

That sounds reasonable to me. I don't have an oscilloscope. I think for my sanity, and the health of my amp, I'm going to have to get a pro involved.  "A man's GOT to know his limitations" ~ Harry Callahan 

[acheld]

"A man's GOT to know his limitations"

Hah!   Not in today's world . . .

[Rich_C]

  Yeah. But experience is a valuable thing. 

[Tone Junkie]

Jump in there give it a shot. 13 yrs ago these guys helped me build a 1987. I’m sure some of them would have liked to strangle me during trouble shootIng. my soldering skills Weren’t quit up to par, but they stuck with Me . just like they will with you. now I have 35 or 40 amps under my belt. Still make dumb mistakes. And they continue to help me. Great bunch of guys.