Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Rich_C on March 05, 2021, 11:52:53 am

Title: Marshall 1987s hum.
Post by: Rich_C on March 05, 2021, 11:52:53 am

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.

Title: Re: Marshall 1987s hum.
Post by: Leevi on March 05, 2021, 12:22:06 pm

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

Title: Re: Marshall 1987s hum.
Post by: Latole on March 05, 2021, 12:34:43 pm

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.

Title: Re: Marshall 1987s hum.
Post by: Rich_C on March 05, 2021, 02:05:53 pm

Leevi ~ Yes, increased after caps were change.

Title: Re: Marshall 1987s hum.
Post by: Latole on March 05, 2021, 02:07:56 pm

Quote from: Rich_C on March 05, 2021, 02:05:53 pm

Leevi ~ Yes, increased after caps were change.

Check you work .

Title: Re: Marshall 1987s hum.
Post by: Rich_C on March 05, 2021, 02:22:50 pm

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.

Title: Re: Marshall 1987s hum.
Post by: Latole on March 05, 2021, 02:40:32 pm

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

Title: Re: Marshall 1987s hum.
Post by: Rich_C on March 05, 2021, 03:06:01 pm

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.

Title: Re: Marshall 1987s hum.
Post by: Leevi on March 05, 2021, 11:24:42 pm

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

Title: Re: Marshall 1987s hum.
Post by: Rich_C on March 06, 2021, 03:47:04 pm

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  :icon_biggrin:

Title: Re: Marshall 1987s hum.
Post by: acheld on March 06, 2021, 09:30:40 pm

Quote

"A man's GOT to know his limitations"

Hah!   Not in today's world . . .

Title: Re: Marshall 1987s hum.
Post by: Rich_C on March 08, 2021, 12:33:11 pm

 :laugh: Yeah. But experience is a valuable thing. 

Title: Re: Marshall 1987s hum.
Post by: Tone Junkie on March 09, 2021, 03:03:09 am

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.