dudes i got a question (ground issue)

[rockstah]

100watt plexi spec (1978 marshall)-

nothing plugged into it and it hums like it has aground issue. when i touch the chassis the hum goes away.

i have checked the input jack and grounds. any ideas why this particular amp is doing this?


Mark

[Structo]

I would double check the input jack.
Sounds like it is not shorting to ground when nothing is plugged in.

I assume it is a Cliff style jack (enclosed black plastic?).
You should be able to see if it is making contact.

[rockstah]

I would double check the input jack.
Sounds like it is not shorting to ground when nothing is plugged in.

I assume it is a Cliff style jack (enclosed black plastic?).
You should be able to see if it is making contact.

i have a ground wire from ground on the jack to the ground buss.
marshall style jacks make contact with the chassis and thus the reason to run a ground from them.

[rockstah]

along with the ground noise going away when i touch the chassis if bring my hand close the input jacks on the front of the amp the noise gets louder?

[dynaman]

Try grounding jacks in a different spot like V1s cathode?

[rockstah]

Try grounding jacks in a different spot like V1s cathode?

this sounds interesting.
i guess my main stop is why on this amp out many many amps does it do this? to do something different than i have done on any other amp is like a can of spray paint on the oil light on the dash.
i must be over looking something. thanks for the suggestion.

[Structo]

Is this a brand new build?

In other words you are just now powering it up for the first time?

[Deric]

Just thinkin' out loud......

Are you using a shielded cable on the input? If so, double check that it is grounded well (at one end only of course).

If you plug a cable in and short it (tip to sleeve) does it go away?

Good luck

[Firestorm]

I still think Tom may be right. It sure sounds like the shorting tab is not shorting to ground w/ nothing plugged in.

[billyz]

Are any of the pots grounded mechanically to the chassis ? could be corrosion on one of the grounds.

[rockstah]

Are any of the pots grounded mechanically to the chassis ? could be corrosion on one of the grounds.

bingo! one volume pot was not grounded well enough!

[billyz]

If you really want to fix it , install a ground buss ala Trainwreck. Solder to the back of all your pots and ground to the buss. Many older marshalls have this feature too.

[rockstah]

i do it that way and it does have a ground buss it just wasn't soldered correctly.
this is a 69 metal panel i'm modifying for someone and whomever had it before me didn't do the ground good enough on that one pot which looks not to be an original pot as well.

[billyz]

Are you doing any SXSoWhat gigs ? As you are in Austin we should run into each other sometime.

[dynaman]

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