MP1 Classic | Hum in Dist. Tube Voicing

[Griphook]

hello guys,

I recently bought a MP1-Classic  :whoohoo!:
The brown-Channel is very nice :) :)

And how could it be otherwise, I got a new problem.
There is a hum, when im in Distortion Voicing. It is not dependant on the OD1/OD2. I also plugged of any other gear, switched the Loop off/on, tried all Ooutputs, but no change of the hum.

Any Ideas? :)
Thanks in advance

[MarshallJMP]

So if you set both OD's on 0 you still have a hum?What happends if you turn down the master volume and the front vol pot?
Just want to figure out where in the circuit the hum is.

BTW try this with nothing plugged into the input.

[Griphook]

Hi MJMP,

tried what you said and heres my result:

I tried the following two times, first with Guitar plugged in, second without:

Turned the following to zero
OD1 --> no change
OD2 --> no change
Master and the Poti on the front --> Hum goes away as more as i turn it down

Both times same results. So the issue is somewhere behind the tube stage?

[MarshallJMP]

do you also have this on the brown voicing?

[Griphook]

Nope, no Hum on the brown voicing

[MarshallJMP]

Can you measure voltage?If so on the upper left corner there is a testpoint,here you should have 200V,can you check it?

[Griphook]

I'll check it tomorrow.

It is a US Version, since i got a Setp-Down-Transformator (220V>110V) delieverd. Maybe this can be part of the issue too?

[MarshallJMP]

Does the step down have an earth connection?

[Griphook]

No It's only 2 Pole, but a grounding issue would affect all Channel wouldnt it?

[MarshallJMP]

Depends,sometimes this gives problems,sometimes it doesn't.

Did you measure that voltage?

[Griphook]

Which point do you mean?

And am I really supposed to have 200V in the US-Model absed on 110 Volts?

I attaced a picture, where do i find the Testpoint?

[Griphook]

Oh, alright. Found it :)

I measured from there to the chassis and got 205,8V

[MarshallJMP]

Mmm a bit too much.I will compare it with mine tomorrow.I'll let you know.

[MarshallJMP]

Mine is 199.9V.So yours is a bit high and it could be R382 is going up in value.(This is like the R913 in the MP-2).So what you can do is try to replace,it might help,it might not.The value is 499k 1%

[Griphook]

Where do I find R382 on the PCB? And in which Schematic is it listed?
There are Several in the Vault:
MP-1 Classic Chorus
MP-1 Classic EQ
MP-1 Classic Input Stage Compressor
MP-1 Classic Output Drivers & Pot Board
MP-1 Classic Power Supply & Peak Meters
MP-1 Classic Processor
MP-1 Classic Tubes
:dunno: