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Temp "Time Format" issue Fix in Discussions

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Author Topic: MP1 Classic | Hum in Dist. Tube Voicing  (Read 5958 times)

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Griphook

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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

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MarshallJMP

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Re: MP1 Classic | Hum in Dist. Tube Voicing
« Reply #1 on: Time Format »

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.
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Griphook

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Re: MP1 Classic | Hum in Dist. Tube Voicing
« Reply #2 on: Time Format »

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?

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MarshallJMP

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Re: MP1 Classic | Hum in Dist. Tube Voicing
« Reply #3 on: Time Format »

do you also have this on the brown voicing?
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Griphook

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Re: MP1 Classic | Hum in Dist. Tube Voicing
« Reply #4 on: Time Format »

Nope, no Hum on the brown voicing
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MarshallJMP

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Re: MP1 Classic | Hum in Dist. Tube Voicing
« Reply #5 on: Time Format »

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

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Re: MP1 Classic | Hum in Dist. Tube Voicing
« Reply #6 on: Time Format »

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?

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MarshallJMP

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Re: MP1 Classic | Hum in Dist. Tube Voicing
« Reply #7 on: Time Format »

Does the step down have an earth connection?
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Griphook

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Re: MP1 Classic | Hum in Dist. Tube Voicing
« Reply #8 on: Time Format »

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

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Re: MP1 Classic | Hum in Dist. Tube Voicing
« Reply #9 on: Time Format »

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

Did you measure that voltage?
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Griphook

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Re: MP1 Classic | Hum in Dist. Tube Voicing
« Reply #10 on: Time Format »

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?

« Last Edit: Time Format by Griphook »
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Griphook

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Re: MP1 Classic | Hum in Dist. Tube Voicing
« Reply #11 on: Time Format »

Oh, alright. Found it :)

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

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Re: MP1 Classic | Hum in Dist. Tube Voicing
« Reply #12 on: Time Format »

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

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Re: MP1 Classic | Hum in Dist. Tube Voicing
« Reply #13 on: Time Format »

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%
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Griphook

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Re: MP1 Classic | Hum in Dist. Tube Voicing
« Reply #14 on: Time Format »

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:
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