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Let's Get Technical => All PREAMP Tech Tips & Support => Topic started by: Griphook on March 12, 2016, 09:16:23 AM

Title: MP1 Classic | Hum in Dist. Tube Voicing
Post by: Griphook on March 12, 2016, 09:16:23 AM
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

Title: Re: MP1 Classic | Hum in Dist. Tube Voicing
Post by: MarshallJMP on March 12, 2016, 12:00:34 PM
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.
Title: Re: MP1 Classic | Hum in Dist. Tube Voicing
Post by: Griphook on March 12, 2016, 02:54:39 PM
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?

Title: Re: MP1 Classic | Hum in Dist. Tube Voicing
Post by: MarshallJMP on March 12, 2016, 03:23:47 PM
do you also have this on the brown voicing?
Title: Re: MP1 Classic | Hum in Dist. Tube Voicing
Post by: Griphook on March 13, 2016, 12:04:10 AM
Nope, no Hum on the brown voicing
Title: Re: MP1 Classic | Hum in Dist. Tube Voicing
Post by: MarshallJMP on March 13, 2016, 07:56:58 AM
Can you measure voltage?If so on the upper left corner there is a testpoint,here you should have 200V,can you check it?
Title: Re: MP1 Classic | Hum in Dist. Tube Voicing
Post by: Griphook on March 13, 2016, 01:32:37 PM
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?

Title: Re: MP1 Classic | Hum in Dist. Tube Voicing
Post by: MarshallJMP on March 13, 2016, 04:23:57 PM
Does the step down have an earth connection?
Title: Re: MP1 Classic | Hum in Dist. Tube Voicing
Post by: Griphook on March 13, 2016, 10:56:37 PM
No It's only 2 Pole, but a grounding issue would affect all Channel wouldnt it?
Title: Re: MP1 Classic | Hum in Dist. Tube Voicing
Post by: MarshallJMP on March 14, 2016, 09:02:30 AM
Depends,sometimes this gives problems,sometimes it doesn't.

Did you measure that voltage?
Title: Re: MP1 Classic | Hum in Dist. Tube Voicing
Post by: Griphook on March 14, 2016, 12:18:43 PM
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?

Title: Re: MP1 Classic | Hum in Dist. Tube Voicing
Post by: Griphook on March 14, 2016, 12:29:16 PM
Oh, alright. Found it :)

I measured from there to the chassis and got 205,8V
Title: Re: MP1 Classic | Hum in Dist. Tube Voicing
Post by: MarshallJMP on March 14, 2016, 02:13:38 PM
Mmm a bit too much.I will compare it with mine tomorrow.I'll let you know.
Title: Re: MP1 Classic | Hum in Dist. Tube Voicing
Post by: MarshallJMP on March 15, 2016, 08:45:47 AM
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%
Title: Re: MP1 Classic | Hum in Dist. Tube Voicing
Post by: Griphook on March 15, 2016, 02:22:08 PM
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:
Title: Re: MP1 Classic | Hum in Dist. Tube Voicing
Post by: MarshallJMP on March 15, 2016, 03:50:19 PM
Schematic;MP-1 Classic Power Supply & Peak Meters section B4

On the PCB,on the right side of C186 (3300µF16V).It's one of those 4 big caps near the transformer,the one closest to the front.
Title: Re: MP1 Classic | Hum in Dist. Tube Voicing
Post by: Griphook on March 16, 2016, 08:46:03 AM
Thnaks for that info.
I just measured both the R382 and also the AC coming from the stepdown Transformator.

R382 has got 503kOhms which is the maximum tolerance of that Resistore right? I first have to find a local dealer who has Resistors with this tolerance.

And just for Interest, I got abot 113V AC coming from the Power Supply. Is this too high or just right?


Are the Voicing Curcuits seperated from each other or is this R382 an "over all" Resistor? And is there anything else I can check, before I start soldering, so I can just do it all in once?
Title: Re: MP1 Classic | Hum in Dist. Tube Voicing
Post by: MarshallJMP on March 16, 2016, 08:50:07 AM
Did you desolder one side of the resistor?You have to do this to get an acurate reading.Else you measure through other circuits.

I think the standard US voltage is 117V??But 113 should be fine.
Title: Re: MP1 Classic | Hum in Dist. Tube Voicing
Post by: Griphook on March 16, 2016, 09:15:08 AM
No, didnt de/soldered anything. Okay, so my measurement is not helpful at all ^^

Wikipedia says 120V O.o
But the Stepdown Trafo only provides 110V.

EDIT: To get that higher volume of TP9 I believe the R382 has to be lower than 499k?
Although I dont thnik so, can too less Power (110V instead of 120) damage my curcuit?
Title: Re: MP1 Classic | Hum in Dist. Tube Voicing
Post by: MarshallJMP on March 16, 2016, 09:39:31 AM
Not helpfull well not correct, I think it's even a bit higher.

The higher the resistance the higher the voltage.

Nah a lower voltage won't hurt.
Title: Re: MP1 Classic | Hum in Dist. Tube Voicing
Post by: Griphook on July 16, 2017, 06:02:58 AM
Since there is another topic with my MP1-Classic, I thought, I just replace this one too.
Do you think, a 499k 5% would be ok, instead of using 1%?
Title: Re: MP1 Classic | Hum in Dist. Tube Voicing
Post by: MarshallJMP on July 16, 2017, 06:22:40 AM
Well try to get as close as 499k
Title: Re: MP1 Classic | Hum in Dist. Tube Voicing
Post by: Adafan on July 21, 2018, 04:23:24 AM

Hi Everybody,

I come on this post because I trhinking to have the same problem...
I have a whistling only on the tube distortion.
And this doesn't disapear when I roll off OD 1 and OD 2... But

I measure the 200V test point And I am 209 V. This seems to be High apparently.

Problem is that I a m a newby in electronic  :dunno:

Impossible to find the R382. How do you do to finding the "right" resistor?

If somebody have pics for me, that will be great...

Sorry for this request, but I don't understand how find the right resistor without pics or putt-off all the resistor...  :dunno: :dunno: :dunno:

Sebastien



PS : Sorry for my English

Title: Re: MP1 Classic | Hum in Dist. Tube Voicing
Post by: rnolan on July 21, 2018, 10:38:27 PM
Hey gibson2612, it's a bit blurry (see attached) but I'm pretty sure R382 is vertical mounted and next to (to the right) of C186 which is the  last big power capacitor in the group of 4 in the top right hand corner of the schematic (C193, C183, C182, C186).  So these are part of the power supply and near the power transformer. As MJMP said earlier  :thumb-up:
Schematic;MP-1 Classic Power Supply & Peak Meters section B4

On the PCB,on the right side of C186 (3300µF16V).It's one of those 4 big caps near the transformer,the one closest to the front.
Title: Re: MP1 Classic | Hum in Dist. Tube Voicing
Post by: Adafan on July 22, 2018, 11:20:58 AM
Yes, thanks my friend. Your document is absolutly what i need. I check this tomorow and coming to you ; ) ; ) ; )

Thanks again.

Seb
Title: Re: MP1 Classic | Hum in Dist. Tube Voicing
Post by: rnolan on July 23, 2018, 06:14:09 AM
No worries, I hope it sorts the issue for you.  Haven't heard if it worked for Griphook yet.
So you need a 499k ohm resistor with a tolerance of 1% (may be harder to get as allot of resistor tolerances are 5%)Tolerance = how much a resister can be up + or down - from it's stated value.  And ADA wouldn't have used a 1% if the didn't think it needed it.
Title: Re: MP1 Classic | Hum in Dist. Tube Voicing
Post by: Adafan on July 23, 2018, 08:06:22 AM

I check it today. And she give me 510/511 k... I going search one with 1% tolerance. But that seems to be hardly...
You think i will checking somewhat else???

Sebastien
Title: Re: MP1 Classic | Hum in Dist. Tube Voicing
Post by: rnolan on July 24, 2018, 03:17:16 AM
Did you desolder one side of the resistor?You have to do this to get an accurate reading. Else you measure through other circuits.

I think the standard US voltage is 117V??But 113 should be fine.
Hey gibson2612, that's a similar reading to the "incorrect" reading Griphook got.  As MJMP says, you have to desolder (and pull out of the PCB) one side of the resistor to get an accurate measurement (or desolder both ends and remove it, then measure).