HELP! Clean/Distortion channels reversed!

[shredbaron]

Hey folks, have an issue after doing some work to one of my units - not sure what happened, but now the "Clean" tube patches are heavily distorted, and the "Dist" tube patches aren't - it's as if the clean/dist channels got reversed.

Any idea how I managed to do this and how to correct it? I did the noise mod, installed an MDRT and battery mod - nothing too fancy. Compared it to my other MP-1 and looks like everything is soldered in the right place.

Ideas?

thanks!

*used to be a member on the Depot many years ago before it crashed...the MP-1 I'm talking about hadn't been turned on since 2010!

[rnolan]

Hey shredbaron, good to have you back with us all  :thumb-up: , hopefully MJMP or others can help you with the issue. Of the cuff it sounds like something got reversed (signal go this way, or this way ? possibly in the voicing switching ?).
So question though:
Is SS voice still normal ? (ie issue is patches with clean tube voice selected in the patch are now routed to dist tubes voice and vis versa) ??

[MarshallJMP]

Did you desoldered the tubeboard?

[shredbaron]

Hi, thanks for the responses. Good to be back, when I was last on the site (before the crash) there had been a project for everyone to record some clips to be patched into one big song - just as that was underway, the site was lost - so disappointing, as it was an incredible treasure-trove of info. Glad to see some of the ol' regulars still here all these years later!

RNolan - yes - SS voice is fine, you described issue perfectly - patches with clean tube voice selected in the patch are now routed to dist tubes voice and vice versa.

MarshallJMP - yes, I had originally pulled the tubeboard, replaced all the filter caps etc. When I wasn't able to get any sound out of it (years ago) I packed it up, only finally got back to it the other day - seemed to me that one of the wires that ran from the tube-board to the mainboard wasn't soldered correctly, so I removed the old solder, redid it, and presto - had sound - but the reversal issue described above.

I haven't had it apart again to play around to try to fix it, I thought someone might have an idea of exactly what it could be before I start playing around and potentially cause myself more trouble

[MarshallJMP]

There is a wire that goes to the tubeboard called the G wire (see the pad's on the boards called G).On that pad you should have -15V when set to clean and 0V when set to dist.

[shredbaron]

Thanks, Marshall - I'll grab my multi-meter and take a look at in the next day or so (I'd do it tonight but have guests coming for dinner.)

I have a feeling I've reversed something, it seems like the most logical explanation. Appreciate your input (as always...kind of trippy to talk to someone who I bought stuff from 8+ years ago...did the MDRT, noise mods and bought some other parts from you back in 09/10)

I'll update once I've had a chance to take a crack at it - thanks again!

[MarshallJMP]

You get a lifetime tech support on it  so no worries :thumb-up:

Ok keep me updated.

[shredbaron]

There is a wire that goes to the tubeboard called the G wire (see the pad's on the boards called G).On that pad you should have -15V when set to clean and 0V when set to dist.

Well, this is interesting, but I'm not sure what it means (or how to fix it?)

Distortion I'm reading 0.14

Clean I'm at -0.48

Voltmeter set to DC volts at 20V scale.

Thoughts?

[MarshallJMP]

Measured to ground right?

Could be a bad fet or a busted cap.

[shredbaron]

Measured to ground right?

Could be a bad fet or a busted cap.

I put the ground pin of my multimeter on the chassis of the MP-1...should I have put it on the green wire ground pad on the PCB?

I'll try again and see if there's a different reading just in case I had a bad ground.

[MarshallJMP]

The chassis is fine.So could be C30 or Q5, or both.

[shredbaron]

The chassis is fine.So could be C30 or Q5, or both.

Thanks! I just ruled out C30 (replaced it with another 33uF cap I had in my parts bin) and I'm getting the same reading.

Hate to ask, but what does Q5 look like and where is it located? Is it one of the transistors on the tube board?

I've just spent half an hour looking at the schematic trying to figure it out and I'm lost. (I'm not an electronics guy by trade, just someone brave/stupid enough to play with a soldering iron )

EDIT: FOUND IT! found a layout file on the web, clearly labelled.

Looks like I will be ordering some parts from Mouser. Going to replace *all* the electrolytics that I didn't replace in 2010, as well as upgrading the LF347N in the EQ section.

Thanks so much for taking the time to reply, it's very much appreciated. Hopefully replacing Q5 does the trick!

George

[rnolan]

Hey George, excellent, if you found a better or more easy to read layout chart (that we don't already have in the schematics section), please post it  or link to it ?

[shredbaron]

Hey George, excellent, if you found a better or more easy to read layout chart (that we don't already have in the schematics section), please post it  or link to it ?

Found a pile of schematics here - the one I found useful was labelled "ADA-MP1-009-027-component-layout-1989.jpg"

this site also has a zip-file with all of the contents in the directory -

http://www.kylheku.com/mp1-files/mp1-schematics/

EDIT: at the bottom of this page are some schematics that seem to be a bit less blurry than others I've seen on the web:

http://www.lydian.ca/ada-mp-1-rebuild/

[MarshallJMP]

C30 is not a 33µF cap, it's only 100nF (small yellow cap)

Q5 is a 2SK364 N-channel FET (don't use anything else)