New Mp-1 Needs Some Love

[Moff40]

Just received a second Mp-1 from a fellow on a Faceb**k group.  This one needs some seious lovin'.  Compared to my other unit, this has very little output, and the OD1 indicator is flashing like crazy, even with the trimpot backed right off.  If I switch voicings, it gets muddy and muffled, and even switching back to the voicing it originally had, nothing changes.  I wonder if swapping the tubes or op-amp would help...

Any ideas for where a non-electronics guy could start?

[Moff40]

OK...  A follow-up.  Tried changing out the tubes for known-good JJs, and swapped out the op-amps.  OD1 clip light still comes on strong with the front panel OD1 set to 5.0

Will not change voicings properly - still muds out with any change, and won't go back intil I recall the preset.

SS drops out entirely if OD 1 is set at 10.0

Tried both the headphone out and FX loop send, no difference.

[Dante]

Man, I wish I could help ya, but I'm the same 'non electronic' guy...somebody in here will chime in and give you a rabbit hole to climb into, no worries!

[Moff40]

More follow-up, as I try ideas (that may be completely useless....):  I swapped the EEPROM and memory module beside it into my working MP-1, and they both work fine the fault is not there...

[MarshallJMP]

Hey, got your emails and mailed back to you.

[Moff40]

Another follow-up, in case it comes in handy for someone else...

I've been working with MJMP on this.  He noticed from the photos I sent to him that a previous owner had replaced some of the original yellow non-polar caps with polarized, and I saw that they had swapped a few opamps.  Thankfully the seller provided some of the opamps he had tried, and so I incurred a very minor expense to replace the caps and buy a 417 opamp.  Swapping these things did not cure my problem, but it made an improvement.

OD1 does nothing on any voicing or preset.

SS is basically broken.  It has very little signal, which I suspect is a residual "background" or "trickle" signal.  The chorus doesn't work, but the tone controls do.  Go figure.  Raising OD2 does little until 7.5, then the level goes DOWN, until 10.0, when it goes completely silent.  Master Gain does nothing.  I had forgotten that switching voices deaults the master gain to zero, but there was a very low signal passing through anyway, which is what I had percieved as getting "quiet and muddy" on all voicings and not changing when I went back to the original voicing - because they were all defaulted to zero.  Sometimes I'm a dope.  :)

On the clean tube voicing, OD2 does little until around 4.6, then incrementally increases as nearly normal to 6.5.  At 7.0, there's a significant volume jump.  The same behaviour is exhibited in tube dist, but that jump is heard as a jump in both volume and gain.

Overall output level is very low.

I've sourced and ordered Vactrols, which I hope will address the issue with OD1, and maybe the weird behaviour of OD2.  Not sure what to do about the SS voicing.

We'll get this going, but I have to say MJMP has the patience of a saint.  I've probably irritated the crap out of him with all my noob questions.

[MarshallJMP]

I don't get easely irritated  ;D

[Moff40]

Latest update...

We measured voltages on various legs of the LF412 JFET, pins 4 and 8 had the expected + 15vdc or -15vdc which was good, but between 5 and 6vdc on pin 2 when OD1 was adjusted.  It should be zero at all settings.

Next, we measured the voltages on the TC9176s.  Pin 2 voltages should decrease from 15vdc to about 5mv on pin 2 as OD1 is increased, and Pin 15 should increase from 5 mv up to 15v as OD2 is increased. Nope.  Pin 2 voltages started at 15vdc at "0" on OD1, and the rest of the settings measured 11v.  Pin 15 measured at 11vdc with OD2 at "0", and stayed there till "10.0", when it went to 15v.  I transplanted the TC9176s from the non-working unit to my working one, and ran the test again.  This time they passed, so they're good.

I have Vactrols on the way.  Crossing fingers!  :)

[Moff40]

Well...  New Vactrols installed, no change.  Voltages measured on the 9176 are the same.  SS Clean doesn't work, OD1 does nothing (it appears to be stuck at nearly closed).  I'm going to guess that, since OD1 is the "volume" control for SS, it not working is the cause for SS not working too.  I'm at a loss as to what might be causing the issue.  Hopefully MJMP has some more ideas...

[Moff40]

SOME SUCCESS!!!!  With MJMP's help, we measured resistance to ground from multiple specific points, and determined some components weren't grounding as they should.  Further troubleshooting showed there was a broken trace between pin 3 on U4 and the negative leg on the U5 Vactrol.  Used some fine hookup wire and ran a jumper between those points, and now everything tested now grounds properly.  Voltages that were funky on previous tests came back to within spec.  Audio tests have tube voicings working properly, but SS Clean is still dead.  Woohoo!  Thanks MJMP!!!  I at least owe you a bottle of your favourite libation!  Two if we get SS going too!   :)  :headbanger:

[rnolan]

Excellent news  , hopefully the SS issue(s) will be solved soon.

[MarshallJMP]

Well then we need to fix that SS clean also  ;D

Now my guess is that you have no ground on the upper circuitry too. Send you a mail with a pic of a trace too follow and some testpoints. We will get there sooner or later. And remember, you did all the work, I just gave you some hints, so well done 

[Moff40]

WOOHOO!!!!  IT'S ALIVE!!!  MJMP gave me a few more things to check, swapping ICs between the working unit and the unit under repair.  All the chips are good.  I reported that back to him by email, and while awaiting a reply, decided to check a few more traces, and found that R75 went to ground as it should, but the other side did NOT have connection to pin 3 of U8.  Closer inspection revealed a missing solder pad under U8.  Installed another jumper, et voila!  I have SS Clean now too!!!!

I cannot state this enough:  Thanks to MJMP for his patience and guidance.  Name you poison for libation!  :)   :headbanger: :canadian-flag-banana-smiley-e

[rnolan]

Well that is great news  .  It begs the question though, how come so many traces were bad  .  Anyway so glad you got it working.  And, as always,  MJMP is such a treasure.

[Moff40]

A previous owner had socketed a number of ICs, and had destroyed or inadvetently removed some of the solder pads in the process.  Not hard to do.