V1.38 gain drop/loss just seconds after startup / warmup (solved)

[Systematic Chaos]

So I have this 1.38 top switch MP-1 (rev D main board) that had this strange issue of dropping loosing gain just seconds after power on / warmup. Not going from 10 to 0 but fading from 10 to 4...to give an impression.
Only happened on the Tube channels, s.s. worked like a charm.
I had done the noise mod, battery mod and the rear jack conversion to it, put in new jacks and pots, basically stuff that I had already done numerous times to a bunch of MP-1s.
First checking the obvious known issues I checked D16 again, checked the input jacks, the pots, my soldering, checked the tubes (even put them in another MP-1 to make sure they work). Problem remained.
I took out the tube board and gave a complete one-over. New Belton tube sockets, all metalfilm resistors, higher/better quality caps, new diodes, complete Super Stock mod.
Double and triple checked all solder joints
Wasn't the input jack, wasn't the pots wasn't D16, wasn't the IC nor the trimpot I changed with the noise mod, wasn't the Anode Resistors on the tube board....
Hell, I was short before hitting up MJMP, printing a DHL label and shipping that thing off to Belgium.
Anyway, while shaking my head and doing some more chopstick poking in that sucker while having a looped track running in the input I accidentally shorted two lugs of one of the two Vactrols (VTL5C4/2) and all the glorious gain was there.
Must have happened that this Vactrol crapped out. Now those VTLs are harder to come by nowadays, especially the high qual Vactec ones (while doing a full restoration of a Boogie MkIV I had to search high and low to find 24 VTL5C9 a couple years ago) ...and those come NOS at quite some steep pricepoint.
Back then you could find XVive branded ones for cheap but those weren't sealed properly so I had to apply my wife's black nail polish and plasti-dip to make them work.
Apparently XVive has somewhat upped their quality and I just ordered two new VTL5C4/2 for this MP-1. For the time being I just soldered the lugs together...new VTLs to be delivered by the end of the week.
BTW, this is a 110v unit and I'm looking for an og 230v PT. I have an MDRT in my other MP-1 but there's something about this old/lower specd og tranny....I kinda dig the squishiness in the sound. Dunno, compared to the MDRT which makes it all the more in-your-face, this one more or less ,,oozes" if that makes any sense....

Here's some pics of the innards (yellow circle is the crapped out VTL):

[rnolan]

Hey SC, good you tripped over the problem , very tidy work BTW.  As suggested in my other reply, (samtheman185) needs to swap out his 230v Classic transformer (which is the same as MP-1 tranny) for a 110v, if he hasn't bought one yet from MJMP, maybe you can just do a swap with him?

[MarshallJMP]

So what you did here is actually setting OD2 to 10.0 by shorting the vactrol led.Don't you have too much gain now? BTW D16 only works for the other vactrol, for this vactrol it is D7. Keep me updated when you change out the vactrol, probably it is the vactrol but it could be something else.

[MarshallJMP]

BTW does it have a blue transformer in it?

[Systematic Chaos]

So what you did here is actually setting OD2 to 10.0 by shorting the vactrol led.Don't you have too much gain now? BTW D16 only works for the other vactrol, for this vactrol it is D7. Keep me updated when you change out the vactrol, probably it is the vactrol but it could be something else.

That's weird because even though shorting the VTLs lugs OD2 works 
I'll check D7 tomorrow then

Tranny is not blue though...it's this one:

[MarshallJMP]

Is the core of transformer 21 or 13.5mm thick? Can you measure the voltage over C131 (3300µF/16V)?

This vactrol controls OD2, so if you short it, the led won't work so the OD2 can't be changed unless something really weird is going on by shorting it.

[Systematic Chaos]

Is the core of transformer 21 or 13.5mm thick? Can you measure the voltage over C131 (3300µF/16V)?

The XFMR core is 13.5mm. Voltage over C131 (unit is noise modded so it's a 4700µF/16V cap) clocks in at 10.8VDC

This vactrol controls OD2, so if you short it, the led won't work so the OD2 can't be changed unless something really weird is going on by shorting it.

I just checked that again. Both OD1 and OD2 work as they should. If I short those same two lugs on the other Vactrol (U5) it turns silent (like OD1 set to 0).

Banzai is usually a tad bit slow with shipping and Reichelt (which is a 5min drive from my doorstep) doesn't have any of those VTLs. Once I get the VTLs from Banzai I'll change both U3 and U5 for new ones. Hell, I might even step by step shotgun that whole section and rebuild it with new parts...slow is smooth and smooth is fast, y'know ;-)


This MP-1 actually sounds real good after the noise mod and the higher gain S.S. I even put new metalfilm resistors in the EQ section and implemented parts from the 3.666 mod to the EQ. Treble and Mid optimization excluding the trimpots. The MP-1 core sound is all there but this (subjectively) took down the noise even more and the EQ seems even more "tonally pleasing" to me. Just also waiting for some WIMA MKS caps for the EQ and swap out those greenies....     

[MarshallJMP]

On U5 that is normal and I'm sure if you short it OD1 doesn't work, or does it? I'm going to try it with one of my mp-1's to see if I get a similar result but I find it very strange that if you short the second vactrol OD2 still works like it should.

Banzai is a good store, they have a lot of things and have good prices but they are OOOHHHHH so slow, still waiting for a transformer I ordered 4 months ago.
That's nice you leave near Reichelt, I order also from them from time to time. Can you buy directly from them at their place?

[MarshallJMP]

As for the transformer, you have an early model "blue transformer", these are different from the later models. They came in a blue isolation tape around the windings for 110V and red for the 220V models. The difference is that the voltage for the 5V regulators and the tube heaters is higher than the later ones, the later ones only have between 8 and 9V over C131 (heater voltage) and these have between 10.5 and 12V (depending on the wall voltage)

[Systematic Chaos]

Ok,good info! So I'm looking for a ,,red" og 220v transformer. How does that one translate (in terms of tap voltages) to the ADA MP-1 XFMR schematic?
http://www.marshalljmpmodshop.net/ADA-MP1-220V-transformer.pdf
If nobody has one lying around I'll might have one custom wound....

Reichelt really comes in handy for me if I need some parts (resistors, caps,....) on short notice. Just order online for shop pickup, wait ~30min and pick it up there.
Another good source is Tube-Town here in Deutschland...not only is their house-brand of tubes really good (plus all other tube brands at a very competitive price point) including V1 selection/testing/matching but they also stock a wide variety of electronics parts and the shipping/turnaround is pretty quick (ordered Silver Micas yesterday, got them delivered today)

[Systematic Chaos]

The VTL5C4/2 in U3 was the culprit. Got some new XVive VTLs delivered yesterday and put a new one in U3 (XVive has indeed up'ed their quality as these ones now look really properly sealed as opposed to the ones I got 4 years ago....).
OD2 is back on line and works as it should. Gain and Saturation for days...

[rnolan]

Excellent  :whoohoo!: