Let's Get Technical > Troubleshooting Problems

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

(1/3) > >>

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 :thumb-up: , 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:

--- Quote from: MarshallJMP on October 18, 2022, 01:48:01 PM ---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.

--- End quote ---

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

Tranny is not blue though…it’s this one:

Navigation

[0] Message Index

[#] Next page