MSR 16 capstan speed fault

[Guerrilla Studios]

So today I mounted a tape from last weeks session, hit play and the speed was all over the place. It’s like someone was turning the pitch control knob randomly speeding up and slowing down the capstan. But the vari function is turned off. The capstan suddenly has a mind of its own!
I’ve had a poke around but it’s pretty complex. There’s a capstan PCB and another small PCB on the motor itself. Then the wiring from the control surfaces goes to the main board and the capstan PCB. I have some suspicions but…

Has anyone encountered/solved this issue before?

cheers

Paul
Guerrilla Studios Manchester

 

[wkrbee]

Are you sure it's not a tension issue? Fluctuating takeup or supply tension can cause it. Verify the capstan motor is actually speeding up/slowing down by putting a mark on the pinch roller, holding up the right tension arm with no tape on the machine and pressing play, and see if the speed is changing by observing the tempo of the mark on the pinch roller/

 

[Guerrilla Studios]

Yeah its definitely the capstan unfortunately. Like you say I tried it without any tape just by running the capstan with the tensioner. Normally it's constant as soon as any tension is applied, but now it's all over the place.
I had a poke around and I'm suspicious of a transistor on the capstan servo pcb. It looks like its been hot and/or changed before. I'll have a crack at changing it I've got a replacement on order. I've also managed to source an entire replacement pcb as well.
Failing that my next stop would be the motor itself.

 

[Guerrilla Studios]

UPDATE: Resolved! The replacement PCB arrived this morning so I swapped it out and now the machine is running perfectly.

On a side note, the old PCB seems to have been modified at some point. There was a small wire soldered to the back which seems to be jumping pins between a couple of the semi conductors. When I first saw it I didn't know if it was a factory mod or not. As the PCB looks significantly older than the rest of the components in the machine. I figured maybe they used the PCB's from an older model and maybe modified them for this machine. However it is now clear that the mod has been carried out by someone post production. What the heck it's doing I don't know, or if it contributed to the problem at all. I've had the machine for years and it's been flawless up until now. So whatever it was doing worked. I'd like to find the root of the problem with the board so I can repair it and keep it as a spare. I'm seeing fewer parts available for these which is a concern given the machines complexity.