B77 HS help please

[Linnovice]

Hi all. I’m in the process of ‘trying’ to renovate a Revox B77 HS machine. It’s been stored in a loft for many years and was absolutely filthy. I’ve spent many hours stripping and cleaning and it’s now much improved. During the course of the cleaning I have removed all the boards and thoroughly cleaned them, replaced rusty and broken trimmers, all Fraco caps and all Rifa caps.
This morning I refitted the last board and fired it up for the first time. All seems to be ok with the exception of the captan motor. When I apply power the motor spins up but after a few seconds stops. It’s quite free and the bearings are fine so it’s electrical not mechanical.
Can some knowledgable member please point me in the right direction to rectify this.
Mike

[Ted Kendall]

When it is running, do you hear the tacho whine? Does it stabilise before the motor goes off?

[Linnovice]

No, there is no audible noise. Capstan rotates three or four times then stops. I’ve checked the wiring and all seems in order. Spooling motors spin ok, haven’t checked the audio circuits yet but all seems good apart from the lamps in the meters. I’m wondering if it’s the motor run caps.

[Ted Kendall]

Could be - also check that the capstan board is getting 21V and that the motor control transistor and rectifier are OK.

[Michael Maurice]

Did you check/replace the trimmer on the motor speed control board?

[Linnovice]

Re the trimmer. No, it looked to be ok so I didn’t change it. I’m waiting for a delivery from Cricklewood Electronics which has the 470k trimmers. When it arrives (today) I will change it then.

[Linnovice]

Well done Michael, great suggestion. I’ve just got around to getting back in the workshop. Changed the trimmer, now all seems fine. I can get on with the rest of it now. Thank you.