Revox B77 take up on play/rec - UK Vintage Radio Repair and Restoration Discussion Forum

Michael Maurice · 13th May 2023, 10:08 am

I was servicing a Revox B77 for someone yesterday. It needed mechanical work to the brakes and a good clean, but the main problem was in play or record.

When play is selected on these machines, the take up spool receives full power for around ½ second to help in accelerating the tape after the pinch roller has contacted the capstan. This wasn't happening.

This pulse is achieved through C7. I had a spare transport board, so I tried that, but the results were the same.

C7 doesn't have a value next to it, but a 10uF was fitted on both my spare board and the original board

I checked all the wiring through the reel size switch but everything checked out okay.

I then replaced C7 first with a 22uF which improved matters but didn't fully cure it, and then with a 100uF which did.

Does anyone know why this should have occurred in the first place?

TIMTAPE · 13th May 2023, 3:13 pm

No idea Michael. How did the old cap measure?

Cruisin Marine · 13th May 2023, 6:40 pm

I don't know the cct. in question at all, but maybe it is meant to be 100uF?

You can make a good guess by using:
t (time in SECONDS) = R (in Megaohms) times C (in uF). This is based on a voltage drop to 63%.
If there is an associated resistor with the capacitor, then you may be able to make some sort of approximation.

Trigon. · 13th May 2023, 7:12 pm

Quote:

Originally Posted by Michael Maurice

C7 doesn't have a value next to it, but a 10uF was fitted on both my spare board and the original board

That is the correct value for C7.

Quote:

Originally Posted by Michael Maurice

I checked all the wiring through the reel size switch but everything checked out okay.

Since the problem was independent of which transport board you fitted it would indeed seem the reel size switch is suspect. Long shot, but did you check the values of R*/R**as well (470R, 8K2 for a standard speed machine) and whether those values are also seen at the board connector?

Cheers

ms660 · 13th May 2023, 7:15 pm

It say's C7 is 10uF in the manual I'm looking at.

EDIT: Post crossed.

Lawrence.

Michael Maurice · 13th May 2023, 9:43 pm

I checked the reel size switch and the resistors are correct. This is the HS version.

Trigon. · 14th May 2023, 2:18 pm

Second thoughts -deleted.

John_BS · 30th May 2023, 9:01 pm

C7 is +5V decoupling? Perhaps have a look at comps around and including Q16 which form the 5V reg. ??
Edit: seem to be s/n variations: . Q18 in another version: BD139

ben · 30th May 2023, 9:46 pm

Interesting. My B77 has the opposite problem - despite the pinch roller arm being nice and free, it tends to 'snatch' the tape!

13th May 2023, 10:08 am	#1
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,223	Revox B77 take up on play/rec I was servicing a Revox B77 for someone yesterday. It needed mechanical work to the brakes and a good clean, but the main problem was in play or record. When play is selected on these machines, the take up spool receives full power for around ½ second to help in accelerating the tape after the pinch roller has contacted the capstan. This wasn't happening. This pulse is achieved through C7. I had a spare transport board, so I tried that, but the results were the same. C7 doesn't have a value next to it, but a 10uF was fitted on both my spare board and the original board I checked all the wiring through the reel size switch but everything checked out okay. I then replaced C7 first with a 22uF which improved matters but didn't fully cure it, and then with a 100uF which did. Does anyone know why this should have occurred in the first place? __________________ Forum Moderator http://www.michaelmauricerepairs.co.uk/

13th May 2023, 3:13 pm	#2
TIMTAPE Octode Join Date: Jul 2006 Location: Perth, Western Australia Posts: 1,969	Re: Revox B77 take up on play/rec No idea Michael. How did the old cap measure?

13th May 2023, 6:40 pm	#3
Cruisin Marine Heptode Join Date: Jul 2018 Location: Worthing, West Sussex, UK. Posts: 992	Re: Revox B77 take up on play/rec I don't know the cct. in question at all, but maybe it is meant to be 100uF? You can make a good guess by using: t (time in SECONDS) = R (in Megaohms) times C (in uF). This is based on a voltage drop to 63%. If there is an associated resistor with the capacitor, then you may be able to make some sort of approximation. __________________ "Behind every crowd, there's a silver Moonshine"

13th May 2023, 7:15 pm	#5
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Revox B77 take up on play/rec It say's C7 is 10uF in the manual I'm looking at. EDIT: Post crossed. Lawrence.

13th May 2023, 9:43 pm	#6
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,223	Re: Revox B77 take up on play/rec I checked the reel size switch and the resistors are correct. This is the HS version. __________________ Forum Moderator http://www.michaelmauricerepairs.co.uk/

14th May 2023, 2:18 pm	#7
Trigon. Hexode Join Date: Jun 2018 Location: Buckinghamshire, UK. Posts: 387	Re: Revox B77 take up on play/rec Second thoughts -deleted.

30th May 2023, 9:01 pm	#8
John_BS Octode Join Date: Dec 2008 Location: Wincanton, Somerset, UK. Posts: 1,780	Re: Revox B77 take up on play/rec C7 is +5V decoupling? Perhaps have a look at comps around and including Q16 which form the 5V reg. ?? Edit: seem to be s/n variations: . Q18 in another version: BD139 Last edited by John_BS; 30th May 2023 at 9:09 pm.

30th May 2023, 9:46 pm	#9
ben Dekatron Join Date: Aug 2006 Location: Madrid, Spain / Wirral, UK Posts: 7,498	Re: Revox B77 take up on play/rec Interesting. My B77 has the opposite problem - despite the pinch roller arm being nice and free, it tends to 'snatch' the tape! __________________ Regards, Ben.