Revox B77 capstan speed control fault - UK Vintage Radio Repair and Restoration Discussion Forum

Michael Maurice · 17th Aug 2014, 11:32 pm

I am working on a revox B77 Mk2 and this has got me really foxed.

When switched on the fast speed (7½ IPS) capstan reaches speed, though not the correct speed, its somewhat fast, if you switch to the slower speed, the capstan slows slightly but certainly not at half speed.

If you then stop the motor it starts again at nearly the correct speed. Ditto if you switch on at the correct speed.

I've changed the 470nF Rifa capacitor and then the uA739 op amp.

Has anyone any ideas apart from changing the control card?

Radio Wrangler · 18th Aug 2014, 12:00 am

Is the variable reluctance tachogenerator close enough to the bell of the capstan motor?

If the discriminator sees fewer pulses, then it thinks the motor is slow and compensates. You can also get an accumulation of ferrous dust on the pole pieces.

I assume the B77 uses the same frequency discriminator and the bipolar transistor in the middle of a bridge rectifier to govern the oomph to the motor?

A shorted transistor or bridge wouldn't notice a change of speed selection, it would just be flat-out.

David

David

Michael Maurice · 18th Aug 2014, 8:19 am

Hi David, the tacho generator is working correctly, neither the bridge rectifier nor the transistor is short circuit. The motor is not flat out, its just not at the correct speed and doesn't change down, though changes up if you start it at the slow speed.

DOFFERY · 19th Aug 2014, 11:22 am

You may well be aware of the following Michael but heregoes.

at 3.75 ips you should get between 35/50mv on P12/P13 Brown/blue on the power board.

Speed calibration, if tacho head in correct position & the above is measured OK, speed control measured on counter, TP1 & ground on speed control unit should read 800 Hz at both speeds within 1%.

Colin.

KeithsTV · 19th Aug 2014, 1:16 pm

I had a speed issue with my A77 which uses a similar type of circuit. It would start on 7.5ips but not on 3.75ips but would run at the lower speed if it had been started at the higher speed. I eventually traced it to the tacho amplifier which had a low level oscillation fooling the discriminator into thinking the speed was too high on 3.75ips and shutting down the speed regulator transistor. The oscillation frequency was lower than the tacho output on the higher speed so it would start when 7.5ips was selected. Once the motor was running the tacho amplifier output was higher than the low level oscillation allowing it to run at 3.75ips.

I never solved the problem of the amplifier in spite of changing several of the components. Instead I built an alternative amplifier based on a dual op amp which has worked faultlessly.

So it's worth checking the output of the tacho amplifier for any noise or oscillation.

Keith

Michael Maurice · 19th Aug 2014, 7:31 pm

Sorted

I blanket changed all the tantalums and the electrolytics and it worked. I've no idea which one was the cause, it seemed easier to replace them in bulk.

Colin: I did know that, duly set up and running ok.

DOFFERY · 20th Aug 2014, 7:15 am

Glad you sorted it Michael.

Colin.

Gomes_74 · 26th Aug 2015, 11:22 pm

I am new on this forum and would like to say Hello for everyone

. I read your posts from years and always found answers for technical issues but not this time

I have a similar problem with my B77 HS, the capstan motor spins too fast and it doesn't react to speed changing (7.5 or 15 ips).

First of all I will let you know what has been done. I got this recorder with a dead capstan motor but all other functions work as they should. I replaced C1, C4 and C5 on the speed control board and the motor started to spin, but too fast (almost twice as fast as it should).

According to the service manual I checked:
1. Supply voltages, regulated reads 21.38 V (on P13 audio interconnection board), unregulated supply reads 27.8 V (on R34 transport control board)
2. Tacho head measuring on P12 and P13 transport control board reads 38 mV
3. All tantalums and electrolytics on the transport control board and speed control board have been replaced with new ones.
4. IC01 (NE555) and IC02 (SN76131N) on speed control board replaced with new ones.
5. All transistors, resistors and and rectifier bridge on SCB have been checked
6. R14 works as it should

Digital counter reads 1370 Hz on TP1 and TP5 (should be 800 Hz), adjusting R14 doesn't make any difference, neither changing speed between 7.5 and 15 ips.

When I change speed to slow (7.5 ips) signal 21 V shows on SCB resistor R8, when selected 15 ips R8 reads 0 V.

On the collector of the Q2 (MJ 411) or "+" of rectifier D01 (B 250 C800) reads variable voltage between 10 and 12 V

, Q2 has been removed and measured (seems to be fine, no short or open)

Revox B77 Schematic Diagram

Transport control board - page 14
Speed control board - page 24

I will be very appreciate for any clues.
Thanks a lot!

17th Aug 2014, 11:32 pm	#1
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,694	Revox B77 capstan speed control fault I am working on a revox B77 Mk2 and this has got me really foxed. When switched on the fast speed (7½ IPS) capstan reaches speed, though not the correct speed, its somewhat fast, if you switch to the slower speed, the capstan slows slightly but certainly not at half speed. If you then stop the motor it starts again at nearly the correct speed. Ditto if you switch on at the correct speed. I've changed the 470nF Rifa capacitor and then the uA739 op amp. Has anyone any ideas apart from changing the control card? __________________ Forum Moderator www.michaelmauricerepairs.co.uk

18th Aug 2014, 12:00 am	#2
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 24,941	Re: Revox B77 capstan speed control fault Is the variable reluctance tachogenerator close enough to the bell of the capstan motor? If the discriminator sees fewer pulses, then it thinks the motor is slow and compensates. You can also get an accumulation of ferrous dust on the pole pieces. I assume the B77 uses the same frequency discriminator and the bipolar transistor in the middle of a bridge rectifier to govern the oomph to the motor? A shorted transistor or bridge wouldn't notice a change of speed selection, it would just be flat-out. David David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

18th Aug 2014, 8:19 am	#3
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,694	Re: Revox B77 capstan speed control fault Hi David, the tacho generator is working correctly, neither the bridge rectifier nor the transistor is short circuit. The motor is not flat out, its just not at the correct speed and doesn't change down, though changes up if you start it at the slow speed. __________________ Forum Moderator www.michaelmauricerepairs.co.uk

19th Aug 2014, 11:22 am	#4
DOFFERY Retired Dormant Member Join Date: Aug 2004 Location: Blackpool, Lancashire, UK. Posts: 1,488	Re: Revox B77 capstan speed control fault You may well be aware of the following Michael but heregoes. at 3.75 ips you should get between 35/50mv on P12/P13 Brown/blue on the power board. Speed calibration, if tacho head in correct position & the above is measured OK, speed control measured on counter, TP1 & ground on speed control unit should read 800 Hz at both speeds within 1%. Colin.

19th Aug 2014, 1:16 pm	#5
KeithsTV Nonode Join Date: Aug 2007 Location: South Bradford, West Yorkshire, UK. Posts: 2,623	Re: Revox B77 capstan speed control fault I had a speed issue with my A77 which uses a similar type of circuit. It would start on 7.5ips but not on 3.75ips but would run at the lower speed if it had been started at the higher speed. I eventually traced it to the tacho amplifier which had a low level oscillation fooling the discriminator into thinking the speed was too high on 3.75ips and shutting down the speed regulator transistor. The oscillation frequency was lower than the tacho output on the higher speed so it would start when 7.5ips was selected. Once the motor was running the tacho amplifier output was higher than the low level oscillation allowing it to run at 3.75ips. I never solved the problem of the amplifier in spite of changing several of the components. Instead I built an alternative amplifier based on a dual op amp which has worked faultlessly. So it's worth checking the output of the tacho amplifier for any noise or oscillation. Keith

19th Aug 2014, 7:31 pm	#6
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,694	Re: Revox B77 capstan speed control fault Sorted I blanket changed all the tantalums and the electrolytics and it worked. I've no idea which one was the cause, it seemed easier to replace them in bulk. Colin: I did know that, duly set up and running ok. __________________ Forum Moderator www.michaelmauricerepairs.co.uk

20th Aug 2014, 7:15 am	#7
DOFFERY Retired Dormant Member Join Date: Aug 2004 Location: Blackpool, Lancashire, UK. Posts: 1,488	Re: Revox B77 capstan speed control fault Glad you sorted it Michael. Colin.

26th Aug 2015, 11:22 pm	#8
Gomes_74 Retired Dormant Member Join Date: Aug 2015 Location: Livingston, West Lothian, UK. Posts: 1	Re: Revox B77 capstan speed control fault I am new on this forum and would like to say Hello for everyone . I read your posts from years and always found answers for technical issues but not this time I have a similar problem with my B77 HS, the capstan motor spins too fast and it doesn't react to speed changing (7.5 or 15 ips). First of all I will let you know what has been done. I got this recorder with a dead capstan motor but all other functions work as they should. I replaced C1, C4 and C5 on the speed control board and the motor started to spin, but too fast (almost twice as fast as it should). According to the service manual I checked: 1. Supply voltages, regulated reads 21.38 V (on P13 audio interconnection board), unregulated supply reads 27.8 V (on R34 transport control board) 2. Tacho head measuring on P12 and P13 transport control board reads 38 mV 3. All tantalums and electrolytics on the transport control board and speed control board have been replaced with new ones. 4. IC01 (NE555) and IC02 (SN76131N) on speed control board replaced with new ones. 5. All transistors, resistors and and rectifier bridge on SCB have been checked 6. R14 works as it should Digital counter reads 1370 Hz on TP1 and TP5 (should be 800 Hz), adjusting R14 doesn't make any difference, neither changing speed between 7.5 and 15 ips. When I change speed to slow (7.5 ips) signal 21 V shows on SCB resistor R8, when selected 15 ips R8 reads 0 V. On the collector of the Q2 (MJ 411) or "+" of rectifier D01 (B 250 C800) reads variable voltage between 10 and 12 V , Q2 has been removed and measured (seems to be fine, no short or open) Revox B77 Schematic Diagram Transport control board - page 14 Speed control board - page 24 I will be very appreciate for any clues. Thanks a lot!