Revox A77 Capstan not turning

[cgehansen]

Hi, I've just got hold of a REvox A77 Mk2 but unfortunately the capstan motor isn't spinning. I've tested the motor itself and it has no issues and the 3 wires feeding it are getting 150v. However there is no voltage on the red or yellow connection points on the speed control board. Could that be the issue? The reel motors work fine, it's just the capstan that isn't running. Any help would be appreciated.
Thanks
Christian

[ms660]

No 21 volts supply = no go for the capstan motor, make sure the board connections are good and the fuse from the 24 volt winding is ok, if ok then check the 21 volt regulator circuit.

Lawrence.

[cgehansen]

Thanks. The fuses are good but no 21 v supply. From where do I measure the regulator circuit? I'm struggling a bit with the service manual.

[Radio Wrangler]

Is the end-of-tape bulb lit?

It runs off of the 21v supply and acts as a quick check.

David

[cgehansen]

No that's dead and the VU meters don't jump when I turn the machine on. The only thing working is the power light and the reel motors.

[ms660]

Originally Posted by cgehansen

Thanks. The fuses are good but no 21 v supply. From where do I measure the regulator circuit? I'm struggling a bit with the service manual.

I've only got a schematic for the MK1 but I'm assuming the circuit is much the same for the MK2, if that's the case then the 21 volt regulator circuit is Q101 and Q102 and the components associated with them, the copy of the schematic I have is a bit fuzzy but it looks like the voltages given are 24 volts AC into the bridge rectifier, and with respect to ground (0v) -ve 6 volts on the emitter of Q102 and +ve 12 volts on the emitter of Q101, might be worth measuring the voltage at those points, if that's any help.

Lawrence.

[cgehansen]

Thanks for that. I just replaced the end of tape bulb which I hadn't checked properly. It looks like the wires on the outside of the bulb had shorted. Now everything turns on as it should - VU meters, capstan and the EOT bulb. I have a Mk3 with the same symptoms so hopefully that's be an easy fix too.

[cgehansen]

...although having said the capstan did stop again momentarily when i was testing it and the light dimmed. I turned it off and on again and it’s working so something odd is going on.

[Michael Maurice]

I’ve had the 2.5K preset pot fail on these giving no or very low volts. It’s usually permanent though, not intermittent.

[cgehansen]

Thanks. I’ll take a look at that. It was working fine for a while but now the pinch roller won’t stay engaged. It’s as though there is no tape in the machine or no dummy remote plug. Could that be a problem with the EOT circuit?

[Radio Wrangler]

You'll probably find al the pots, if still original, on the threshold of crumbling.

David

[cgehansen]

Quite a few have been replaced but some are original. The EOT sensor reads around 11kOhm when the bulb is lit so from reading the service manual that sounds like it's an issue.

[Welsh Anorak]

Check the 220uF capacitors on the switching PCB (by the vlume control). If the one on the right shorts there's no capstan rotation but no other ill effects.

[cgehansen]

It died again and a little pressure on the 2.5Kpot on the power supply board got it working. I've disconnected the EOT sensor and the relays now stay engaged but when it's connected they don't.

[ms660]

Originally Posted by cgehansen

I've disconnected the EOT sensor and the relays now stay engaged but when it's connected they don't.

That suggests that the EOT sensor has gone low resistance or there's a short/leakage across the connections to it, or for whatever reason there's to much light getting to it.

Also with the EOT sensor disconnected measure the voltage on the collector of Q111 (A77 schematic ref.) just to make sure there's nothing suspect with that transistor, the voltage on its collector should be somewhere around 20 volts.


Lawrence.

[cgehansen]

Thanks. I'll take a look at the transistor later and check for shorts. I measured again and the resistance on the sensor now 7k Ohm when lit (not 11k Ohm which I previously measured) and 16k Ohm when dark.

[ms660]

Originally Posted by cgehansen

Thanks. I'll take a look at the transistor later and check for shorts. I measured again and the resistance on the sensor now 7k Ohm when lit (not 11k Ohm which I previously measured) and 16k Ohm when dark.

16k dark resistance is to low and the resistance with light is to high, according to the A77 service info I'm looking at it should be less than 2k with light and more than 20k in the dark.

Lawrence.

[cgehansen]

That’s what I thought. If I connect the green wires the resistance is 3kohm across the EOT terminals with the machine off.

[cgehansen]

I've checked the voltage on the collector of Q111 and it's 23v with the EOT sensor disconnected, 1v with it connected and the lamp lit and 23v volt with the lamp covered. I'm not sure what that means so any further advice would be greatly appreciated.

[ms660]

Originally Posted by cgehansen

I've checked the voltage on the collector of Q111 and it's 23v with the EOT sensor disconnected, 1v with it connected and the lamp lit and 23v volt with the lamp covered. I'm not sure what that means so any further advice would be greatly appreciated.

You're confusing me now.....so when you press the play button with the EOT sensor connected and a tape installed do the solenoids work?

The circuit, in its basic form is quite simple, the EOT circuit is in effect a switch in series with the stop control contacts, Q112 acts as the switch, Q112 is turned hard on when the collector of Q111 is high, the play relay A would normally be activated when the play button is pressed, when you let go of the play button Relay A is kept activated via its holding contacts a12 & a8, the stop contacts and the switch action of Q112, the operation of the solenoids is via relay A's contacts a11 & a7, if that's any help.

Lawrence.