Trouble with Revox A77 speed control

I have a Revox A77 that has full voltage applied to the Capstan motor at all times. The 10v pp square wave is okay at the discriminator transformer but at Q209 (which is basically in series with the motor) the readings at the test points suggest that that transistor is "saturated" (their term). Getting test-lead access to individual components on the board seems impossible. I have changed Q209 as this was a failure on another machine. Does anyone have a suggestion short of removing the whole speed control board.

I only had one problem with an A77 varispeed and that was in the discriminator itself. The 1% caps appear to drift after many years even though they are  film types. . I replaced them and that fixed my problem.

Cheers

Ian

I don't know much about the A77, but to start with,  on all Studer and Revox machines, track down the X2 capacitors and replace them.  These are typically 0.1uF 250 volt, look like rectangular toffees and are used accross the motor driver transistors and transformers.

They cause a plethora of wierd and  obscure problems. 

Regards

Mike

I think removing the speed control board might be your best approach.

There is a thread on Tapeheads where it is suggested that the tants on the speed board might be the culprit.

http://www.tapeheads.net/showthread.php?t=63195

madswitcher said:

track down the X2 capacitors and replace them.  These are typically 0.1uF 250 volt, look like rectangular toffees and are used accross the motor driver transistors and transformers.

They cause a plethora of wierd and  obscure problems. 

Click to expand...

They may, but not on the capstan motor. They are surge arrestors on the spool motors and solenoids.

MagnetoSound said:

I think removing the speed control board might be your best approach.

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Do you mean doing without it, or removing it for troubleshooting? Certainly, the capstan motor can't work without its control board.

abbey road d enfer said:

Do you mean doing without it, or removing it for troubleshooting? Certainly, the capstan motor can't work without its control board.

Click to expand...

For troubleshooting!

MagnetoSound said:

For troubleshooting!

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OK. Troubleshooting the control board usually requires an oscilloscope.

I just thought the board should be removed and the components checked, given that a power-up test appeared problematic.

Bernard Buller said:

Getting test-lead access to individual components on the board seems impossible.
...
Does anyone have a suggestion short of removing the whole speed control board.

Click to expand...

I want to thank all who took the time to share their wisdom re this problem. The consensus of this and other groups is that the caps on the board need to be replaced. Due to the difficulty of getting the board out of the machine I have shelved this project for another time. I had chosen this A77 to repair because it had minimal head wear -- I'll just move on to another I have around and hope that the tape motion circuits are in better shape. Again THANKS to all.