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Revox B710 left capstan spinning like crazy!

R

renault5

New Member
Hello all!

I was "given" a Revox B710 a couple of years ago and now I want to refurbish it. The problem is: when I put the power on, the left capstan starts spinning REALLY fast!

The unit will still rewind and fast forward but as soon as I hit play, the tape gets mangled all around the left capstan. I suspect a bad transistor on the board behind the motors or on the capstan board.

Anyone experienced this issue or can help me?

Thanks!

Guillaume
 
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Something more complicated is happening. The B710 has a protection circuit that should prevent the head-block being engaged unless both capstans are PLL locked. That clearly isn't working, so I might suspect the shift register ic.
However , all electrolytic capacitors must be replaced before you look anywhere else!
 
I had the same problem on a B795 turntable. I know that's not a cassette deck, but IIRC, the motor drive principle is the same: Power goes to the motor, servo signal returns to the motor control and dictates if power should be increased or decreased.
If the servo signal isn't coming back or isn't being fed to the comparator circuit, the drive "logic" thinks the motor isn't starting so it's feeding more power to the windings. In my case it was a matter of a broken servo track pcb. It could be a nicked wire or a fault in the servo signal processing.It was causing the direct drive motor to run at 300rpm or so.
But, as always, when troubleshooting Revox equipment from that era showing weird behavior, you should observe rule #2: Replace all golden FRAKO caps.

PS
Rule #1: remove and replace the RIFA surge suppressor across the AC input
Rule #2: replace all yellow FRAKO caps
Rule #3: if you still have weird issues, replace all remaining electrolytics, especially blue Philips
Rule #4: to reduce risk, replace all blue tantalum capacitors (never had a problem with one of them going short but when they do, they create great troubleshooting puzzles)
Rule $5: only use PDP61 lubricant in capstans and motors.
 
What Steerpike said. If it’s a Revox, all electrolytic and suppression caps have to be replaced first. No two ways around that.
 
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