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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 11th Dec 2017, 3:09 pm   #1
Michael Maurice
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Default Revox B77 capstan speed issue

I am servicing a Revox B77. One issue I have is with the speed control, if I switch the machine on with 3¾ selected, the motor runs at that speed. Switch to 7½ and the capstan goes to the higher speed.

BUT

If I switch the machine to 7½ or switch the machine on with 7½ selected then switch to 3¾ then the capstan remains at 7½.

I have replaced the tantalum capacitors but haven't made any progress. Other than board swapping is there anything else that would cause it?
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Old 11th Dec 2017, 7:24 pm   #2
TonyDuell
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Default Re: Revox B77 capstan speed issue

I've just looked at the service manual (not having such a machine) and the speed selector circuit is quite interesting.

The tacho head signal is divided by 2 (discrete transistor flip-flop) For the high speed, one output is used, looking for the -ve going edge. For the low speed, both outputs are used thus giving twice as many tacho pulses per revolution. To keep them coming at the same speed, the motor must run half as fast.

So that's where I'd look first. Particularly at the transistor controlled by the speed switch and the diode gates associated with it.
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Old 12th Dec 2017, 1:42 am   #3
Michael Maurice
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Default Re: Revox B77 capstan speed issue

I have thoroughly checked the transistor and diodes and they are functioning as they should.

I'll try a new 555 timer chip, otherwise, I'll replace the board.
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Old 12th Dec 2017, 3:08 am   #4
TIMTAPE
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Default Re: Revox B77 capstan speed issue

I've never had this problem. Could it be a poorly contacting speed switch?
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Old 12th Dec 2017, 11:42 am   #5
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Default Re: Revox B77 capstan speed issue

Unlikely Tim as there is a clean 21V - 0V switch to the board and the switching transistor is operating.
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Old 13th Dec 2017, 12:26 am   #6
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Default Re: Revox B77 capstan speed issue

I'm afraid I chickened out on this one by changing the board.
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