Revox A77 MkIV Dolby problem

[cgehansen]

Hi, I've picked up an old A77 MkIV dolby in poor condition. Apart from the usual problems the VU meters don't appear to work. I've swapped the record cards but that doesn't help and there doesn't appear to be anything mechanicaly wrong with them. The VU board is wired up correctly and record works (sort of). Is there something on the Dolby cards could be affecting the VU meters? Any help would be appreciated. Thanks, Christian

[Michael Maurice]

I’ve had some of the capacitors go short circuit on the Dolby boards, but AFAIK the VU meters are connected to the record boards.

Have you checked the wipers of the presets on the boards? The wipers have a nasty habit of disintegrating.

If they need replacing, the machine will need recalibration.

[cgehansen]

Thanks for the advice. I did manage to get hold of a good meter for testing and it worked when connected to the board so both of my meters are blown unfortunately. They mechanically look in good shape but the circuit is open so I assume the windings are broken.

[Michael Maurice]

Its odd that both have failed, have you done a continuity test on them out of circuit?

[cgehansen]

Yes I did check and there’s no continuity. Both red and blue wires are connected to the coil but there’s no continuity from one side of the coil to the other. I guess replacing them is the only option.

[Michael Maurice]

Yes replacing them is your only option, but you can get newly manufactured ones from Nagravox, though they're expensive. There should be plenty on ebay.