Revox tape machine one channel not recording

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I'm having trouble with a B77 Revox where one channel is not recording. Erasing is fine, input before the record is fine, just will not record. I searched around a bit and found that people generally have trouble with the oscillator relays. So I tried to spray these with a contact cleaner. This worked for a short while but now the situation is back. I suspect the relays are the problem but how does one go about cleaning them? The contacts are very small and look rather delicate. I have put a piece of paper in between to get out some of the gunk, but it appears to need a heavier cleaning.
Any help would be appreciated...
Hi again, thanks for your help. It turns out not to be the relay after all. I switched the two around and the right channel is still out.
I am beginning to suspect there may be something wrong with the playback amp/head. I put a different tape on the machine and only the left side plays.
Is it possible that there could be a problem with one side of the head and not the other? Or it is probably more likely a trouble with the repro electronics?
Hello Hugo,

Well the switches on the front include the tape monitor and the record enable. The tape/input monitor switch affects both sides of the inputs L and R so I think that can be eliminated as an option. The R record enable is lit and I see similar signal on the L and R relays so I think this is also not an option.

I have tried initially to clean and wiggle the switches to no avail.

I appreciate your help if you think of something else please let me know.
Of course I have a copy already thank you. My point is that I have a signal on both relays when in record mode which means that a signal is then transmitted to the record head according to the schematic. So unless there is a problem with one side of the record head which I find highly unlikely the problem would lie in the reproduce amplifier.
As well, when I recorded a signal on the left channel then flipped the tape, the signal was not playing back on the right side.

Am I misinterpreting something here?
Thanks again for your reply
Joined 2003
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Indeed, most likely the playback circuit is the troublemaker.
It should be possible to trace the signal while playing a tape and compare left and right playback channels. Or try to inject a signal or sinewave (low amplitude) just after the first head amp or as close to the head as possible.

Joined 2004
Paid Member
I had terrible trouble with all the trim pots in the A77s I've owned. Had to reaplace all of them. Don't remember that problem in a B77, but they were a lot newer at the time.

As Hugo says, see if you can inject a signal a various places along the playback path. Even if it's just hum from the tip of a small screwdriver. Good luck - report your findings!
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