[gaetan8888]

Hello

I have a Revox A77 MK3, sr: G170460, two tracks, with 7 1/2 and 15 ips speeds. I did not used it since some times.

I have a problem, wen I turn it on, even wen I did not push any recording buttons, and without any input signal, the vu-meter are topping to the maximum and still topping to the maximum until I turn my Revox off.

And wen I record music or sine signals (7 1/2 and 15 ips ), I did push all the buttons for recording, but there is no sound on the recorded tape (Scotch 250)

All the motors run good.

I've look in the service manual but I did not find a solution in that manual.

Any hints or solutions ?

Thank you

Bye

Gaetan

[FullRangeMan]

If the heads are OK, would be nice to read the Service Manual output circuit logic to some light on the prob.
By what you said seems a prob at the output signal section, not on power suppy or motors.

[gaetan8888]

Hello

I have take out the bias board and the vu-meter are still topping to the maximum.

I have take out the playback amp board and the vu-meter are still topping to the maximum.

But wen I have take out the 2 record amp board, the vu-meter was no more topping to the maximum.

So, I think it's the 2 record amp board who are the problems.

Which parts on the record amp boards who could cause the problems ?

Thank you

Bye

Gaetan

[FullRangeMan]

Surely you find the prob.
Do a visual inspection on this PCB and if there not sing of a burn part, as smoke or melt part, you will need to use a multimeter for search the damaged part.
Unfortunatelly the PCB must be in working position and powered to proceed this test.

Due to electrical risk it must be perfomed by a tech.

P.S.:>  Just remember there is some short-circuit tests you can do in this PCN without it be working/power on.

[gaetan8888]

Hello

There is lot of tantalum capacitors on the 2 record amp board, could it be the defect parts ?

I will do some short-circuit tests anyway.

I do mostly diy amps and cd players mods, so I have a scope, dmm, etc,   but I don't usualy do analog tape recorder repair.

Thank

Bye

Gaetan

[FullRangeMan]

Hello

There is lot of tantalum capacitors on the 2 record amp board, could it be the defect parts ?

I do mostly diy amps, so I have a scope, dmm, etc,   but I don't usualy do analog tape recorder repair.

Thank

Bye

Gaetan

The capacitors may be gilty, as they are prone to leak or lost storage capacity with age.
Good you are a repair man.

[gaetan8888]

Hello

I say to a friend that I would fix it for him this week-end, so I was a bit short in times and axious not finding the problem fast enough, if I can fix it there is still lot of adjustments to do to this old revox.

Thank

Bye

Gaetan

[FullRangeMan]

If a part is damaged you will need a replacement one, which may take time to order and arrive.
Good Luck

[gaetan8888]

Hello

I think I have found the problems. I've tests some capacitors with my capacitance meter.

First, the two recording amp C509 electrolytic capacitors, who connect to the vue meters, are short, so dc are going to the two vue meters.

And the record relay circuit C602 electrolytic capacitors are dry and do have near 0 uf, so the relay was not working since C602 need to be full to switch on Q601 and the relay.

I will do more tests, but it seem that I will need to replace all electrolytic and tatalum capacitors in my Revox, just in case they are dry or defects.

Thank

Bye

Gaetan