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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 5th Apr 2020, 8:08 pm   #1
pentoad
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Default Revox A77 recording problem

I have a problem which has just suddenly occurred with my Revox A77 and was wondering if there was any member on the forum who maybe an expert on this tape machine who could point me in the right direction?
Maybe this could be a commonly occuring fault with this tape recorder?
I made a stereo recording this evening from my PC at the correct levels but on playback was barely perceptable on playback although audible in stereo, yet previous recordings playback fine. So it's a fault which occurs during the recording process but affects both channels.
So the erase is working but the recording process is weak.
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Old 6th Apr 2020, 11:23 am   #2
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Default Re: Revox A77 recording problem

I'm thinking it maybe the oscillator board or record relay board needs recapping with new electrolytics
I will carry out a few tests later

Last edited by pentoad; 6th Apr 2020 at 11:50 am.
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Old 6th Apr 2020, 2:16 pm   #3
TIMTAPE
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Default Re: Revox A77 recording problem

Is the tape path clean? On the A77 you need a dentists mirror to see it.
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Old 6th Apr 2020, 3:38 pm   #4
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Default Re: Revox A77 recording problem

Yes - try the obvious first - clean, clean and clean again. And someone I know (!) managed to wind the tape backwards with very low sound. Took me - I mean him - a long time to find that one!
Recapping is often seen as a 'cure-all' but usually paves the way for re-alignment on an A77 which you might not want to get involved with. Also if the presets are original they will obligingly fall to pieces even before you've dropped the proverbial hat.
Don't forget the RIFAs - they WILL need changing if original.
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Old 7th Apr 2020, 7:51 pm   #5
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Default Re: Revox A77 recording problem

I have cleaned the heads but problem persists. Doing a bit of research!
BTW Reefer Caps changed
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Old 7th Apr 2020, 8:10 pm   #6
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Default Re: Revox A77 recording problem

Is the issue the same on headphones output, line output to external amplifier and internal/external speakers (assuming the unit has the optional power amp boards) ?
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Old 7th Apr 2020, 11:05 pm   #7
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Default Re: Revox A77 recording problem

Why would it be any different between the various outputs, considering they all come from the same point?
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Old 7th Apr 2020, 11:19 pm   #8
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Default Re: Revox A77 recording problem

The plug in modules will likely have disintegrating pots (the crude open type) which may explain the loss of signal in your case. Replace with Piher types and you should be okay for another 40 years use!

If the pots are already changed and are okay, reseat the cards as I have had intermittent audio due to poor connections here.
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Old 8th Apr 2020, 8:24 am   #9
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Default Re: Revox A77 recording problem

Quote:
Originally Posted by Michael Maurice View Post
Why would it be any different between the various outputs, considering they all come from the same point?
Yes precis, if the playback fault is before the output of the driver amp stage of the Playback amp board, then yes one would expect to get the same results at all the outputs.

In this case it is not known/was not stated what output was being used, so it would be good to know if the fault symptom exists in all the outputs to better aid troubleshooting.

One scenario where it could help, is if the fault was experienced while using the internal or externally connected speakers, but good results obtained if headphones or line output used, then that would strongly point to a problem with the loudspeaker amplifier boards or much less likely a problem with the speakers or associated wiring/connections.

It may be academic here because it was stated by pentode that a good previous recording plays back fine, so suspected the recording process.

However I would always keep an open mind, sometimes fault fixes do not tie up with the presented fault symptoms.
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Old 8th Apr 2020, 8:44 am   #10
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Default Re: Revox A77 recording problem

"Yet previous recordings play back fine" in the original post eliminates everything from the playback head right through to all the outputs from consideration.

This does look like purely a recording issue though it could be the tape itself.

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Old 8th Apr 2020, 10:52 am   #11
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Default Re: Revox A77 recording problem

Could the tape in use have been twisted at some point, resulting in the head contacting the plastic surface instead of the oxide?
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Old 8th Apr 2020, 12:40 pm   #12
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Default Re: Revox A77 recording problem

If the erase is working it's hard to see what might be the problem causing both channels not to record unless switch contacts on both channels happen to be faulty, a functioning erase means that the record relay is kicking in on record, and that the record output should be disconnected from ground, anything like PSU rails would affect other functions so far as I can male out, I would double check the tape path again, failing that I would do some signal checking, not too much to check as there's not a lot after the VU meter take off unless, as Ben mentioned, both preset wiper contacts are duff.

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Old 8th Apr 2020, 2:57 pm   #13
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Default Re: Revox A77 recording problem

Quote:
Originally Posted by Welsh Anorak View Post
Recapping is often seen as a 'cure-all' but usually paves the way for re-alignment on an A77 which you might not want to get involved with.
I am generally against global re-capping in more 'modern' equipment but in the case of changing electrolytics, surely their function in most circuitry would generally not require a re-alignment?
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Old 8th Apr 2020, 3:50 pm   #14
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Default Re: Revox A77 recording problem

Quote:
Originally Posted by stevehertz View Post
Quote:
Originally Posted by Welsh Anorak View Post
Recapping is often seen as a 'cure-all' but usually paves the way for re-alignment on an A77 which you might not want to get involved with.
I am generally against global re-capping in more 'modern' equipment but in the case of changing electrolytics, surely their function in most circuitry would generally not require a re-alignment?
No but changing the presets would!
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Old 8th Apr 2020, 5:35 pm   #15
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Default Re: Revox A77 recording problem

For sure (and pretty obvious), but not related to my question/point.
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Old 8th Apr 2020, 5:58 pm   #16
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Default Re: Revox A77 recording problem

Recapping on its own would lead to the machine needing recapping, BUT those presets are very fragile and may not survive when the board is handled.

One other thing, if the fault occurs on both channels on both speeds, its highly unlikely that the presets are faulty as it would require all four of them to fail.
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Old 8th Apr 2020, 6:07 pm   #17
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Default Re: Revox A77 recording problem

DJ on pin 8 of the Osc transformer or defective print.....poor ground return from heads...if it's not the tape path then need to check levels.

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Old 12th Apr 2020, 4:13 pm   #18
pentoad
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Default Re: Revox A77 recording problem

I tested the relay (REL601) on the record relay board, pins 14 and 13 which appear to connect to the winding. I could not get any resistance reading on both of my DMMs.
The purpose of this board is to switch the oscillator on " softly " according to the manual.
Could this be the source of the problem?
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Old 12th Apr 2020, 4:31 pm   #19
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Default Re: Revox A77 recording problem

So far as I can make out an O/C record relay coil would mean no erase, no bias and no record signal to the heads.

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Old 17th Apr 2020, 3:37 pm   #20
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Default Revox A77 recording problem channel 1 low/poor

Earlier I posted about a recording problem with my Revox A77.
Since cleaning the heads it only appears on channel I, channel II is fine but channel I is very low when recording through the Aux input.
Monitoring the input and tape output through headphones I swopped the playback amplifiers around to see if the fault transfered to channel II which it didn't. Then I did the same with the record amplifiers with the same result.
I then recorded using a microphone inputs separately via the the two mic inputs on the front panel. Channel I appeared slightly lower in level than channel II but channel I meter's is broken so it's difficult to judge but nowhere near as significantly low as the Aux input.
I'm wondering if it's the input selector switch or maybe the the record preselector or am I barking up the wrong tree?
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