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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 17th Nov 2018, 12:10 pm   #21
Raving Loony
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Default Re: Revox A77 Stops Playing (Transport Fault)

No worries. I know one or two of the symbols etc but it's all Chinese to me. I can follow some of it on the diagram but couldn't locate the parts on the unit itself. I did want to train when I left school but colours stopped me.
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Old 17th Nov 2018, 1:32 pm   #22
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Default Re: Revox A77 Stops Playing (Transport Fault)

Youi have actually picked a good time to do yours because I now have 4 in a row to repair and service so ideal for pics ..lol .
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Old 17th Nov 2018, 3:13 pm   #23
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Default Re: Revox A77 Stops Playing (Transport Fault)

Quote:
Originally Posted by Ted Kendall View Post
As to the fault itself, have we established what is meant by "stop"? Does the pinch roller drop out or does the capstan stop with the pinch still engaged?
All motors continue to work. The fault is the same as pressing the stop button but without being able to restart.
Sorry I misread above. Yes pinch wheel does disengage

Last edited by Raving Loony; 17th Nov 2018 at 3:29 pm.
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Old 17th Nov 2018, 3:17 pm   #24
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Default Re: Revox A77 Stops Playing (Transport Fault)

Quote:
Originally Posted by NorfolkDaveUK View Post
Youi have actually picked a good time to do yours because I now have 4 in a row to repair and service so ideal for pics ..lol .
Thanks for the pic.
I have checked the voltages and the red and yellow have 21volts
The White, Violets and black all have 18.5 volts.
This is before any transport error.

Last edited by Raving Loony; 17th Nov 2018 at 3:17 pm. Reason: spelling error
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Old 17th Nov 2018, 3:34 pm   #25
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Default Re: Revox A77 Stops Playing (Transport Fault)

The only part of the power supply that could be responsible is the unregulated 24V supply. It may be that, the only way to tell is to switch the machine on at which point it should measure 28V (Its high because there's no load on it.) When you engage any function that voltage drops because the solenoids and relays place a load on it. I've had the that on a B77 but it happened when the machine was cold.

In this case the capacitor in question is C104 which is the large capacitor.

However capacitors usually perform better when they're hot, not cold.

Look at the attached picture of the power board, hook your meter with the red lead to any tag that I've marked red, connect your black lead to any tge that I've marked blue.

Switch on and check the voltage should be around 27V. dont worry if its a volt or two down (or up). Load a tape, press play and leave it till the tape stops. Measure the voltage. Then operate any mode and check that voltage, if it goes below 24V then replace the cap.

You have to be very careful if you proceed to replace the cap as it may involve removing the transformer to do so.
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Old 17th Nov 2018, 3:42 pm   #26
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Default Re: Revox A77 Stops Playing (Transport Fault)

Many Thanks
Does the machine need to be off and cooled before I Start the check. I can't find the attached picture. Am looking forward to further investigating.
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Old 17th Nov 2018, 3:51 pm   #27
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Default Re: Revox A77 Stops Playing (Transport Fault)

Sorry forgot to post with picture, picture now added, machine should be cooled down but in working mode before tests are made.
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Old 17th Nov 2018, 4:18 pm   #28
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Default Re: Revox A77 Stops Playing (Transport Fault)

Just powered on the machine and the voltage is reading 18.3 on the tags you suggested
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Old 17th Nov 2018, 4:42 pm   #29
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Default Re: Revox A77 Stops Playing (Transport Fault)

The Voltages on the black / violets / white is way too low . this is the issue . That should be 27v in parked mode (stop) .

You now need to have a look at the AC and make sure that that`s ok ...so ....change you meter to AC and measure the AC coming from the transformer .....on mine (Mk4) they are just behind the connectors you just measured the DC on ....the are just little dots on the board . they have 2 black wires and 2 blue wires connected to them on the other side of the board . Tell me what you are reading from those please . probe the blues , then the blacks .,...i.e. red on one black and black on the other black ,....then repeat for the blue wires . You only really need to test the blues , they are the ones we are interested in but its good practice to check both . Is your transformer getting hot by any chance ?
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Old 17th Nov 2018, 4:58 pm   #30
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Default Re: Revox A77 Stops Playing (Transport Fault)

I'm not so confident on this one. I'm a bit unsure of which wires you are referring to and not sure if the ends of my cheap meter are good enough to get into awkward spaces safely. If you can help with a more accurate location of the wires in question I can work out if I can access them safely
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Old 17th Nov 2018, 5:03 pm   #31
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Default Re: Revox A77 Stops Playing (Transport Fault)

The voltage is low so dont worry , its about 30ish volts ac not mains bud ...ok ...have a look where you checked all those voltages before ...peek over the top of those and you will see them

here you go , they are marked black and blue on my board

Blues should read about 21vac
blacks should read about 24vac
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Old 17th Nov 2018, 5:20 pm   #32
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Default Re: Revox A77 Stops Playing (Transport Fault)

Finally got there.

Black = 23.6
Blue = 21.9

Machine on but not playing

Last edited by Raving Loony; 17th Nov 2018 at 5:23 pm. Reason: Spelling
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Old 17th Nov 2018, 5:24 pm   #33
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Default Re: Revox A77 Stops Playing (Transport Fault)

OK that's good news for the transformer, it's putting out what it should.

So just check for me one more time that the voltage on the violet violet black white supply is 18.5 volts when in STOP please (back to DC).
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Old 17th Nov 2018, 5:29 pm   #34
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Default Re: Revox A77 Stops Playing (Transport Fault)

Voltage on the violet violet black white supply is 18.3V.
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Old 17th Nov 2018, 5:31 pm   #35
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OK so clearly there is an issue there so what we need to do now is see if there is a fault with the supply or whether something else somewhere else is causing this fault. Unplug the black white violet and violet and check those terminals again. See what you get on the connectors with the wires removed.

Be careful not to short anything with the wires flapping around, there's no juice flowing though them of course, just watch what they are touching.

Last edited by NorfolkDaveUK; 17th Nov 2018 at 5:41 pm.
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Old 17th Nov 2018, 5:50 pm   #36
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Default Re: Revox A77 Stops Playing (Transport Fault)

OK I get 19.1 volts
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Old 17th Nov 2018, 5:53 pm   #37
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Default Re: Revox A77 Stops Playing (Transport Fault)

OK ...so the fault is on the power supply board its self ....congrats you just made your first diagnosis ...there are now only 2 possibilities .....

1) the bridge rectifer is borked
2) the 2500Uf cap is leaking like a seive....

My money is on the cap ....but we will check the rectifier now .....

OK this is a little tricky so bear with me ok ...
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Old 17th Nov 2018, 5:56 pm   #38
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Default Re: Revox A77 Stops Playing (Transport Fault)

Actually I`m not sure its a good idea for you to check the bridge rectifier , they are tricky to get too and its more than likely going to be the cap . So what I would suggest you do . Is go back to my very first post ...and recap the power supply board ...

If you WANT to measure the rectifier , you measure the AC going in , and then the DC coming out . If the DC coming out is LOW then thats the issue here . If not its the cap ...You measure it as in the pic ....there should be symbols on the rectifier its self .wae are interested in the DC mainly . we know what the AC is ....
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Old 17th Nov 2018, 6:07 pm   #39
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Default Re: Revox A77 Stops Playing (Transport Fault)

Quote:
Actually I`m not sure its a good idea for you to check the bridge rectifier , they are tricky to get too and its more than likely going to be the cap . So what I would suggest you do . Is go back to my very first post ...and recap the power supply board .....
Have enjoyed what I have done so far and couldn't have done any of it without the very kind help of yourself and all the others who have given their valuable time and assistance.
so my next question is how difficult is it to recap the power supply board and what do I need to purchase. Any info on this would be very much appreciated.
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Old 17th Nov 2018, 6:09 pm   #40
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Default Re: Revox A77 Stops Playing (Transport Fault)

There is a significant possibility that a bad capacitor takes out part of the bridge rectifier, or that a failure of a diode in the bridge rectifier damages the capacitor. I wouldn't replace either without checking the other.

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