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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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9th Sep 2020, 7:13 pm | #21 |
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Re: Revox A77 Help
It looks to be on the servo board in the picture right where it is closest to the bell of the spool motor. A green wire and a yellow wire with a short rubber sleeve on them go just over it. There is something dirty round there.
My A77 is of this era with the pot core transformer in the frequency discriminator. It seems less troublesome than the integrated circuit version David
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9th Sep 2020, 7:18 pm | #22 | |
Triode
Join Date: Sep 2020
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Re: Revox A77 Help
Quote:
I'm not familiar with cap specs so I don't know what X Class type means. I see it's a 0.47mF cap. Could you provide me with a make a model number? I'll likely order from Digi-Key. |
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9th Sep 2020, 7:36 pm | #23 |
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Re: Revox A77 Help
Some observations:
The pilot lights, including the record lights are fed from the unregulated 24V, the 21V rail feeds the End Of Tape lamp on the head block. If there is a short on the 21V rail, the pilot lights will remain lit but the EOT lamp will be extinguished or very dim. What I've found is that as these machines are now 50 years old, all the small electrolytic capacitors will need replacing. When I service these, I remove the servo board completely and replace all the small capacitors, they wont have burst, or look tired, but they will be. You could test them if you have an ESR meter, but its cheaper and easier to replace them all. There are a number of 125uF capacitors on the main board, they can go short causing similar problems. Replace all the 'Rifa' caps including the ones on the transport board (The one with the relays) with X class polypropylene types rated at 275V. |
9th Sep 2020, 7:47 pm | #24 |
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Re: Revox A77 Help
Couldn't navigate the digikey site but here's the type you need, X2 Class 0.47uF (470 nF):
https://uk.farnell.com/kemet/phe840m...ial/dp/1572160 Other makes and suppliers are available. EDIT: Post crossed. Lawrence. |
9th Sep 2020, 7:59 pm | #25 |
Triode
Join Date: Sep 2020
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Re: Revox A77 Help
Thanks Lawrence! I found the same PN on Digi-Key.
Is there a complete list somewhere I can reference on which caps need replacing? Might as well buy everything I need in one shot I guess. |
9th Sep 2020, 8:31 pm | #26 |
Dekatron
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Re: Revox A77 Help
To replace all the old capacitors in your machine would take a lot of doing, I wouldn't recommend it for a beginner.
However I would order four of those 0.47uF X2 Class capacitors so that when you're ready you can replace the three on tape drive control board (the F board) the circuit refs. are.....C113, C114 and C115. You could replace the electrolytic capacitors on the servo board (the E board) if you want but replace them one at a time and check your work as you replace them because electrolytic capacitors are polarized. The electrolytic capacitors on the E board are C204, C209, C210, C211 and C212 (per the all transistor schematic) the 25uF and 250uF ones can be replaced with 22uF and 220uF, the voltage ratings for them aren't given in the info I have but in the absence of that info I would go for a voltage rating that's the nearest to 50 volts. Lawrence. |
9th Sep 2020, 8:49 pm | #27 | |
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Re: Revox A77 Help
Quote:
David |
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9th Sep 2020, 8:53 pm | #28 | |
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Re: Revox A77 Help
Quote:
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9th Sep 2020, 8:57 pm | #30 | |
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Re: Revox A77 Help
Quote:
David |
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9th Sep 2020, 9:03 pm | #31 |
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Re: Revox A77 Help
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9th Sep 2020, 9:05 pm | #32 | |
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Re: Revox A77 Help
Quote:
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9th Sep 2020, 9:08 pm | #33 |
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Re: Revox A77 Help
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9th Sep 2020, 9:18 pm | #34 |
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Re: Revox A77 Help
We don't know for sure yet, but bear in mind that it's the bridge rectifiers load current that dictates the current through the motor windings, the symptoms were that the motor starts for a short moment and then stops.
Lawrence. |
9th Sep 2020, 9:33 pm | #35 |
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Re: Revox A77 Help
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9th Sep 2020, 9:55 pm | #36 | |
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Re: Revox A77 Help
Quote:
C204 = 25uF 25 volts C209 = 250uF 3 volts C210 = 25uF 25 volts C211 = 10uF 35 volts C212 = 250uF 25 volts |
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9th Sep 2020, 10:32 pm | #37 |
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Re: Revox A77 Help
The Rifa cap that appears to have blown, is a suppression capacitor connected across the output transistor.
If the motor starts for a short while then stops, it means the transistor is being turned fully off. You could short its collector to ground and see if the motor starts to spin (at very high speed), assuming it does then it rules out the motor and the run capacitor. There were a number of versions of this board including all transistor ones. My advice is change the capacitors, use good quality, Nichicon or Panasonic types rated at 105° then test the machine. You can use the nearest preferred value: 25uF use 22uF 250uf use 220uF, 10uF is a preferred value and easily obtainable. |
9th Sep 2020, 10:42 pm | #38 | |
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Re: Revox A77 Help
Quote:
The other one you also will come across is class Y which is an altogether tighter safety class for EMC filter capacitors intended to go between lin or neutral to ground or to some touchable floating parts. These have to be designed to fail open circuit at all costs because any short-circuit failure becomes extremely dangerous. Most of these capacitors are paper based and so are susceptible to failure from moisture ingress, and the cases used by Rifa have proven to crack or craze over periods of 10 years or so. The idea is that if over-voltaged and the dielectric punctured, enough heat is generated to melt-back the metalised electrodes, leaving an insulated barrier around the puncture. David
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9th Sep 2020, 10:47 pm | #39 |
Octode
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Re: Revox A77 Help
I don't have the details to hand, but I recall fixing a fault many years ago with similar symptoms, on my A77 which has the FM disciminator type speed control.
It was caused by a leaky (both electrically and physically) capacitor in the discriminator. It was passing DC to the detector which responded as if the motor was over speed and stopped it from turning. When switching the machine on the capstan would give a little kick, but wouldn't spin. I think it was a Tantalum type in a small white plastic can, replaced with a modern low-leakage aluminium electrolytic. |
9th Sep 2020, 11:22 pm | #40 |
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Re: Revox A77 Help
Good input, C204 is the capacitor that leads directly into the discriminator coil.
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