Revox A77 Help

[Radio Wrangler]

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

[sergiop]

Originally Posted by ms660

That's good, now replace the capacitor that's top left in the photo, the one with the transparent (ish) case, if it's the one that's in the all transistor schematic it will be C213 which is connected between the collector of the motor driver transistor (the big transistor that's mounted on the heatsink) and ground, replace it with an X Class type.

Lawrence.

Thanks Lawrence. Is there any way to check this cap needs replacing before doing it? (I know these Rifa caps need replacing but I was hoping to find the root cause before replacing anything if possible).

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.

[Michael Maurice]

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.

[ms660]

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.

[sergiop]

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.

[ms660]

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.

[DMcMahon]

Originally Posted by Radio Wrangler

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.

David

Yes can see it now, certainly looks suspect, well spotted. I think this is the 4th 0.47uF RIFA of the 4 referenced in Post 9, the other 3 being on the Relay/Tape Drive board.

David

[DMcMahon]

Originally Posted by ms660

That's good, now replace the capacitor that's top left in the photo, the one with the transparent (ish) case, if it's the one that's in the all transistor schematic it will be C213 which is connected between the collector of the motor driver transistor (the big transistor that's mounted on the heatsink) and ground, replace it with an X Class type.

Lawrence.

Well if C213 has failed (shorted) that would certainly explain the capstan motor issue.

[Michael Maurice]

Although expensive, you could try Nagravox They have a good reputation and a number of parts are manufactured especially for them.

[DMcMahon]

Originally Posted by Michael Maurice

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.

Yes agreed, can now see that lamp LA101 which I thought was the Power on lamp is the EOT lamp.

David

[ms660]

Originally Posted by DMcMahon

Well if C213 has failed (shorted) that would certainly explain the capstan motor issue.

If it was shorted the motor would be running flat out.

Lawrence.

[DMcMahon]

Originally Posted by Michael Maurice

Although expensive, you could try Nagravox They have a good reputation and a number of parts are manufactured especially for them.

The capacitor upgrade kit also includes the motor run capacitors which are another potentially weak area on old machines.

[DMcMahon]

Originally Posted by ms660

If it was shorted the motor would be running flat out.

Lawrence.

OK so what sort of failure mode do you think could cause the problem symptom ?

[ms660]

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.

[DMcMahon]

Originally Posted by sergiop

How can I ensure I have the right schematic for my model?

This is a guess but I wonder if the relevant 1.077.xxx board number may be written on the boards, for example 1.077.725 for the all transistor Servo/Speed Control board.

David

[DMcMahon]

Originally Posted by ms660

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.

Values for the all transistor board.

C204 = 25uF 25 volts
C209 = 250uF 3 volts
C210 = 25uF 25 volts
C211 = 10uF 35 volts
C212 = 250uF 25 volts

[Michael Maurice]

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.

[Radio Wrangler]

Originally Posted by sergiop

I'm not familiar with cap specs so I don't know what X Class type means

Class X means that it is a capacitor specifically designed to be connected across the mains, between line and neutral, principally for parts to do EMC filtering. To stop RF noise getting out, or for that matter, getting in. They have to be self-extinguishing and self-healing because it's pretty certain that they will get flashed-over internally with the almost limitless transients occasionally seen on mains networks.

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

[m0cemdave]

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.

[DMcMahon]

Good input, C204 is the capacitor that leads directly into the discriminator coil.