Revox A77 Help

[sergiop]

Hello from Canada!

I've been poking around this forum for a couple of days and get the impression a lot of you know what you're doing

My aunt just gave me her Revox A77 (I believe it's MK1) and a few tapes (she was going to toss it out!). Super excited, brought it home, downloaded the user manual and service manual. Put a tape on it (following the instructions), press play...nothing.

Some notes:

1. The remote plug is installed and intact (I confirmed pins 1 and 2 are shorted)
2. I made sure the pre-tape was not under the optical sensor.
3. When I press play, the button won't stay engaged and the machine buzzes (solenoid) while trying to move the reels. Afraid to hold it for too long.
4. FF and RW work.
5. With no tapes installed and something to block the optical tape-end sensor, the play button will hold and the reels will spin.
6. So, being a mechanical engineer, the next step was to open it up and hit the web (where i was very quickly overwhelmed with information and quickly realized I was in way over my head )
7. Everything inside appears to be plugged in correctly. No obvious signs of damaged components or overheating. I work with circuit cards day-to-day so I have a general idea of what I'm looking for.
8. I've read A LOT about original caps needing to be replaced before they blow up but I'm still unsure about which ones specifically need to be replaced and I've read that I should find the root-cause before replacing these so as to not introduce other issues.

THE ACTUAL PROBLEM:

With the cover off and switching the power from off to any of the reel speed settings (not from speed to speed, only from the off position first) I noticed the capstan will spin momentarily and then stop. I cannot get it to spin again and I suspect that's why it won't run any tape. I'm guessing the motor is fine if it's able to run at startup but that something in the power or control circuitry is not working.

My goal right now is to just get this thing running so I can hear what it sounds like and decide on how I might use (I'm an amateur music producer). I'd love to fully restore this thing one day but that will have to wait for my kids to leave me alone, my wife to leave me alone and more disposable income.

Happy to provide photos or videos to help with the debug. I can order parts online (like a big boy!) and have access to IPC grade soldering equipment and trained personnel (who I can bribe with beer to fix my stuff).

Anyone here want to help a Mechy from Canada fix his A77...eh?

Thanks in advance,
Sergio

[ms660]

The capstan motor is servo controlled, the relevant board in the schematic is the one labelled E.

The power for the motor is supplied from the 130 volt winding on the mains transformer via a bridge rectifier (D203) the current flowing through the bridge (and thus the motor) is controlled by transistor Q209 which is controlled by the rest of the servo circuit on that board, there's a tacho input to the board from from the capstan motor, the supply voltage for the E board should be approx. 21 volts.

First I would make sure that all the relevant connections are good to the E board, eg: the 21 volts supply, the tacho input and the AC supply to the bridge.

Lawrence.

[Welsh Anorak]

On the function board (the one which the modules plug in to) there are two large electrolytic capacitors. The one on the right as viewed from the back can short stalling the capstan motor. Worth replacing the two.

[DMcMahon]

I think the capacitors Glyn refers to are C306 & C307 on the Switch Board (Function board) which in the Service manual are listed as 250uF 25 volt electrolytics.

These are across the +21 volt DC supply and if dead short the front panel power on pilot lamp would not illuminate.

David

[sergiop]

Thanks David and Glyn. I've located these but the pilot lamp illuminates. Does this imply that these caps are okay?

[sergiop]

Originally Posted by ms660

The capstan motor is servo controlled, the relevant board in the schematic is the one labeled E.

The power for the motor is supplied from the 130 volt winding on the mains transformer via a bridge rectifier (D203) the current flowing through the bridge (and thus the motor) is controlled by transistor Q209 which is controlled by the rest of the servo circuit on that board, there's a tacho input to the board from from the capstan motor, the B+ supply voltage for the E board should be approx. 21 volts.

First I would make sure that all the relevant connections are good to the E board, eg: the 21 volts supply, the tacho input and the AC supply to the bridge.

Lawrence.

Thanks Lawrence. I'm sorry but you lost me a bit. I've located D203 but that board only lists up to Q205 on my schematic (I can't find Q209).

Also, I'm not sure how to find or verify "B+" at 21V

Your help is appreciated. I'm just not very familiar as a mechanical engineering with reading and verifying schematics.

[ms660]

No problem, the set of schematics I was working from showed only the fully transistorized version of the E board, there's another version that uses integrated circuits and transistors, the bridge loading transistor being circuit designation Q205, I now have a copy of that schematic.

You can check the "B+" voltage on that board by measuring the voltage across capacitor C221, the voltage across that should be approx. 21 volts DC.

B+ is just another way of saying the DC supply voltage.

Lawrence.

[sergiop]

Okay. I found C221 and Q205 in the schematic. The problem is that when I go look at the physical board it's different from the schematic I have. Perhaps that other schematic you referenced is the right one. Here is a pic of the board (if that's helpful at all). How can I ensure I have the right schematic for my model?

[Welsh Anorak]

Not 100% sure, but ISTR the front panel light does illuminate if C307 shorts. I went round in circles the first time I had this, expecting a fault in the regulator circuit. Anyway, it's worth replacing these two capacitors while the machine's apart.
Though not relevant to your fault the four 0.47uF RIFA capacitors on the relay board need to be replaced with class X 275v components. You can see them in your photo - they are the semi-transparent ones, and when they fail (and they will) there's clouds of evil smelling smoke!

[DMcMahon]

Originally Posted by sergiop

Thanks David and Glyn. I've located these but the pilot lamp illuminates. Does this imply that these caps are okay?

No not 100%. If I have followed the circuit correctly then if one or both of these capacitors were fully shorted down, then the lamp would not illuminate. But the the lamp being illuminated does not mean the capacitors are 100% good. Measurement of the +21 volt supply will somewhat help to prove that the capacitors are not really bad.

Electrolytic capacitors can and do degrade with age, sometimes severely where they will can cause real problems, there are various electrical measurements of the capacitor that can be done to quantify further but for the average person easier just to renew them if any doubt (as a preventative measure for future reliability). So as Glyn suggests would be a good time to replace them while machine is open, also the 0.47uF Rifa capacitors that are renown for failing sooner or later.

David

[ms660]

Originally Posted by sergiop

Okay. I found C221 and Q205 in the schematic. The problem is that when I go look at the physical board it's different from the schematic I have. Perhaps that other schematic you referenced is the right one. Here is a pic of the board (if that's helpful at all). How can I ensure I have the right schematic for my model?

I've not seen a servo board like the one in your photo, but so far as I can make out it's fitted with a discriminator transformer, that being the case it should be the all transistor version and not the one with IC's and transistors, below is a link to the schematics I have access to:

https://www.vintageshifi.com/repertoire-pdf/Revox.php

If you open that and scroll down to Revox-A77-Service-Manual-4 opening that PDF gives the schematics for the two servo circuit versions that I'm aware of.

Lawrence.

[DMcMahon]

I forgot to mention that a visual check of electrolytic capacitors is always worthwhile doing to ensure that their cases/cans have not ruptured and leaked electrolyte or just having bulging/swollen cases/bodies.

[Radio Wrangler]

If you look on that photo, close to the Papst spool motor bell, just on the far side of the little rubber sleeve holding a few wires together, there seems to be mess on the board. Some capacitor has spilled its guts and right next to the mess looks to be a Rifa.

I think this would be a good area to start investigations in if the capstan motor isn't working.

David

[DMcMahon]

A photo of the front (assembled) and/or its serial number would help to establish its Mk. #

[DMcMahon]

You must have good eyesight David, I am struggling to see it, I am not doubting you for one minute, certainly needs investigating.

Sergio - when replacing the 0.47uF Rifa capacitors with 0.47uF 275 VAC X2 (safety spec) make sure they are Metal Polypropylene.

David

[sergiop]

I've been studying the photo and can't see what you're referring to Dave (sorry). Can you identify it in the photo?

Lawrence I've downloaded Manual 4 but I'm lost now on where to check the 21V - We said C221 originally but that's no longer on this schematic. I'm guessing C212 should show 21V?

Serial Number is in the photo I think actually: No. G 39939

Thanks

[ms660]

Yes C212 on the all transistor schematic.

The capacitor being referred to in the photo is towards the top left as seen in the photo sitting below the three left most tags, those types are very common to fail.

Lawrence.

[sergiop]

Okay I've located it and checked it with the power on. Good news (I think?) 21V!

To be clear I've put a red arrow on the cap I checked. From what I can tell from the manual this is C212. What fun!

[DMcMahon]

Originally Posted by sergiop

Serial Number is in the photo I think actually: No. G 39939

Thank you Sergio, I should have spotted it in the photo.

This indicates that it is a Mk. 2

David

[ms660]

Originally Posted by sergiop

Okay I've located it and checked it with the power on. Good news (I think?) 21V!

To be clear I've put a red arrow on the cap I checked. From what I can tell from the manual this is C212. What fun!

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.