Revox A77 Help

[sergiop]

Originally Posted by Michael Maurice

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.

You lost me

I'm a mechy remember. I speak screw threads and torque, not collector and ground. Happy to short stuff just not entirely sure what you mean.

[Michael Maurice]

On the servo board, you'll find a large transistor bolted to a heatsink, there are three terminals to this transistor, two are soldered directly to the transistor, one is on a tag which is secured by a bolt, that is the collector. The ground is the chassis.

CAUTION: The collector of this transistor will be approx 100V and could give you a shock, please be careful.

[sergiop]

CAUTION: The collector of this transistor will be approx 100V and could give you a shock, please be careful.

I just measured it with a multimeter at 212V. Not sure I feel comfortable jumping that (plus it's hard to access).

New caps showed up in the mail today! I'll start by swapping the one on the speed control board.

It looks like I might be able to do this with the board installed. I've heard it's a real pain to get this board out. Any advice? I'm not sure how to remove these with one iron. Can I de-solder one side and bend the cap up and de-solder the other side?

[ms660]

That voltage measurement suggests that at the moment the capacitor (C213) is doing its job but replace it anyway because at some point it will fail big time, you should be able to remove it using just one soldering iron but take it easy because you don't want to damage the copper foil track on the solder side of the board, try using some proper solder wick if you can.

Lawrence.

[sergiop]

Understood. Thanks Lawrence. I won't get my hopes up with the replacement then. Is it possible that one of the three caps on the transport card is causing this or should I expect to change those without any improvement either (other than eliminating old dangerous caps)?

[ms660]

Replace that capacitor first, it'll be good practice, be aware that capacitors hold their charge if there's no discharge path, looking at the schematic there's no quick discharge path for it so before touching it's connections make sure it's fully discharged, say by leaving your volt meter connected across it with the power off, given several seconds it should discharge ok.

Lawrence.

[sergiop]

Alright gents, new caps installed. I didn't burn myself or burn down my house! Thank you for everyone's help thus far.

Haven't created any new issues that I can see but the original problem still remains.

What's next?

[Radio Wrangler]

You've changed all the capacitors?

The ones expected to be at risk are those golden Rifa ones and the three big motor capacitors. It's routine to change these when working on an A77, the originals if not already failed, will be close to going.

Were any of the ones you changed bad? Looking for physical damage is obvious, but testing the ones coming out can give clues which become valuable if the machine still doesn't work after you've changed them. In some cases a capacitor gone short circuit can damage other components around it. Replacing the capacitor fixes the cause, but leaves its collateral damage.

While trying to find faults, information is very valuable.

David

[sergiop]

Thanks David. I changed the caps that were recommended in this thread. In fact, it was recommended I change them and unlikely that it would fix the problem. Just a good place to start.

The 4 RIFA caps I replaced looked to be in good condition and given that the functionality of the unit hasn't really changed I'm guessing they were probably still functional as well. I feel better having replaced them as I know it's a very common thing to do (and generally safer).

The capstan motor still continues to spin at power up for a second or two and then stops. I'm just not sure what to investigate or change next.

[DMcMahon]

It gets difficult Sergio without more fault finding diagnostics which with respect may be a step too far for you.

The suggested shorting down of the collector of the transistor as suggested by Michael is a good diagnostic as it would rule out to a large extent the Capstan motor (fairly unlikely to fail normally) or its Run capacitor being suspect, but I fully understand your reluctance to do it because of the > 200V and of course your personal safety is of prime importance.

The Servo/Speed Control board (Board E) previously listed electrolytic capacitors would be good to change both as a hopeful fix and a preventative future failure measure. As mentioned by David if you had the right capacitor/ESR measuring equipment you could measure the capacitors but depending upon actual results it may not be conclusive so easier just to renew them in this case

As mentioned by David the motor run capacitors are another area worth renewing, there is 1 run capacitor for each of the 3 motors. These are the large metal can stud mounted capacitors which you can see in your first photo. Definitely worthwhile giving them a good visual check over to make sure they are not bulging or leaking.

The Run capacitors are non polarised and the Capstan motor one is 3.5uF (C151) and the other two (C152 & C153) for the 2 reel motors are 4.3uF, all are rated at 220 volts AC.

Any replacements should be AC rated for continuous operation (and state suitable for Motor Run operation) and be Metalized Polypropylene and best to go for higher rated AC volts than originals, most suitable replacements would be rated > 400VAC

Most modern replacements would be plastic type bodied, some people may argue better to go for metal type, but would be more expensive and more difficult to source.

David

[sergiop]

Thanks David. I was going to try the shorting of the collector next to be honest. I just want to make sure I do it the right way. My plan was to use a piece of 14 gauge insulated wire (that's what we use in our houses here). Hold one end on the collector bolt and then touch the chassis with the other end. As soon as I see the motor spool up, remove the wire. That's it?

If I understand correctly, doing this will rule out the motor as being faulty in which case I can turn to the components you listed above.

[Radio Wrangler]

14 gauge will be too thick to be easy.

DO not try holding it in place.

Get some thinner wire and tack solder it in place with the power off and the board out. Reassemble with the short in place. The capstan motor should spin up to a higher speed than normal and stay at that speed until you turn the machine off.

What this test does is quickly show whether the fault lies in the motor/switching/run capacitor or whether it's on the board. On the board is the most likely outcome, shown by it running. If it doesn't run the board isn't entirely exonerated, there are still solder joints, connections and a bridge rectifier.

Capstan drive failed on my A77 some years ago, the plastic body of the speed sensor head had cracked and the head had shifted. A bit of araldite epoxy fixed it.

David

[sergiop]

Okay good and bad news. I soldered a jumper wire in and powered it up. The motor ran continuously! Then something started smoking so I powered it off. I can't figure out what it was though. Perhaps I just let the motor run to long with this jumper in place?

Anyone have suggestions on next steps?

Thanks,
Sergio

[DMcMahon]

As you say a mixture of good and bad news. It largely exonerates the motor.

Do you think the motor speed once spun up was constant or was it varying ?

Any idea if the smoke might have come from the shorted board or maybe the Run capacitor or elsewhere ?

David

[sergiop]

Speed of the motor appeared to be constant.

The smoke appeared to originate from the speed control board but I'm really not sure.

Question: Is the system expected to run for an extended period with that short in place? In other words, would you expect the short to cause something to overheat?

Trying to figure out if the jumper I installed is meant for a quick power up and power off or if it could technically be left in place and powered up for 15-20 seconds without issue.

Part of me wants to power it back up and let it smoke again so I can find the root of the problem. Not sure if that's wise.

Thoughts?

[DMcMahon]

I have never personally done this shorting test so I am unsure.

I can imagine that the transistor and/or the bridge rectifier may not be happy for too long with a hard short applied. Someone who has done the test will be able comment better.

David

[Radio Wrangler]

It just puts the full 130v on the capstan motor. Current goes up but not out of control. Some people bodge normal A77s to get 15ips by changing the discriminator and their capstan motors get there pretty much flat out, and they take more current because they put theirs into play.

Normally the capstan motor is run starved of voltage to bring down and to regulate the speed. These motors normally run heavily into slippage.

David