Revox B77 issues

[skanker]

Hi Folks

I'm trying to repair my B77 and have run into some issues. I fitted a new power supply yesterday and now everything is operating in one direction (play/FF/RW) for some reason.

Play is the opposite to what it should be as is FF, only RW spools in the corect direction. On top of this PLAY is very fast compared to normal.

I've gone over my work and I cant see anything glaringly obvious so would really appreciate some advice if anyone can assist?

Thanks

[Ted Kendall]

All right. There are any number of places where this operation could have come to grief, so let's go back to basics. Why did you replace the power supply? What was wrong with machine before? Are you sure the new power supply is beyond reproach? Did you replace the transformer and board, or just the board? Did you replace the slide tags in the correct order? A photo of the relevant edges of the board would be useful.

[skanker]

Hi Ted

Thanks for chiming in mate, it's much appreciated.

First off, I've had the unit approx six years and used it regularly up until two years ago when I sent it to Brian Reeves for a once over. He came back with a quote that scared me so I took it back from him unserviced and with an additional surprise of the power supply having burnt out whilst with him.

I bought a reconditioned power supply soon after and the whole lot has sat there these two plus years just waiting for me to have a go at it.

Removed the old one and fitted the new and replaced every connection as it was previously and now my unit functions as mentioned in my original post.

As for the supply, it came as a whole unit with transformer and side card attached.

I've uploaded a couple of images as requested, hope they make sense!

[skanker]

To add, i've just noticed this new supply has a jumper as seen with the image which the old one does not. It's the only thing between the two that doesn't match

[Ted Kendall]

Well, the jumper could just be bridging a bit of lifted PCB track. What worries me more at this point is why the original PSU went pop, and what damage it caused in the process. If I understand you correctly, the thing hasn't run since the PSU burned out. To be basic, are there any smells of burning or other signs of things getting hot?

[skanker]

Thats rights, it's not run since burnout

I have no idea what the underlining cause for the burn out was but looking at the image attached i can see the cap is well shot and Brian obviously cut the leg whilst with him.
I can't see or smell any other smoke/burn related damage.

I think i'll put the old one back in with the cap replaced from the new one and see if i get anything different.

Im no tech as you can probably tell but if it's not too hard a job i should be able to fix it myself with a bit of advice on here (fingers crossed)

[Ted Kendall]

Ah, the Rifa! The bad news is there are two or three of them in this machine, and the good news is that the cap in this position, though it goes off like a firework when it fails, seldom causes other damage. There are a couple of threads about these little jokers elsewhere on the forum. Best to change them all...but if this is the "burn out", your original PSU may be OK. You can run it without this cap for test purposes.

[Radio Wrangler]

That looks like a 'Rifa' capacitor on the mins input. They are special 'safety' types designed to not fail short circuit by having a metallised-paper dielectric which melts back when extreme voltage spikes cause an internal burn-through of the dielectric.

But after a few decades their plastic cases craze, allow moisture in and they fail spectacularly with foul smelling smoke.

These things are now failing at a remarkable rate in old equipment. So I'd not blame whoever the recorder was with, these parts are at best now just sitting waiting for their moment. I've had a few fail in test gear from the same era.

But it's just one capacitor, not the entire power supply. Cutting one leg stops the pyrotechnics so the machine can be investigated for other problems. I certainly wouldn't replace an entire power supply for just a Rifa letting go.

There are a few of these capacitors in the A77/B77 machines. Treat them as unexploded stinkbombs. There are also some phase shift capacitors for the motors which are also well known for degrading. For any Revox tape machine now, it's serious advice to replace the Rifas and the motor capacitors as a preemptive precaution. Some firms sell complete kits.

The other known age-related failure is that the trimmer pots Revox used tend to fall apart if you touch them.

David

[Radio Wrangler]

I must type faster! Ted pipped me at the post.

David

[skanker]

So that's what they're called. I had wondered, good news then hopefully.

I'll have a look online for some modern options and let you know how i get on...

Thanks for the advice gents, much appreciated!

Neil

[skanker]

BTW, I'm on the lookout for a new capstan shaft (7.5/15 ips) for this unit if anyone has any leads?

[Ted Kendall]

Are you looking for a replacement for the existing shaft, or attempting a conversion of a standard machine? If so, be aware that the shaft is not the only mechanical difference between versions.

[skanker]

I'm looking to replace an existing rod so all good on that front mate.

This morning i swapped the RIFA over from the new supply to the old one and also swapped some wiring over and the unit is now playing and FF in the correct direction but is now RW incorrectly. Visually looking at the speed of the nabs they appear to be playing to fast with no difference when either 7.5 or 15 option engaged and the FF/RW appears slower than i'd expect.

I ordered some new caps and replacement for the RIFA's but i don't see these helping with whatever is going on sadly, any ideas?

[Radio Wrangler]

Do the motor phase shift capacitors as well. They have about the same life expectancy!

David

[skanker]

Will do mate, thanks for the tip

[Radio Wrangler]

Oddness in spooling speed makes the phase shift capacitors the first port of call.

Wrong playing speed and no change from the speed switch means you're going to have to take a look at the capstan motor servo control board. Be wary that Revox didn't sell that many high speed machines, and that there seem to be more around nowadays. Some people changed everything necessary to modify one, but there were several levels of bodging done.

Check that this one has a real high speed larger diameter capstan (some people just wound up the servo to speed up the normal motor but 15ips isn't reliably in range)

Check that the right motor casting is used.... this offsets the larger capstan centre so that the surface contacting the pinch wheel lies in the usual place. (you guessed it, some people didn't notice the difference, some people filed holes...)

So don't trust that it's a real HS one until you've checked.

David

[skanker]

Thanks for the info David, i've already ordered the phase shift caps so im getting there.

As for the HS authenticity, my unit deffo is as i bought it from a collector and Brian Reeves has also looked at it since i bought it so im confident on that front.

I just need to do some reading and figure out what the hell could be up with the servo control board now

[Radio Wrangler]

It's the A77 i know my way around, not the B77.

The speed controller is a frequency discriminator. Later A77 moved to a 'pulse-count' (bad name, pulse averaging would be better) discriminator using a 555, so i suppose this is in the B77 running from the magnetic reluctance sensor on the capstan motor bell. My pet A77 went silly on speed because the sensor had cracked off its base and was dangling on its wires. Araldite to the rescue.

The motor speed is controlled by a power transistor living in the middle of a bridge rectifier. If the transistor or rectifier fails short, the capstan goes to full speed. To allow the power transistor to interface with the discriminator which runs off of ground-related supplies, the capstan motor and the bridge rectifier run from their own isolated secondary on the mains transformer. The transistor and bridge are a bit exposed to mains transients and can get taken out by things which go bump in the night. Mains transients also use up life expectancy of those self-healing Rifas.

Back in the day, those Rifa capacitors were seen as the bee's knees answer to new RFI regs and safety regs. Designed loads in, and kept a lot in the junk box for years thinking they'd come in useful. I need to track them all down and dump them, or have a mains powered fireworks display?

David

[skanker]

Thanks again for the info, i had to read up on what the servo board was so any additional info is much appreciated

I've found a reconditioned servo board online for £30 so i may just grab that for ease. £30 don't seem to bad unless you can advise differently?

Am i correct in thinking the servo board was a standard part for all B77 units or do they differ depending on the configuration?

Also, do you or anyone else happen to know what this little round spacer is? I found it lying loose when i took the unit apart and i cant figure out whether it's part of the machine or a rouge bit of bebris

[Radio Wrangler]

According to the manual, there are several versions of the capstan servo board and each one has different components loaded in some places depending on the version of the B77 and on the range of serial numbers.

So it may be difficult to determine if the available reconditioned board is right for your machine. It may be easier to just repair the original board.

I don't know what the spacer is, but if the B77 is in a cabinet then it's unlikely to have found its way inside. It probably belongs somewhere.

David