Revox B77 pinch roller adjustment

[jpmorris]

The pinch roller has gone out of whack on my mixdown machine again. As before, manually holding the roller in place fixes it so the pressure has gone down. Last time I adjusted it until it stopped slipping, but I think the time has come to do it by the book.

I've now got a 50N spring balance on order, but the service guide is terse and so I'm a little confused about how you actually set it up to measure the force. If anyone has actually done this, could they post a clear description or photo or something?

 

[Tim Gillett]

The pinch roller has gone out of whack on my mixdown machine again. As before, manually holding the roller in place fixes it so the pressure has gone down. Last time I adjusted it until it stopped slipping, but I think the time has come to do it by the book.

I've now got a 50N spring balance on order, but the service guide is terse and so I'm a little confused about how you actually set it up to measure the force. If anyone has actually done this, could they post a clear description or photo or something?

I've done it enough times using the instructions on page 4-3.

Obviously you have to remove the front grey panel first, and to do that you have to remove the pinch roller from the shaft. Watch those white plastic washers! There should be one on either end of the pinch roller bearing. Also remove the pitch control knob. It's just a push fit.

The part about " attach a nylon thread" is a bit vague. I loosen the Philips screw holding the pinch roller end cap on, and put the thread under the screw head. Maybe substituting a longer screw temporarily can help.

It's easier with a tape on the machine as you dont have to hold down the play button and you also know when the roller has been lifted slightly off the capstan as the tape stops turning, or changes speed.

Also when it says "shift pressure roller solenoid to the right" it doesnt show a picture of this or the two screws you need to loosen and tighten. It's a pretty small adjustment. It can help to make some marks on the chassis to help identify where the screws were before they were moved. I scribe a line following the outline of the screw washers. Also best to use a closed socket rather than a flat blade.

Check that your roller rubber hasnt gone too hard. No amount of adjustment will fix it if the rubber is hard. And while there, I clean the roller bearing and shaft, and relubricate with a little light oil.

It's easier on the B77 than the A77!

Hope this helps. Tim.

 

[jpmorris]

I've done it enough times using the instructions on page 4-3.

Obviously you have to remove the front grey panel first, and to do that you have to remove the pinch roller from the shaft. Watch those white plastic washers! There should be one on either end of the pinch roller bearing. Also remove the pitch control knob. It's just a push fit.

Yeah, I'm no stranger to that bit. I've replaced the counter belt and various other goodies. Indeed, the machine is open and ready to go as soon as the balance arrives.

What was confusing me was the whole "nylon thread" thing, but I think I understand now. The idea is to engage play, and then see how much force it takes to pull the thing away from the capstan shaft, correct?

And if there is insufficient force, adjust the nut on the solenoid shaft until it's about 13 Newtons?

Also when it says "shift pressure roller solenoid to the right" it doesnt show a picture of this or the two screws you need to loosen and tighten. It's a pretty small adjustment. It can help to make some marks on the chassis to help identify where the screws were before they were moved. I scribe a line following the outline of the screw washers. Also best to use a closed socket rather than a flat blade.

I'm not sure I have one of that diameter but I'll check.

Check that your roller rubber hasnt gone too hard. No amount of adjustment will fix it if the rubber is hard. And while there, I clean the roller bearing and shaft, and relubricate with a little light oil.

It should be okay as it was replaced late last year. I'm not even sure the old one was bad - it was most likely the pressure and/or solenoid causing the slippage.

Hope this helps. Tim.

Yes, thanks. I think I have a clearer idea of what needs to be done.

 

[jpmorris]

When I bodged the machine earlier I had no idea that the solenoid moving was a problem, so I basically cranked up the pinch roller pressure to compensate. I don't know what it was set at but it must have exceeded 20 Newtons. I don't have any nylon thread so I used multiple strands of polyester - basically I stopped when it went above 16 and showed no signs of budging.

I believe it's now more like 14 which is a bit on the high side but within tolerances.
Before I fine-tune it, what benchmark is used to tell when the pinch roller is lifting away from the capstan?
I'm getting a small crack of light between them at about 14. Is that a sensible way to tell, or is there a point just before that which I'm interested in?

 

[Tim Gillett]

When I bodged the machine earlier I had no idea that the solenoid moving was a problem, so I basically cranked up the pinch roller pressure to compensate. I don't know what it was set at but it must have exceeded 20 Newtons. I don't have any nylon thread so I used multiple strands of polyester - basically I stopped when it went above 16 and showed no signs of budging.

I believe it's now more like 14 which is a bit on the high side but within tolerances.
Before I fine-tune it, what benchmark is used to tell when the pinch roller is lifting away from the capstan?
I'm getting a small crack of light between them at about 14. Is that a sensible way to tell, or is there a point just before that which I'm interested in?

It's important to get both adjustments right as they interact. The solenoid only has maximum force when the plunger is fully engaged. If you keep tightening the spring you actually get less force because the plunger cant seat correctly and it cant compress the spring.

Sometimes just adjusting the solenoid just a little to the right is all that's needed, and I mean just a little.

With a tape on the machine you can tell when the pinch roller has slightly lifted, as the tape stops. But be careful. If the tape is at the beginning of the reel, lifting the pinch roller can cause the tape to actually speed up, not slow down. In that case, better to FF the tape on to a point a lot further on where lifting the PR causes it to stop. Maybe about half way through the reel.

Just make sure the plunger stays fully seated, even when the pinch roller is lifted slightly of the capstan. If lifting it causes the solenoid to lose its hold you can be sure you've set the spring too tight and/or the solenoid clearance is too big.
Satisfy both conditions and you're laughing.

BTW it doesnt have to be nylon thread. I use whatever's available in my drawer, usually string.

Cheers Tim

 

[jpmorris]

Thanks. Doing this with a tape instead of jamming the light sensor has given rather different results. How I have it set up at the moment, it is starting to slow around 8N and halting at 12N, with the roller starting to lift away at around 14. Is that the sort of figure you were getting?

The reason I'm doing this at all is because the machine slows down towards the end of 5" tapes unless you actually poke the pinch roller in, which seems clear enough to me that there is insufficient force or clearance.

 

[Tim Gillett]

Thanks. Doing this with a tape instead of jamming the light sensor has given rather different results. How I have it set up at the moment, it is starting to slow around 8N and halting at 12N, with the roller starting to lift away at around 14. Is that the sort of figure you were getting?

The reason I'm doing this at all is because the machine slows down towards the end of 5" tapes unless you actually poke the pinch roller in, which seems clear enough to me that there is insufficient force or clearance.

Aha! 5" reels. I'm not at home at present but I'm almost certain the B77 is only designed to handle minimum 7" reels (or the normal hub size of a 7" reel).
Even with the tension switch on small there's too much backtension at the end of a 5" supply reel.

So maybe there was nothing wrong with your B77 in this respect?

Cheers Tim

 

[jpmorris]

Aha! 5" reels. I'm not at home at present but I'm almost certain the B77 is only designed to handle minimum 7" reels (or the normal hub size of a 7" reel).
Even with the tension switch on small there's too much backtension at the end of a 5" supply reel.

So maybe there was nothing wrong with your B77 in this respect?

May I just say "Oh shit" since I have a box of 50 of them?

For some reason this unit came with the manual for the low-speed model instead of the HS, so I could be wrong, but the manual is saying "7 inches or less"... either way, the backtension is weak in that configuration so you may have hit the nail on the head there.

 

[jpmorris]

Well, it seems to be far better behaved with 5" reels than it was before - when I left it recording ambient sounds the other day only to find that it was barely moving towards the end of the reel, that was when I reasoned something had to be done.
I'm going to put it back together now...

 

[Tim Gillett]

That's good.

I looked up the manual. Minimum hub diameter 2.36" or 6cm. So even a 7" standard reel is strictly not suitable. I remember reading years ago the B77 worked best with 10.5" NAB spools and LP tape.

Cheers Tim.

 

[jpmorris]

That's good.
I looked up the manual. Minimum hub diameter 2.36" or 6cm. So even a 7" standard reel is strictly not suitable. I remember reading years ago the B77 worked best with 10.5" NAB spools and LP tape.
Cheers Tim.

Thanks. Yes, I usually use 10.5" tape for the more critical work anyway, but I have been using smaller reels fairly regularly for other things, typically applying tape effects to digital voice material.
(The continuous varispeed on the lead voice in this clip, for example:

)

I'll have to remember that mixing or working on longer segments is a bad idea when using small spools.

 

[jpmorris]

This machine is still unwell. I've spent about 9 hours trying to make it play back a 1khz tone. If the pinch roller is pushed in it reads a nice, steady 1.02khz, but left to its own it wobbles from about 920-980hz with a sort of warbling sound like the Vogons in HHGTTG (which were created by forcing the capstan to slip IIRC).

I also note that wind speeds are way down, so I'm thinking that there is some underlying problem, though the pinch solenoid seems to be getting its full 24v. I do have a spare PSU assembly, and indeed spare spooling motors which someone parted out from a PR99 mk1, so I might try swapping things out, but unless anyone has any bright ideas, I'll probably have to get it professionally serviced.

 

[RRuskin]

Have you thought aboust replacing the pinch roller with a new one? There comes a time when there is too much wear to be adjusted out.

 

[jpmorris]

Have you thought aboust replacing the pinch roller with a new one? There comes a time when there is too much wear to be adjusted out.

I did that a few months ago when the problem first manifested itself. I'm assuming that the replacement is itself good, of course.

 

[RRuskin]

I did that a few months ago when the problem first manifested itself. I'm assuming that the replacement is itself good, of course.

Was it new or just new to that machine? Have you kept it clean and conditioned?

 

[jpmorris]

Was it new or just new to that machine? Have you kept it clean and conditioned?

It was new and unused from the Italian Revox service kits that appear on e-bay from time to time. You bring up a good point that since then it has not been cleaned very thoroughly.

What substances would work for this, bearing in mind that it doesn't look like I can source the Caig stuff? (Deoxit and various similar substances are available, but that's about it)

 

[RRuskin]

It was new and unused from the Italian Revox service kits that appear on e-bay from time to time. You bring up a good point that since then it has not been cleaned very thoroughly.

What substances would work for this, bearing in mind that it doesn't look like I can source the Caig stuff? (Deoxit and various similar substances are available, but that's about it)

Never use head cleaner or anything with an alcohol base. Any vendors still have the old TEAC Rubber cleaner? That stuff works great. Sony used to recommend dish washing liquid for the pinch rollers on their digital reel to reels. The part must be thoroughly dry before you load any tape on the machine. BTW - pinch rollers should be cleaned as often as you clean the rest of the tape path.

 

[jpmorris]

Sony used to recommend dish washing liquid for the pinch rollers on their digital reel to reels.

That improved things marginally, but it is still far from perfect. In any case I'm heading off for a week so I'll have to leave it until I get back.

 

[jpmorris]

It looks to me as if the diode across the pinch solenoid has gone open-circuit. Any idea what effect that would have?

 

[jpmorris]

Oops, that should have read "short-circuit". As in 'no longer rectifying'.