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Old 2nd Jun 2024, 12:52 pm   #1
Linnovice
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Default Revox PR99 Bearing problem

Good afternoon all. I am experiencing another problem with my PR99 MkII. Mechanical this time. The bearing on the supply side of the headstock rattles in ff or rw mode. I've tried three different bearings all mic up at 19mm x 6mm x 6mm (well, .004" under in each case. I have it packed out on either side with copper shims to fit the centre bearing and it is tightened sufficiently. Not trials of strength tight but enough. In every case the bearing rattles as the tape passes. If I put my finger on the non tape side it stops rattling so it must be the bearings.

Question. In the service manual it just give a Revox part number, no dimensions. Am I using the wrong size bearings? Being slightly under the metric dimensions, albeit only .004", would that be causing the rattle? Its very odd and annoying. They are silent in play mode though. Has anyone else experienced this problem, if so, how did you cure it. The bearings I have are from SFK so are reputable bearings. I'm a little stumped at the moment so before I order some different bearings it would be good to know I'm not unique.

As always, any advice would be gratefully received.
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Old 2nd Jun 2024, 1:45 pm   #2
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Default Re: Revox PR99 Bearing problem

The same bearing appears to be in Revox B77's. The SM doesnt seem to list the bearing as such, just a Revox part number for the complete assembly. When removed the old bearing seems to be a 626XX. Obviously the original spacer washers are important to allow a small clearance so 1/4" tapes dont become jammed between the outer discs.

I've had the same problem many times of a bearing squealing in fast wind. Sometimes I've added a little light oil which settles it down for a while, but I've seen bearings which were obviously worn as shown by the play in the bearing. They really needed replacement.

But I've also had the problem of a newly installed 626XX bearing barely rotating with the tape. May as well be a stationary guide! I put this down to a lubricant too viscous for the application, which effectively stalls or slows the bearing. There must be an original lubricant inside but as to which it is who knows? I suspect the issue might be that the bearing is designed for heavier average loads requiring a heavier lubricant whereas in this Revox application the bearing has very little load on it and unless the lubricant is of lower viscosity the bearing wont rotate freely. The tape just slips across it.

A bit of experimenting with lighter lubricants may be a solution, but not sure how easy it would be to remove the factory lubricant and replace it when the bearing has a seal on either side!
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Old 3rd Jun 2024, 7:48 am   #3
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Default Re: Revox PR99 Bearing problem

Hi Timtape, thanks for the response. My thoughts entirely. I'm wondering if originally, the Revox bearing was the 626XX variant but with a lighter grease installed at manufacture. The amount they would have ordered justified the change in production to the manufacturer. That doesn't help now though. I could soak the bearings in a solvent to wash out the original grease but the difficulty would be in getting a light grease into the bearing after. Can't use oil as it would just run out.
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Old 3rd Jun 2024, 9:05 am   #4
TIMTAPE
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Default Re: Revox PR99 Bearing problem

Yes that's my guess too, that Revox had the manufacturer use a lighter grease when assembling the bearing. That might also make sense of Revox's listing the spare part (at least as I found it in the B77 SM) as the complete assembly rather than listing the bearing as a standard 626XX. But it would have been nice if Revox had clarified in the SM if or that the bearing contained a non standard lubricant. On the other hand maybe the bearings I purchased had heavier than standard lubricant and I need to shop around! Maybe even the drag imposed by the bearing seals varies with different brands.
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Old 3rd Jun 2024, 3:15 pm   #5
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Default Re: Revox PR99 Bearing problem

I have a bearing of the same dimensions but with rubber seals either side of the race. I've removed the rubbers either side as they add drag to the outer bearing ring. I rinsed out the existing grease and replaced with a much lighter silicon crease. Overall that is much better. The bearing still rattles for the last 30 seconds at either end of a RW/FF cycle but mainly silent throughout the run.

I notice that during the RW/FF cycle the bearing does not rotate according to the speed of the tape going over it. ie. the tape barely seems to run on the bearing surface. It seem to just brush over it and lightly drag the bearing round. On play though it has a firm contact with the tape and rotates at a constant speed.

All seems very odd. Its as if the positioning of the tape/bearing in RW/FF is not firm enough to rotate the bearing at speed.
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Old 3rd Jun 2024, 9:28 pm   #6
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Default Re: Revox PR99 Bearing problem

At higher speeds air is drawn between the tape and the roller creating an air bearing which allows a degree of slip between the two.

The tape position roller on an EMI TR90 has grooves to minimise this effect to maintain accuracy of readout during its rather alarmingly fast spooling!

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Old 4th Jun 2024, 1:20 am   #7
TIMTAPE
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Default Re: Revox PR99 Bearing problem

Yes good point. The PR99 itself has grooves milled into the alloy roller on the take up side of the head block. Not surprisingly the roller forms part of a tacho.

Maybe slipping on the supply roller guide doesnt matter here as the bearing metal is relatively hard, and the bearing does rotate some of the time, helping to average out any possible differential wear at the circumference.

High speed fast wind is not good for magnetic tapes. It results in a poor, uneven "pack" of the tape winds, partly because of the same "air bearing" effect you mentioned. The best wind is a slower and smoother "play" wind where tape tension is controlled and the air has time to escape, allowing the tape to locate itself at the same height as the previous tape winds, rather than floating randomly.

Last edited by TIMTAPE; 4th Jun 2024 at 1:29 am.
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Old 4th Jun 2024, 9:37 am   #8
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Default Re: Revox PR99 Bearing problem

Here's a spare of the type l used in my B77. I haven't got my notes to hand at the moment but l chose this particular bearing because it had a low "stiction" specification, /C3GJN.

PM me your address and I'll stick it in the post foc.
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Old 6th Jun 2024, 9:31 am   #9
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Default Re: Revox PR99 Bearing problem

Thanks Ken, PM sent.
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Old 10th Jun 2024, 1:28 pm   #10
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Default Re: Revox PR99 Bearing problem

Thanks to the assistance (and generosity) of Kentode my noisy bearing problem is now solved. I didn't realise that bearings were made in different rolling resistant specs. The bearing with the 626-2Z /C3GJN is the correct bearing to install as due to its low stiction ie force required to move from rest as opposed to friction ie rolling resistance.
A subtle difference but an important one!
Thank you Ken, very much appreciated.
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Old 10th Jun 2024, 2:18 pm   #11
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Default Re: Revox PR99 Bearing problem

You're welcome!
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Old 11th Jun 2024, 9:41 am   #12
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Default Re: Revox PR99 Bearing problem

Yes thankyou Kentode. I've worked on these machines for decades so this important detail about the bearing's stiction specification is for me most welcome. Wish I'd known it in the 1980's!
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Old 11th Jun 2024, 10:14 am   #13
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Default Re: Revox PR99 Bearing problem

Quote:
Originally Posted by Trigon. View Post
At higher speeds air is drawn between the tape and the roller creating an air bearing which allows a degree of slip between the two.

The tape position roller on an EMI TR90 has grooves to minimise this effect to maintain accuracy of readout during its rather alarmingly fast spooling!

Attachment 298469

Cheers
I have never seen a machine which spools as fast as a TR90 - I think they used 2 pole motors (circa 2850 rpm max)
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Old 11th Jun 2024, 7:06 pm   #14
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Default Re: Revox PR99 Bearing problem

Quote:
Originally Posted by barrymagrec View Post
I have never seen a machine which spools as fast as a TR90 - I think they used 2 pole motors (circa 2850 rpm max)
The motors actually only run at about 1450rpm unloaded. I think what makes it seem so fast is that the motors are powerful enough to run near that speed for the entire length of the tape rather than slowing as the going gets tough.

2400ft on 10.5" NAB reels will rewind in about a minute at full speed, with the supply reel doing well over 3000 rpm at the end. With 5.75" cine spools that becomes over 4000rpm...

I've always assumed most operators manipulated the spooling control toward the end to be a bit more gentle on the tape!

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Old 11th Jun 2024, 7:39 pm   #15
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Default Re: Revox PR99 Bearing problem

I don't know what the RPM of my Revox B77 actually is, but l calculated that it maxed out at a rate of 26 feet per second, which is quite enough when it's only 18 inches from my head!
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