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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 16th Jan 2017, 5:06 pm   #1
saxmaniac
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Default General question on pinch wheels

Does anyone know if a pinch wheel and capstan should both be absolutely cylindrical so they meet parallel to each other or should there be a slight barrel shape in order to make the tape self centre? I ask because I have an Akai machine which is prone to the tape occasionally riding off the pulley. This is particularly bad with double play tape. The pinch wheel is cylindrical and I have tried another just in case but no different
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Old 16th Jan 2017, 5:26 pm   #2
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Default Re: General question on pinch wheels

I have seen machines with designs that use cylinder shaped pinch rollers and others that use barrel shaped ones. The most important factor apart from the general condition of the pinch roller is that the axis of the pinch roller is parallel to the axis of the capstan shaft. Check that the capstan shaft bearing is not worn allowing it to move out of parallel.

I haven't worked on many R2R machines but very many cassette decks and video machines where the same mechanical principles apply.
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Old 16th Jan 2017, 5:30 pm   #3
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Default Re: General question on pinch wheels

The capstan spindle should be cylindrical and from memory I think the pinch roller should be also.

Sometimes tape ride up can be caused if the capstans top bearing is badly worn or if the pinch roller isn't parallel to the capstan spindle for whatever reason or dirt, is the pinch roller pressure adequate? I assume this is a cassette deck.

EDIT: Woops...Post crossed.

Lawrence.
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Old 16th Jan 2017, 5:54 pm   #4
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Default Re: General question on pinch wheels

I think this is a R2R machine.

This sounds like an alignment problem, maybe as a result of wear or damage. The tape guides should determine where the tape goes, not the capstan and pinch wheel which just move it forward. Poor tension from the takeup spool can also cause this.
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Old 16th Jan 2017, 7:32 pm   #5
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Default Re: General question on pinch wheels

This is an Akai 1700 r2r with the capstan sleeve. It's more prone to problem when sleeve is fitted. There's no discernable bearing play. I've held the pinch wheel very close to the capstan and shone a light through the gap but they look parallel as far as I can see. The only thing is that I do have take up and feed tension at the minimum to avoid head wear so Paul's suggestion is worth following up. Incidentally this machine is the subject of an earlier thread about too much flutter particularly when the sleeve is fitted. I'm still struggling with that one! I 'll see if I can post a link in case anybody's interested. Thanks for replies so far
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Old 16th Jan 2017, 7:35 pm   #6
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Default Re: General question on pinch wheels

https://www.vintage-radio.net/forum/...d.php?t=128542

Here's the link mentioned above
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Old 17th Jan 2017, 1:55 pm   #7
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Default Re: General question on pinch wheels

Probably the pinch roller. If it is the orginal one, the rubber is probably past it.
This in my experience causes most cases of tape skewing. If it is shiny, barrel shaped or cracked, then you almost certainly have identified the culprit.
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Old 17th Jan 2017, 4:31 pm   #8
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Default Re: General question on pinch wheels

Not audio related however...

The Sony C9 Betamax machine used pinch rollers with an offset internal bearing so that all things being equal, the pinch roller had a natural tendency to allow the tape to wander in the desired direction against the guides. Generic replacements did not have this offset bearing and the tape path alignment could be problematic if these were used.

Never come across anything like that in audio...

Carry on
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Old 17th Jan 2017, 7:06 pm   #9
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Default Re: General question on pinch wheels

I agree with Ben, the pinch roller is probably too hard and glossy, or possibly dusty. Either causes too little friction, and then the tape can wander.
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Old 17th Jan 2017, 11:52 pm   #10
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Default Re: General question on pinch wheels

I've tried pinch roller in another machine and vice versa and the problem doesn't transpose. Rollers appear in reasonable condition but I will try giving them a light skim in the lathe to ensure they are parallel. The rubber is still fairly soft. I am suspicious about the capstan sleeve as I think that may be behind a flutter problem at flywheel speed that I have on this machine. I'm thinking of putting out a wanted request for a flywheel off a 1720 which doesn't use sleeve so is just a two speed machine. I can live without the 1 7/8 speed on the 1700. It's a long shot that that orrible sleeve design is behind problems even though there's nothing obviously wrong with it.
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Old 18th Jan 2017, 2:08 pm   #11
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Default Re: General question on pinch wheels

Quote:
Originally Posted by saxmaniac View Post
This is an Akai 1700 r2r with the capstan sleeve. It's more prone to problem when sleeve is fitted. There's no discernable bearing play. I've held the pinch wheel very close to the capstan and shone a light through the gap but they look parallel as far as I can see.
I wonder if it could be that the azimuth is off for the pinch roller, relative to the capstan. I.e. they appear parallell when looking along the tape, but not when looking across the tape - the difference may be too small to see with the naked eye.

I've noticed a natural tendency for the tape to wander off to one side when the take-up tension is very low (or rather, nonexistent, as when stopping the take-up reel by hand), even in recorders (e.g. Tandberg) where the pinch roller is spring loaded so that it aligns itself to the capstan. Of course, the spring loading in these machines only allows automatic alignment in one plane, the azimuth of the pinch roller can still be off which I think is the reason for the tape wandering more than anything else. Or can it be the azimuth of the capstan, if it is not perpendicular to the tape travel?
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Old 18th Jan 2017, 5:05 pm   #12
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Default Re: General question on pinch wheels

I'm a bit thick and don't understand the first paragraph of your reply above. The pinch wheel in this case is a lot wider than the width of the tape and surely a bit of azimuth error should not make a difference if everything is parallel. I'm going to look at the take up tension as I have that set as low as poss to be kind to the tape so that may provide a solution or a cure. It may not be quickly as I have very limited time for these things and projects run into months and years!
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Old 18th Jan 2017, 7:25 pm   #13
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Default Re: General question on pinch wheels

I think ricard means it has to be parallel in both planes. That is to say the roller has to be parallel to the capstan when viewed in the direction.

Capstan is black, tape green and the roller red.
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Old 18th Jan 2017, 7:48 pm   #14
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Default Re: General question on pinch wheels

Ah I understand thanks!
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Old 18th Jan 2017, 11:17 pm   #15
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Default Re: General question on pinch wheels

Check the capstan is indeed 'true'. I had this not long back with a Tandberg deck someone had fiddled with. the tape was riding badly.It was a case of aligning the upper bearing.

I know some decks have a capstan surface that is sort of matt as it was 'textured', many 8 track decks had this. Before condeming the sleeve maybe try running it against some fine sandpaper just slightly, to take the shininess off a bit, see if that helps with the wandering.
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Old 19th Jan 2017, 4:12 am   #16
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Default Re: General question on pinch wheels

I've found reel tensions can contribute to tape skewing at the capstan/pinch roller. Decreasing take up tension can help, as can increasing back tension.

So the problem should be worst at the beginning of a reel where take up tension is highest and back tension lowest. Often the problem decreases and then disappears as the tape progresses and the relative tape diameters on the reels change.

Flutter can occur on a machine with felt type clutches (like I believe your 1700) which have gone a little sticky and at certain speeds you get a "stick, slip, stick, slip" condition. Then as above, as the tape progresses and the reel speeds change, the condition may occur or disappear.
I guess when the clutch is in the "stick" mode (limiting friction) the take up tension may be above specs and could also cause the tape to skew.

I've dismantled the clutches and cleaned the felts and tables in warm water and detergent, and then readjusted the tensions.
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Old 19th Jan 2017, 9:12 am   #17
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Default Re: General question on pinch wheels

My Teac X10 has a switch to alter the torque on the payout reel for 7" or 10" reels. It started misbehaving with the tape wandering off the first capstan when using a 7" reel and the 7" setting, but changing to the 10" reel setting cured the symptoms. Thoroughly cleaning the pinch roller with cellulose thinners cured the problem, at least for a while. One of the problems with that machine is that the pinch rollers are not well protected from falling dust
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Old 19th Jan 2017, 9:46 am   #18
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Default Re: General question on pinch wheels

Hi Tim
flutter problem is more of a flywheel speed wow. I had thought the capstan was slightly bent or the flywheel out of balance but have discounted those theories (checked capstan with dial gauge as well as visually and experimented with trying to re balance the flywheel) I have already cleaned felts etc and clutches are smooth and set for minimum tension. No setting of clutches makes any difference to the flutter. It seems worse with the sleeve fitted but I can't see anything wrong with the sleeve. To make progress with the tape path problem I am going to experiment with tension adjustments and skim the pinch wheel to ensure it's parallel plus the suggestions above about ensuring wheel perfectly aligned. Thanks to all for suggestions so far!
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Old 19th Jan 2017, 10:56 am   #19
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Default Re: General question on pinch wheels

Quote:
Originally Posted by ben View Post
I know some decks have a capstan surface that is sort of matt as it was 'textured', many 8 track decks had this. Before condeming the sleeve maybe try running it against some fine sandpaper just slightly, to take the shininess off a bit, see if that helps with the wandering.
Good advice. Again referring back to Beta decks, this was something I used to do. The very fine black wet and dry carborundum type paper was good for this as it didn't leave scratch marks but did fully restore the matt sheen and finish (and the same for upper drums too).
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Old 20th Jan 2017, 7:18 am   #20
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Default Re: General question on pinch wheels

I know it doesnt tally with the flutter being at capstan shaft revolution speed but the other thing the larger diameter capstan sleeve sometimes does is lift the tape a little off the play head on the exit side. In some situations this can cause dropouts (due to spacing loss) which can be mistaken for flutter. Just a thought.

Tim
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