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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 3rd May 2021, 1:14 pm   #1
jmcilkley
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Default Cassette capstan speed.

I have been restoring a Marantz 5000 cassette deck and having replaced belts, lubricated the mechanism, de-ox'ed the record switch and cleaned the capstan roller and heads all seemed good to go. Except there is a clear speed oscillation. I recorded a 3000Hz tape on another deck and when I play it back there is a frequency oscillation of with a beat of about 4Hz. I'm wondering if this is related to the capstan rotation speed but I can't find online the rotational speed (or the diameter) of the capstan. I could work out the rotation speed from the diameter so does anyone know either the capstan diameter or the rotation speed of the capstan?
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Old 3rd May 2021, 1:45 pm   #2
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Default Re: Cassette capstan speed.

I don't know either of those specs... just saying that first.

If the playback speed is varying as you say at 4Hz then I would look at such things as any auxiliary drives of the main capstan flywheel such as take up clutches for the take up spool and so on.

Don't discount something like a problem with back tension. Try unspooling a few seconds of tape from the supply spool so that the spool doesn't turn for a moment or two and see if that alters anything.

Also see if the take up torque is smooth and constant and not 'grabby'

Make sure any brake pads are not binding. Pinch roller in good condition and not showing any flat spots?
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Old 3rd May 2021, 9:36 pm   #3
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Default Re: Cassette capstan speed.

Check the rear capstan flywheel plate/thrust bearing (if fitted) is not too tight - been caught by that one before. Also there is no debris on the flywheel / the groove where the belt sits - must be spotless.
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Old 3rd May 2021, 9:49 pm   #4
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Default Re: Cassette capstan speed.

Are you saying that you measured the 3kHz signal with a frequency meter and it was varying by 4Hz? That doesn't sound too bad to me. Domestic cassette decks aren't precision laboratory instruments, even good ones.
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Old 4th May 2021, 7:51 am   #5
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Default Re: Cassette capstan speed.

Use a commercially recorded test tape and measure the deviation % on a wow and flutter meter and compare this with the spec for your deck. This will tell you whether you have a fault or not.
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Old 4th May 2021, 8:08 am   #6
jmcilkley
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Default Re: Cassette capstan speed.

Thanks for the advice so far. When I say that the frequency oscillation is at about 4Hz that is not the amount by which it changes but the up and down of the frequency oscillates at 4Hz (approx) - like a 4Hz vibrato!
I was hoping to work out the rotation speed of the capstan to see if this 4Hz warble is related to that.
The flywheel is not too tight and has been cleaned with ipa, so I don't think that is the problem. I'm still thinking it is a capstan problem as the warble is so constant. The tape speed is 1 7/8 in/s which is 47.624 mm/s. So the capstan circumference would be 47.624 divided by the number of times the capstan rotates in 1 second and the diameter would be 47.624 divided by pi divided by the rotations.sec (=15.157 per revs/s). To match the warble of about 4Hz the capstan would have to rotate 4 times a second which would make the capstan diameter 15.157 divided by 4 = 3.79mm.
Clearly the capstan is smaller than this - using a mm rule it looks about 2mm. So it seems it is not in time with the capstan itself. Unless an eccentricity makes 2 cycles in 1 rotation which would give a diameter of 3.79/2=1.9mm which looks about right.
Any mathematicians out there to check my working?
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Old 4th May 2021, 9:31 am   #7
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Default Re: Cassette capstan speed.

For 1 7/8 IPS I make the capstan revs for 2mm diameter 454.782 RPM (7.5797 RPS)

Lawrence.
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Old 4th May 2021, 10:44 am   #8
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Default Re: Cassette capstan speed.

I have seen a bent capstan on a couple occasions. The effect on the sound was very pronounced, but it was also very easy to spot the problem.
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Old 4th May 2021, 12:13 pm   #9
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Default Re: Cassette capstan speed.

Quote:
Originally Posted by Nickthedentist View Post
I have seen a bent capstan on a couple occasions. The effect on the sound was very pronounced, but it was also very easy to spot the problem.
You beat me to it, Nick. I came across a handful of bent capstans in my time on the bench, always caused by the owner desperately trying to release a tape which had wound itself around the capstan due to a loss of take-up.
The audible result is a 'fast wow'.
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Old 4th May 2021, 1:14 pm   #10
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Default Re: Cassette capstan speed.

Access to a Wow and Flutter meter such as Kenwood FL180A or alike would help along the way to monitor any changes or fault finding your undertaking and can help show if your making progress or its getting worse since the last attempts to trace and resolve on going.
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Old 4th May 2021, 3:08 pm   #11
jmcilkley
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Default Re: Cassette capstan speed.

Unfortunately I don't have a wow & flutter meter or a proper test tape. I'm using a tape recorded on a known good deck from a 300Hz signal.
I will get the capstan out again and check if it is true. Is it possible to correct a bent capstan or does it need specialised equipment?
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Old 4th May 2021, 5:11 pm   #12
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Default Re: Cassette capstan speed.

I doubt you would ever straighten one satisfactorily. A close visual inspection with it running and a finger tip should be good for detecting if it is out of true.
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Old 4th May 2021, 5:14 pm   #13
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Default Re: Cassette capstan speed.

It will almost certainly snap if you try.

I managed to replace one though. I tapped the old one out of the flywheel using a small hammer, some heat and an improvised punch. Then replaced it with one retrieved from a scrap deck of the same make and from a similar era. Amazingly, it worked well!
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Old 5th May 2021, 10:10 am   #14
HECTOR63
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Default Re: Cassette capstan speed.

Quote:
Originally Posted by jmcilkley View Post
Unfortunately I don't have a wow & flutter meter or a proper test tape. I'm using a tape recorded on a known good deck from a 300Hz signal.
I will get the capstan out again and check if it is true. Is it possible to correct a bent capstan or does it need specialised equipment?
Well I have 2 as use them frequently and are a very good tool indeed, try Stewarts of reading Dwayne or Beryl as I picked mine up at a very good price and no where near eBay etc prices either!
They really are worth there weight in Gold-if I was nearer to you would have been happy to lend you one but sadly not the case.
But its worth an enquiry and as they have built in Frequency counter and Ref Oscillator outputs you could record your own test tape on a good stable machine and check it against the machine and also all the other parts in situ with the meter range etc.
I have a pdf of the user guide which I can post up by thread or email if its of any help for references.
http://www.stewart-of-reading.co.uk/

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Old 22nd May 2021, 6:20 pm   #15
jmcilkley
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Default Re: Cassette capstan speed.

I think I resolved it. After trying the tension in the take-up spool and recleaning everything; I remembered that when I replaced the belts I didn't have the right size for the capstan belt and used one that was too small. It seemed to work ok but I guess it was pulling the capstan out of alignment. I ordered some more assorted belts and put in the right size one and now it works so much better. The wow and flutter is no longer audible so I rate that a success!
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Old 22nd May 2021, 6:35 pm   #16
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Default Re: Cassette capstan speed.

The capstan belt is certainly the critical one. Good result.
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Old 22nd May 2021, 7:40 pm   #17
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Default Re: Cassette capstan speed.

Quote:
Originally Posted by jmcilkley View Post
I didn't have the right size for the capstan belt and used one that was too small. It seemed to work ok but I guess it was pulling the capstan out of alignment.
Yes it would and it will also cause excessive wear on the capstan bearing. I once came across such a bearing that had worn oval....The customer complained that it was running 'a bit slow....'!

It is quite amazing what some people will listen to before they decide that something is wrong!
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