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Tape speed

Geoff.1

May 8, 2021
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Hi, I have replaced the drive belts on my late 70's Sony cassette tape deck but it is now running too fast, could it be it is because the new belts are approx. 1.3mm square whereas the old one was 2mm. square section, thanx, Geoff.
 

bertus

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Nov 8, 2019
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Hello,

From private messages from the TS:
Thanx, Bertus, as I have not used the deck for a number of years that could be the reason, I think I will have to check all the mechanics while I'm at it, Cheers, Geoff.

Second private message from the TS:
Hi, thinking about it being the pressure roller (pinch roller ?), the actual tape would scrunch up and gather, which does'nt happen, think I will have go back to my text books, if I can find them, to see if the drive belt is the cause, I think I remember something along the lines about actual belt thickness.

As I do not reply to replies made in private messages, I posted the posts here.

When the pressure roll is not pressing the tape will move faster on the speed of the pickup spindle.

Bertus
 

PETERDECO

Dec 19, 2019
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The thickness of the belt does have an effect on motor speed. When looped around the motor pulley, a thicker belt will effectively increase the diameter of the pulley and increase the speed. Some tape deck motors have a speed adjustment on the back. Look for a small hole with a trimmer resistor inside. Adjust with a small jeweler screwdriver.
 

Harald Kapp

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Hmmmh, doesn't make much sense to me. The "gear" ratio between driving motor and driven pulley should be given by the ratio of the pulley diameters on both sides as this is where the belt touches the surface of these pulleys and gets the speed from the motor or transfers that speed to the driven pulley. Having a thicker belt means the outside of the belt is a tad faster around the pulley (same angular speed, bigger diameter), but not the inside which is what counts.
But I may be wrong.
@Geoff.1 : How much faster does the tape run? Just a tad, which would be consistent with a small change in belt diameter and accordingly @PETERDECO ' observation? O quite a bit faster, which would be consistent with @bertus ' observation (and to which I'm leaning to)?
 

Harald Kapp

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There are so called "speed calibration" casetrres available.
Rather expensive, aren't they?
As a DIY solution: record a tone of known frequency (e.g. 440 Hz or 1 kHz) on a known good recorder. Play back this cassette on the faulty recorder and adjust the speed until the tone's frequency is correct. You can do this by ear by listening to the original tone and the played back tone simultaneously and reducing the beat frequency to (near) zero.
 

PETERDECO

Dec 19, 2019
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Trust me, I used to repair tape decks. A thicker belt will not fully descend into the groove of the motor pulley and change the "gear" ratio. As for speed adjustment, I used a 1KHZ test tape and a frequency counter. Believe it or not, the "norm" is to adjust the playback frequency to 1015 HZ.
 

Harald Kapp

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A thicker belt will not fully descend into the groove of the motor pulley
That is a valid argument. Partly, because this will happen on both sides and the effects cancel. How much they cancel will depend on the difference in size between driving pulley and driven pulley. Not much if the difference is huge.
But wouldn't that e an issue mainly if the new belt is wider than the old belt? If the groove in the pulleys was designed for a snug fit of a 2 mm belt, the 1.3 mm belt would also fit snuggly, If it were the other way round, it were understandable that a 2 mm belt doesn't fit a 1.3 mm groove and rides on top of it.

Believe it or not, the "norm" is to adjust the playback frequency to 1015 HZ.
Interesting. Do you have an explanation for this?
 

PETERDECO

Dec 19, 2019
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Back in those days I was told that manufacturers set the speed a little fast to compensate for the normal slowing of the motor with age. Also if you make your own recordings with a machine running about 1.5% fast and played back on a machine not running 1.5% fast, it will be discernible.
 

Geoff.1

May 8, 2021
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Thanx every one for the interest, the way I see it is that a thinner section belt will make the tape go faster as it tucks into the drive at it's smaller dia. whereas a thicker belt runs on the outer dia. which takes longer to to make one revolution therefore slows it down, now, does anybody know where I can get a 1.34mm (approx.) square section x 86mm (approx) dia. belt from please ?, thanx., Geoff.
 

davenn

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Thanx every one for the interest, the way I see it is that a thinner section belt will make the tape go faster as it tucks into the drive at it's smaller dia. whereas a thicker belt runs on the outer dia. which takes longer to to make one revolution therefore slows it down, now, does anybody know where I can get a 1.34mm (approx.) square section x 86mm (approx) dia. belt from please ?, thanx., Geoff.


I have never seen a tape player that doesnt have a speed adj.
so find it on or near the motor and adj. the speed :)
 

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