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Old 13th Jul 2006, 9:10 pm   #1
radiogammon
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Default Slipping drive cord

Has anyone any tips on stopping drive cord slipping on the small drive spindle pulley. I have been overhauling a Unitra K-100 and everything is now working well except that the drive cord is slipping when I try to tune in one direction. It seems taut enough and I have lightly oiled the pulleys and tuning cap spindle. It is possible that a small drop of oil may have got on to the cord. Is there any powder or such which I could put on the cord to improve grip.I don't want to habve to re-string it except as a last resort.
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Old 13th Jul 2006, 9:22 pm   #2
Nickthedentist
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Default Re: Slipping drive cord

Hi John,

First make sure that the cord is tensioned enough, that the cap is really free to turn, and that the pulleys are all nice and clean (use meths or IPA).

Failing that, try some rosin, as used by string players to make the hair on their bows grip the strings of their violin etc.

Nick.
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Old 13th Jul 2006, 9:52 pm   #3
jim_beacon
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Default Re: Slipping drive cord

I've had this problem when using new dial cord witha smaller diameter than the original (eg using "spiderwire" braided fishing line). I found that an extra turn was sometimes needed around the drive spindle.

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Old 13th Jul 2006, 10:04 pm   #4
Radio_Dave
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Default Re: Slipping drive cord

I've found that if the cord is wound around the tuning spindle in the wrong direction it can work one way but then wind over itself when turned the other and slip...Might be worth a look?
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Old 13th Jul 2006, 10:11 pm   #5
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Default Re: Slipping drive cord

I had this problem on an HMV 1375 and it was caused by a stiff bearing in the tuning capacitor. Careful cleaning and lubricating cured the trouble.


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Old 13th Jul 2006, 10:16 pm   #6
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Default Re: Slipping drive cord

try pulling the cord through a block of beeswax, that often does the trick.
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Old 13th Jul 2006, 10:21 pm   #7
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Default Re: Slipping drive cord

I usually find it is also worth roughening the surface of the drive spindle with some emery cloth or similar, as the action of the drive cord tends to polish the surface after years of use, causing slipping. .

Regards, Mick.
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Old 13th Jul 2006, 10:30 pm   #8
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Default Re: Slipping drive cord

I've met this problem with 2 of my Bush VTR330s, the brass hub of the drive pulley is of a very small diameter and had worn in both cases so was little more effective than wrapping half-inch steel rope around a sewing needle and expecting that to turn!

On one set, I simply replaced the cords (it was the pointer drive cord that was slipping in both, so at least both sets could still be tuned) with new cord that was about twice as thick as the original. I roughed up the surface of the brass hub (I know, I know....) and that was one sorted.

On the other, the cord was less worn, so was still quite thick enough. This one was approached rather more scientifically.
Having found a bit of thin (24swg) brass sheet that had been used in the restoration of something else, I cut from this a strip whose length would allow it to fit snugly around the hub of the 330's tuning control. After sticking it in there with good old "JB Weld", the whole wheel was attached to a face plate and spun in the lathe to remove (by turning down) the excess sticky product, leaving a less worn surface for the cord to run on. The point of doing it this way was simply to establish whether it was feasible.

It certainly proved so, the very slight amount of extra material on the pulley only required 3mm more stretch from the spring and so far, so good.
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Old 13th Jul 2006, 10:56 pm   #9
Nickthedentist
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Default Re: Slipping drive cord

What a lot of useful replies, must try to remember them
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Old 14th Jul 2006, 8:29 am   #10
radiogammon
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Default Re: Slipping drive cord

Thanks everyone, as Nick says, for so many useful suggestions. I will start with the easiest, rosin or beeswax and go from there. The drive cord on this Unitra K-100 has quite a number of pulleys as it operated both the tuning condenser for MW and LW as well as slug tuning for FM. I have lightly oiled all the bearings etc but it made no difference. The drive pulley is quite small so may have become too smooth. I only want to remove the cord as a last resort as re-stringing is one of the jobs I hate most (no patience). The teak veneered cabinet of the set is in A1 condition, however, so worth persevering. I have already sorted the sound output by adding a bit more selective feedback, as Polish sets in particular always seem to sound very harsh to my ear.
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Old 14th Jul 2006, 9:01 am   #11
Mike Phelan
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Default Re: Slipping drive cord

If the spindle is easy to remove, another way is to "roll" a small rat-tail (round) file around the groove on a flat surface, effectively knurling it.
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Old 15th Jul 2006, 1:33 pm   #12
radiogammon
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Default Re: Slipping drive cord

Nick,
Got a small block of rosin this morning. It is pretty hard stuff and I wasn't sure what to do with it, so filed some into a powder and applied it to the dial cord. It worked a treat and gave the cord a great grip round the drive spindle. No slipping and certainly an easier solution than re-stringing. Many thanks for the idea.
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Old 15th Jul 2006, 1:39 pm   #13
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Default Re: Slipping drive cord

Glad it worked, John.

Musicians would use it by rubbing the hair of their bow backwards and forwards across the block a dozen or so times. This may have worked for your cord, and it may even be possible to apply it like this in situ too.

Nick.
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