Advice needed - slipping tuning drive cord

[westcliff]

A friend has asked me to fix a Ferguson 329A radio, which won't tune.  I expected to find that the drive cord had snapped and that restringing would solve the problem.

What I found was one of the two cursors was jammed at the end of its travel, so I freed it, removed both cursors for cleaning and greased the track they run along,  The drive cord was intact

This restored tuning and I was able to tune from the tuning capacitor being fully closed to fully open.  Unfortunately, when operating the tuning knob in the opposite direction, when the tuning capacitor is around 80% closed, the tuning drive spindle turns and the dial cord doesn't move.  Is there a way to make the spindle more "grippy"?

Gus.

[Crackle]

What is the cord made of, is it normal braided type or is it wire. I have seen some use mono filament fishing line, this never works satisfactorily.
Applying the correct tension is important, and having the correct number of turns on the drive spindle.
If it is normal braided cord and it is still slipping then I rub some stiff wax to the cord whilst turning the drive spindle. The wax helps create a controlled amount of friction between the cord and spindle.
If all else fails then an extra turn on the spindle will help a lot, but this may involve re-stringing.
Can you release tension on the end spring, and put another turn on the spindle.
It looks a complicated drive cord.
Mike

[Terry]

Rosin? The stuff that string players use in their bows to make them grip the strings?

Also, as everything seems to work OK for 80% of the range, could the tuning gang need lubrication? Try releasing the screws that secure the drum to the spindle so that it moves freely, then check both the gang and the cord drive to see which is causing the problem.

[Refugee]

I have restrung with necklace cord with success several times including a Phillips set.
A dried up cord that is otherwise OK can be painted with a mixture of capacitor wax and alcohol to make it grip again. The alcohol dries off and leaves the wax on the cord.

[Murphyv310]

I've never had great success with wax. What I have found is the spindle on the tuning knob shaft gets very shiny so the cord slips, a rub with emery paper and the cord cleaned with IPA always works for me. As Terry says the tuning cap itself could be stiff and in need of adjustment and lubrication.

I'd advise you clean off the grease off the cursor track, just make sure it's clean and use a coupl of drops of engine oil instead. Grease drags and can add to the resistance and make the cord slip.

[Crackle]

It is important the cord has a controlled amount of slip on the drive spindle.  I have learned some important lessons from using hand winches to operate the sail ropes (sheets) on sailing boats. (although these were never waxed, the grip between the rope and winch drum is controlled by tension on the lazy end and the number of turns.)
During the process of turning the tuning knob on a radio the cord works its way up the side of the drive capstan, if there is no slip preventing the turns from sliding down the side,  then the high end of the cord is liable to tumble back and cross over the cord causing it to lock on the capstan drive. Also when you get to the end of the travel the cord needs to slip so that it cant be over turned to the point of breaking the cord.
I have only ever used wax to create the controlled amount of slip that is required. It needs to be the correct type of wax, traditionally beeswax was used, it is firm yet easily kneaded.
The wax found on the surface and ends of waxy capacitors can sometimes be a bit soft, it can work, but it is not as suitable as beeswax.
Sometimes harder candle wax may be ok to use, did you ever make those little toy tanks with an elastic band, cotton reel, slice of candle and a matchstick. They worked by the controlled friction caused by the wax on the side of the cotton reel.

Mike

[Murphyv310]

Absolutely Mike.
I don't go daft on the spindle, just enough to give it a "key" for the cord to grip on. Making the surface too rough will make to cord bunch and of course it then gets tight, all we want to do is remove the glaze. The surface area is small on these spindles intentionally so at the end of the travel of the tuning cap you cannot snap the cord by excessive torque applied to it. The whole arrangement is a compromise.

[boater sam]

Philips are famous for their complicated cord drives, but they often used steel wire in part to get them to work well.
I thought they were possibly the worst to replace, with the plasic drum pulleys and bowden cables, but having to repair a PYE  P76F this week I have change my mind. If you think that is a bit extreme, have a look at the stringing instructions on the service sheet.
There are 2 sets of cord on the same spindle, but its all one piece of cord!

[boater sam]

Philips are famous for their complicated cord drives, but they often used steel wire in part to get them to work well.
I thought they were possibly the worst to replace, with the plastic drum pulleys and bowden cables, but having to repair a PYE  P76F this week I have change my mind. If you think that is a bit extreme, have a look at the stringing instructions on the service sheet.
There are 2 sets of cord on the same spindle, but its all one piece of cord!

[westcliff]

Thanks everyone for the advice. I went with the wax solution, as there was a lot of wax underneath the radio's chassis (under the mains transformer to be precise). The tuning drive is now working.

There will probably be more to come on the subject of this radio, as my friend has now asked if I can get it working.

Gus.