Tuning Drive and Scale Restoration

Tuning Drive Cord

The tuning drive cord often breaks or becomes weakened. Replacement cord is available from component suppliers. Normally the broken cord will retain some of its shape, and this should be carefully noted before disturbing it. Bear in mind however that it could have been replaced previously, and may not be correct!

On many sets, the drive cord arrangement is fairly simple, but a few can be more complex. If you cannot work it out, obtain the service sheet for the set.

If you are not thinking clearly, it is easy to get the pointer moving in the wrong direction, or the knob working backwards. The pointer should move towards the high waveband (low frequency) end of the scale as the vanes of the tuning capacitor close. In addition, the pointer should move to the right as the knob is turned clockwise, otherwise it is confusing to operate!

Nigel Hughes adds this suggestion:

If you cannot find a supply of tuning drive cord, try to find strong linen cord from e.g. John Lewis. Linen cord has virtually no stretch, which is vital for some complex drives. If the cord is at all elastic, the pointers can stay still while the tuning condenser is rotated slightly. Linen is best, cotton next and nylon is awful.

I have also seem recommendations for certain types of fishing line. Try taking a piece of the old cord to a local fishing tackle shop and see of they have anything suitable. The cord must have the right amount of grip, and must not stretch.

John Buckly has been trying to obtain some drive cord, without much success. He says:

I have contacted a number of vintage radio restoration specialists, and in the main they tell me that no component supplier stocks this now - the principal reason being that is is no longer manufactured. They all suggest the use of a suitably chosen length of fishing twine. There are a number of different gauges of this, so it is wise to take along a piece of the original for comparison. I am told that the end result is "excellent" if the correct twine is used.

Whatever cord or twine is used, it's characteristic must display little or no stretch, and good grip or the receiver scale calibration won't correctly track the tuning mechanism when in use.

I was lucky in my search - one specialist was able to supply me with a length of proper tuning cord, but even his stocks are very limited, so probably can't generally supply more than a very short length as he obviously needs it for his own use. I dare say there may be other restorers I didn't contact, who may be able to supply small lengths, but I guess they will also be in a similar situation.

If anyone knows of a specific type/brand/thickness/whatever of fishing twine to use, please let me know so I can add the details here. It'll help those of us who know nothing about fishing!

Doug Baird (VE4XZ) said:

I hit upon two materials that are not too bad for drive cords. One is the Dacron backing used on flyrods, and the cheaper actual flylines. You DON'T want the soft vinyl types but the real cheap ones that are not quite so..... soft.

Some Philips sets, such as the 310A and 341A, have sections of the tuning drive cord threaded through tube sections similar to the outer of Bowden cables (such as those used on bicycle brakes). This allows the tuning capacitor mounting on soft rubber grommets to be damped properly (with most sets the cord tension defeats the purpose of the grommets to some extent). The nylon fixings for the bowden sections tend to break, and repairs require some ingenuity - I have used epoxy resin.

Dave Mycock has a better suggestion:

I would like to make a suggestion as to the repair of the ends of outer bowden cable retaining fittings.

Take a 1/8" pop rivet and knock out the steel "nail" from the middle, this will leave an aluminium "top hat" section. With a drill just slightly larger than the outer diameter of bowden cable, drill from the top hat brim side partly through the rivet. The bowden cable then fits into this hole and the shank of the rivet fits into the support brackets a treat, making a neat repair without using epoxy resin.

The tuning pointer can be repainted if necessary. Enamel paint intended for plastic construction sets (such as Airfix or Humbrol paint) is ideal for this. You may also wish to repaint the plate behind the tuning scale. Car spray paint is ideal for this, and its use is described later. You will need to remove the plate or mask the chassis carefully.

Tuning Scale

The printing on the tuning scale glass is usually very soft and can easily be removed if the wrong cleaning product is used. The best approach is not to clean the printed side at all unless it is really necessary and you are sure the printing is sound. Even then, you should only use a dry duster or possibly a tissue slightly dampened with water. Be very careful, as a replacement tuning scale will be almost impossible to obtain.

If the printing is flaking off, it is worth spraying it with a clear lacquer to hold the printing that remains in place. DIY stores sell Plasti-Kote Special Purpose Polyurethane Varnish (Gloss Finish) or something similar which is generally suitable, but try a small amount in a corner first to make sure it does not soften the printing.

Gary Tempest commented:

I tried a lacquer on a pierce of old glass. One sputter from the spray can and it looked awful. I am glad it was not the real scale! What is bad about the tip is that it goes against good restoration and conservation in that it is irreversible. What I did was go to a picture framer and get a pierce of glass cut of the same size as the scale. The glass was the thinnest he had - about 3mm. Then after cleaning everything up I backed the scale with this glass using cloth adhesive tape to hold the two firmly together. Had to pack out the scale fixings a little but this was no problem.

If some of the printing is missing, and you are reasonably artistic, you may like to repaint it. Remember that you are working backwards, and that the first layer of paint is the one that will be seen. Enamel paint is ideal, but if you have previously sprayed the glass with lacquer, you should try the paint in an invisible corner to make sure the two do not react.

The outside can be cleaned with Auto-Glym car windscreen cleaner or another glass cleaning product that dries to a powder which is then polished off. Make sure this does not get onto the printed side.

Nigel Hughes suggests:

I am in process of restoring a 1936 Philips 585U radio. This has a thin translucent plastic tuning scale, sandwiched between two layers of celluloid. The scale is very fragile and I dared not even wash it. In case of disaster later, I decided to try to reproduce the scale by scanning it with a flat bed scanner and touching up the existing damage using a photo editing program on my PC. I use Corel PhotoPaint, but many other programs do similar things. I then printed out the result on Hewlett-Packard transparency material in my Deskjet 850C colour printer. This gave me a very usable copy which I have kept in case of need. It occurs to me that a cracked or broken painted glass scale might be rescued this way, provided that the scale was still fairly complete. The photo editing programs would certainly enable a crack to be edited out. Alternatively, if a glass scale is completely missing, it might be possible to find somebody who could lend you one to copy.

The only problem with this is that inkjet printers don't print in white. So if the dial has white text or markings you will have to use a different colour and hope it doesn't look too wrong.

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No part of this website may be reproduced in any form without prior written permission from Paul Stenning.
All details are believed to be accurate, but no liability can be accepted for any errors.
The types of equipment discussed on this website may contain high voltages and/or operate at high temperatures.
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Last updated 14th April 2006.