Dial Cords and Drives PDF Print E-mail
Written by Bryce Ringwood   
Friday, 18 April 2014 17:40

Radios have a dial drive mechanism to translate motion of the tuning knob into motion of the dial pointer and increase the number of turns required to move the vanes of the tuning capacitor. A system of cords and pulleys is used to move the dial pointer horizontally. This article explains how to reair broken drives and the principles of operation.

The simplest radios do not have a dial drive. The tuning knob connects straight on to the tuning capacitor and a pointer on the knob points to the frequency marked on the radio cabinet. Many transistor radios adopt this simple system. At the other end of the (er.) scale we have complicated arrangements to make tuning easier for the user. Such complexities are by no means confined to military and "communications" type receivers.

 

Tuning drives considered here are:

  1. Dial cord
  2. Friction Drive
  3. Gear

or any combination of the above. The Eddystone drives below uses all three.

Eddystone 898 Dial Drive (Viewed from the Rear)

Dial Cord Drives

If you are reading this, it's probably because you have a dial cord problem. Either the cord is slipping, or broken. If the cord is slipping, take a moment to photograph the cord stringing (or reeving) because you are almost certainly going to re-string it. Do this also if the cord is broken, but otherwise intact.

This simple precaution will save many hours of time trying to second-guess how the dial cord was assembled.

Most of the time the dial cord will be missing, or lying at the bottom of the chassis in a most diagreeable mess

In quite a number of cases, the cord will have been replaced by someone who lost patience, or lacked the skill to correctly string the cord, and the assembly works incorrectly, slipping, tuning partially, or "going backwards".

In this case, try to find the correct stringing from the radio's service manual. Sometimes the manual for a related radio is available, which can provide insight into how the stringing was intended.

A typical stringing diagram for the National NC109 receiver is given below.

NC109 Dial Stringing Diagram

The lower stringing diagram is quite tyical. Note that the cord passes round the spindle attached to the tuning knob, then to a drum attached to the tuning capacitor often via a pulley. The cord is tensioned by one or two springs inside the drum. The cord runs from the drum to another pulley in a horizontal line, behind the dial scale then over another pulley and back to the spindle. Normally, the cord has no more than 1 1/2 turns round the spindle, and the amount of cord round the drum is only just sufficient to unwrap in a tangential direction.

The pointer is attached to the horizontal portion.

The cord itself is made from nylon fibre covered with a woven nylon sheath. It comes in various diameters from 0.3mm to 0.7mm, or more. If you see a pile of steel wire, it probably does not come from this arrangement, since wrapping wire round the spindle doesn't work. Wire is sometimes used to drive the scale pointer from the drum, with the normal cord driving the drum round. At this point, I would like to be able to suggest alternatives to dial cord, since you have to make the effort to purchase the correct material from specialist electronic stores. Unfortunately, I have had no success with any of the suggestions made on the Internet and elsewhere.I have found that using smaller diameter cord than the original often works, but the reverse is not true.

The dial cord operates under tension provided by springs inside the drum, or elsewhere. A tension spring on the horizontal portion is a bit suspect. Too much tension and the dial cord may slip or break. Too little tension will also cause the cord to slip.

 

Re-Stringing the Cord

Presumably the set worked when it was new. (Not always a valid assumption). After removing the old cord you need to check that all the pulleys run smoothly (the NC109 didn't use pulleys that move - just fixed studs. Not good) and that the tuning capacitor also turns freely, without the moving vanes shorting the fixed ones. Check the pulleys aren't sticking or broken. Use a very light oil for lubrication. I generally use sewing-machine oil.

Check the old cord and try to figure out why it broke. You need to piece it together (if it was the original) so that you can make the new cord exactly the correct length. If the spring(s) are broken or missing, they can be replaced by modifying springs available at your local hardware store.

Now try to visualise how the cord arrangement will work. You want the pointer to move to the right when you turn the tuning knob clockwise. At the same time, you want the indicated frequency to increase (wavelength to decrease) as the tuning capacitor vanes open.

The actual re-stringing will probably require an enormous amount of patience. It can take quite a few DAYS to get it right. When finished, check the pointer and tuning capacitor all move in the correct direction and that the arrangement doesn't stick, slip or bind at any point. The cord should not roll over itself on the spindle or the drum. If it does, it will be unreliable, so you will have to start all over again. Tensioning the cord requires that it be just the correct length. Each end of the cord should be finished with a small loop. The original set makers would have used a brass ferrule to secure the loop. A non-slip knot will also do the trick. I generally put a dab of paint over the knot.

During the stringing process, you can sometimes anchor the free end using a bulldog clip.

If the resulting drive seems to be good in every respect, except that it still slips, avoid the temptation to add more turns than stipulated round the spindle. You will know if you have too many, because the cord will travel from one end of the spindle to the other and of course, once it rolls into the spindle bearing, it will probably snap. The same thing will happen if you use too big a diameter cord with the correct number of turns. Sometimes you have to roughen the spindle with coarse emery paper, but most often the problem is caused by a stiff pulley, a tight tuning capacitor or the dial pointer failing to slide. If the cord is too tight, it will slip - it will also do so if too tight!

Philips radios require special mention. Their dial drives are often quite confusing, involving dial wire travelling inside spring cord and many other interesting features. Try to get the manual at all costs. Some Grundig and Radionette also deserve special mention - they have a clutch arrangement that comes into play when you switch to FM. The tuning knob then operates a separate FM tuning capacitor, or inductive tuner.

Friction Drives

This section covers Muirhead slow motion drives, Jackson Brothers Ball drives, and so on.

Jackson Brothers Ball Drive - Side View Jackson Brothers Ball Drive - Showing Ball Bearings

 

The above shows two views of a ball type slow motion drive.

Planetary drives consist of a spindle with usually three disks that roll against it. The disks also rotate inside an outer stationary ring. The output shaft is attached to a frame carrying the disks. The disks may be split, a raised portion of the input shaft sits in the split. In all honesty, I have never had one of these go wrong.The Eddystone receivers also use a single large disk riding on a raised section of the tuning spindle. This disk is concentric with a large gear that turns the dial cord pulleys. I seem to remember that on at least one of their sets, you have to tighten the tuning knob so that the disk remains correctly engaged. Some of the radios use a wire (770R) and others use a dial cord. (898 Dial, for example. Not sure about the 940 - mine has a cord).

Ball drives use ball bearings lubricated with viscous grease in a planetary arrangement. If they get dirt in the mechanism, they become lumpy and uncomfortable to use. You can try thouroughly de-greasing the drive, hopefully dislodging any dirt particles in the process. The drive will then have to be re-packed with special viscous grease (available from Rocol). If you attempt disassembly, the metal lugs holding the thing together may break off - then you will have to throw it away.

Other friction drives (e.g. Atwater Kent) use rubber rollers that are either perished or missing. You will have to use a bit of ingenuity to replace these parts. An adhesive like "Wonda Fix" sets like hard rubber - but may be too hard. It can be machined quite easily. It can also be used to hold a hard rubber sleeve in place. I found the best way to machine rubber was to have a normal cutting tool that you might use for brass, freshly ground so that it was as sharp as possible, then to run the lathe at high speed removing minimal amounts of rubber at each pass. Try freezing the rubber prior to machining . Good luck.

Gear Drives

Radios with gear drives include the AR88, B40 (Huge gears and a chain drive), R-390 etc. and the Pilot H664 domestic radio set. The Eddystone radios have gears, but they actually use a combination friction and gear drive (see above).

Slow Motion Tuningg Drive from a Marconi SF/IF Unit

 

A gear drive from a Marconi SF/IF Unit.

The best plan here is to avoid trouble in the first place. If you're planning on buying a radio and the gear train looks grimy and unkempt - then providing nothing is broken (try it) - that's OK. If the radio is 50 years old and the gear train looks bright, shiny and new - be wary. Be extra wary if it doesn't feel right. Someone might have polished it up a bit with a Dremel - this could spell trouble.

I'm going to recommend nothing harsher than a toothbrush and degreaser - such as petrol. Sometimes its best to degrease using lots of sewing machine oil. Most domestic radios have the gear drive built in to the tuning capacitor. You may, however, encounter mechanical miracles, such as the Pilot H664, with its worm drives, quadrant gears, rack and pinion mechanism and so on. This is far and away more complicated than any communications receiver, bar the R-390

If you MUST - and sometimes you have to - disassemble a complicated drive - be sure to photograph everything first. You will NOT remember how to put everything back together. Make notes on how you took it apart, and be sure to reassemble in the correct reverse order. This is not enough-you also have to understand how the mechanism works. Also try to remember or note what lubricants were (or seem to have been) used.

Gear drives use anti-backlash gears. These can be clearly seen in the above image.Two identical gears on the same spindle are forced apart with a small spring. When re-engaging the gears to the matching pinion, the teeth need to be forced together compressing the spring slightly so that there is no 'play' in the mechanism. Backlash is annoying because you overcompensate when tuning one way, then there's an annoying fraction of a turn on the tuning knob when you try to go back. It can also be caused by a gear being loosely attached to its shaft.

The gear train in the R-390 is used with the band-change mechanism. In order for the radio to track correctly, the whole mechanism is also linked to the tuning drive. The R-390 has to be mechanically as well as electrically aligned. I would not attempt to dismantle this gearbox.

Modern Radios

Modern radios, such as the Drake R8, iCom R2 and so on have a tuning knob, but its not connected to a tuning capacitor. As you turn the knob, a device called a shaft encoder sends pulses to the tuning circuitry. This tunes the set up or down by the number of Hertz or kHz you have programmed the set for. The Grundig 700 also has this arrangement. I guess that if the shaft encoder goes wrong, it must be replaced with the correct part.


Resources

Try to find a copy of Rider's perpetual service manuals. There is also a book of stringing diagrams from oldtubes.net. The chances are that you won't find what you want, but you may get a good idea.

 

Last Updated on Saturday, 19 April 2014 16:51
 
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