How to do a basic refurbishment of a rotary dial?

S&H Modell 36
S&H Modell 36

I like working on dials. Many collectors do not, some even hate it and there are those who are afraid of it. So here is an article on doing a basic, simple and safe clean up.

This method applies to most dial designs, as they all have basically the same parts and work in a similar manner. So it does not matter if it is a dial from a ZBSA 24, Western Electric 302 or French U43.

In this case I used a German dial, the so called NrS (Nummernschalter) 36. It is a 1936 design, used for many decades and on many telephones. It first appeared on the Siemens and Halske Modell 36 from 1936, which was the forefather of the famous W48, standard telephone for the Deutsche Post after the war.

This dial was also used on the Heemaf 1955.

First make sure it works!

Heemaf 1955 dial
Heemaf 1955 dial

I cannot stress this enough. There is no point in working on a dial that is broken. I will not magically start functioning properly after a quick clean and polish. You’ll need to do a more in depth restoration or do a proper repair first. If you do not know what you are doing, you may want to take it to someone who does.

Of course you may need to make a small correction or adjustment. For example the finger stop is often bent, inhibiting free movement of the finger wheel.

You may need to oil it first, if it is very dirty and grimy and it does not run smoothly.

So, test it first on an outside line or a small private exchange if you have one. Make sure you test the higher numbers (8, 9, and 0) too.

What are you going to need?

Stuff you need
Stuff you need

Good oil: use good quality oil! Fine machine oil or clock oil. Make sure the oil is silicone free!

 

Brushes: good hard brushes, do not use wire brushes

Toothpicks: very versatile

Oiler or oilstick (you can use a toothpick if you do not have one of those)

Q-tips

Tissue paper

A rotary dial is a precision instrument

When the finger wheel is released after it is turned clockwise, the dial will return to its starting position giving off a number of pulses. These pulses are generated with a certain frequency, 10 pulses per second.

Typically exchanges will accept frequencies between 9 and 13 pulses per second. If the dial runs too slow or too fast, the exchange will not accept the numbers being dialled.

There is a little more to pulse dialling, like make/break ratio. But this is a basic refurbishment, so we’ll leave it at pulse rate for now.

Basic anatomy

Front

Dial anatomy front
Dial anatomy front

1. Spring housing, in this case it contains a leaf spring

2. Fingerstop

3. Finger wheel

 

 

 

Rear

Dial anatomy rear
Dial anatomy rear

1. Shunt contacts

2. Pulse contacts

3. Pulse wheel

4. Governor

5. Wormgear

Disassembly, be careful

This is quite enough!
This is quite enough!

To give a dial a proper clean, you’ll need to partly disassemble it to reach all the nooks and crannies. The mechanical, rear side, usually is accessible, so there will be no need to remove parts there. But the front is a different matter.

Ideally the finger wheel should be removed, but this is not always possible without the spring unwinding.

Uhoh!
Ohoh!

DO NOT UNWIND THE SPRING.

If you unwind the spring, you will need to recalibrate the dial, after rewinding the spring. That is hard to do without specialist equipment.

Dirt and encrustation
Dirt and encrustation

Older dial designs often do not allow for easy removal of the finger wheel. If that is the case, leave it in place. Otherwise this basic refurb will turn into a total rebuild. So be careful and make sure you can remove the finger wheel without too much trouble. When in doubt, leave it in place and clean it as best as you can.

If you can remove the finger wheel, remove the finger stop too. There is usually a lot of encrustation on the inside of it.

Cleaning

Brush those teeth!
Brush those teeth!

Wash the parts you have removed and polish the parts that go on the face of the dial. Warm water and a little soap work well, leave to soak overnight if needed. The cleaner you get them, the better the dial will look. Polish the parts before putting them back on.

Pick those teeth!
Pick those teeth!

The mechanism and electrical components should not be cleaned with water. We are doing a basic refurbishment, not a full-fledged restoration.

Clean all the gear teeth with a brush, toothpick etc. until they are nice and clean. If the black stuff on the teeth does not come off, put a little drop of oil on it, leave overnight and try again. Do remember to clean of any excess oil afterwards with a q-tip or a piece of tissue paper.

Cleaning contacts

Cleaning contacts
Cleaning contacts

The dial mechanism has a number of contacts that are made or broken as the dial is operated. These contacts are made of leaf springs with a small point, that is coated in a special black substance. This substance ensures conductivity and prevents the contacts from arcing electricity.

Clean the leaf springs by brushing them and do not use a metal brush. And old tooth brush is just the thing for this job.

Do not use a file or sandpaper, or you will damage that special coating. To clean the contacts, I run a piece of coarse paper along them. This is quite enough to clean them. You can imagine that drenching the dial in WD40 or another oil, will not be good for the contacts.

Where to oil and where NOT to oil

Oiling away
Oiling away

Firstly: use good quality fine machine oil. Make sure the oil is silicone free. Low quality oil or the wrong kind of oil will coagulate and turn to a sort of sticky yellow glue. You can probably imagine that this will adversely affect the proper working of the dial.

Secondly, use as little oil as you can. Oil attracts dust and that will clog up the dial. Do not spray it with WD40! (Yes, some people actually do that).

Please mind that this refurbishment has to last for decades and not just a couple of weeks: cheap oil will become sticky after a while and too much oil will attract dust, inhibiting the proper working of the dial.

Put a drop of oil on all the points where parts move: ends of shafts, etc. If there is a leaf spring, it is wise to put a little drop of oil on it, if you can reach it. This will ensure smooth winding and unwinding of the spring.

Please note that there is a very important part that should not be oiled.

DO NOT OIL THE GOVERNOR!
DO NOT OIL THE GOVERNOR!

DO NOT OIL THE GOVERNOR

The governor is the bit in the next picture on the right. It has weights mounted on a shaft. When the dial is operated, these weights move outward by centrifugal forces thus acting as a friction brake. This part makes the return of the dial smooth and even, thus ensuring a steady and even stream of pulses. Oiling this part really messes up the pulse frequency of your dial. So do not oil this part.

Checking the dial speed, quick and rough

After cleaning, oiling and reassembly the speed of the dial will be altered and the pulse frequency too. You will need to check if the dial runs at approximately 10 pulses per second and that the exchange will accept the number.

This can be done by simply installing the dial in a working phone and test it on an private exchange or telephone line. Please make sure that you also check the high numbers like 0,9 and 8 and not just the low numbers.

3 2 1 let go!
3 2 1 let go!

You can also compare the speed of the dial with another dial (one that works properly of course), by dialing the highest digit (usually zero) on both dials simultaneously and releasing the finger wheels at the exact same time. This gives you an idea if your refurbished dial is running slower or faster that the test dial.

Keep in mind that as the years passed and dirt accumulated in the mechanism the dial the slowed down a bit. Refurbishing will make the dial run closer to its original factory adjustment. It will not suddenly make the dial run too fast (unless you did oil that governor).

My experience is that after a refurbishment as described above a dial will still function properly, but it will run a lot smoother. And of course it is ready for another couple of decades of use.

If you want to read about restoring a entire telephone, you may want to read how I restored an Ericsson type 1951 bakelite telephone.

25 Comments

  1. Hi, I have just purchased a BELL TELEPHONE VINTAGE M.F.G Company, Belgique..From around the 1950s. I’m able to accept incoming calls with it, but I’m having problems dialing out. It works sometimes but not all the time as the dial appears to be sticking/ slow movement. It’s an intermittent problem, it will work when dialing a number then it won’t. I’m in the UK and need some help/ advice…What kind of brand name of oil would you recommend as I have been looking for this ‘silicone free’ kind of oil that you have mentioned but can’t seem to find anything when I ‘google’ it… And how much would it cost to have this restored as I’m in the UK and don’t use euros…Where are you based, or is there anywhere local ie in the UK that you can refer me to…Thanks in advance Barry.

    • Hi Barry, thank you for your comment. BTMC used a number of dial designs and I am not quite sure which one you have there. The erratic behaviour of your dail my very well be caused by dirt or the dial speed is just a little out of spec. In any case, some cleaning will be needed.

      I am not sure which brands are available in the UK but a good quality sewing machine oil will be good for oiling a dial.
      An example of a lubricant with sillicone is waterproof bike chain oil. I do not recommend that for dials.

      I am based in the Netherlands. I can of course refurb your dial, but shipping costs to and fro will set you back at least 30 e.

      I will send you this reply by email too. I suggest you send me a picture of your dial, so I can at least give you some pointers for cleaning it.

      Regards, Arwin

  2. Hello, when you disassemble all that is behind the dial, regarding the spring, it is enough to go slowly by lifting it, to count the number of turn that will do by releasing it. Once cleaned, it’s simple, you count the same number of laps that it did by releasing it and you put everything back in place correctly. This should be enough once everything is cleaned up. I never had a problem with my dial phones .

    • Hi Yuri, thanks for sharing your view on this matter. Please mind that this article is about a basic restoration, so I wanted to keep things simple and for the novice easy to do.
      Besides that, with many dial designs, especially older ones, it is impossible to remove the finger wheel without unwinding the spring.

      In the future I intend to publish more article on this matter, like articles on specific dial designs, how to calibrate a dial without a dial measuring instrument, and how to do full restorations of dials.

      Best regards,

      Arwin

  3. Hello I am trying to change a telephone dial card in a rotary phone for use in a production of ‘Dial M for Murder’ The phone we have in the props cupboard has a slightly broken plastic cover of the dial card and consequently I can’t see the hole to help remove the face. Do you have any suggestions? I have photos I can send you.
    Any advice, greatly appreciated!

      • Arwin, thank you for your response. Other rotary phones on that same line work fine. It’s just this rotary phone that does not work. I can get dial tone with this phone, but the dialing does not work.

        Again, thank you for your response! I await your comment!

        Susan

        • Hi Susan, either the telephone is wired wrong or the dial does not run at the right speed and needs adjusting.

          The first problem is hard to solve, via this medium. The possibilities are endles.

          The 2nd problem is easy to test. You can help the dial along with your finger, on the return stroke, making it run a little faster. Or you can hold it back a little with your finger on the return stroke, making it run a little slower. If you do this dialing 2 or 3 and it breaks tone, you know it is running too slow or too fast.

          Regards,

          Arwin

  4. Arwin –

    Thank you for your response. The phone jack at the location where I would like the black rotary to be placed is currently a white rotary phone (which works, by the way). I tried the black phone on another rotary-accepted phone jack and I run into the same problem as above. Get dial tone, but cannot dial the phone. To me it seems like something’s askew with the rotary itself. Are there websites (yours perhaps) where I can get instructions on how to look inside the phone and make sure it is working/connected correctly?

    Thanks again!

    Susan

    P.S. I it normal to have only three of the four colored wires? Red, yellow & green and not black? Could that be the problem?

  5. Many ex-BT engineers are on the Facebook page GPO Telephones and will be willing to offer advice for the phones they used to work on. In the past, we used to change the dial if we thought it would be quicker than trying to work on a dial at the customer’s house or office.

  6. I have a 1910 Rotary dial phone Danish phone I need to know if you have shematics for this type of phone or some idea how to convert from pulse to tone so that I can use it, it is in good working condition, thank you very much

  7. Arwin, I got another fg.tist 282, and it has two issues:

    1) How do I loose the finger wheel to align it with the numbers?
    I have to unscrew that hexagonal central piece? With which tool?

    2) Nobody hear me, and I got a big noise (a crack) on receptor when
    the hook goes up (when I pick up the phone). It seems a power
    overload in some component. Even if I unplug it from the wall, I hear
    that “discharge” (once) if the hook goes up.

    Don’t know If I made it clear, sorry. Any tip please? Thanks in advance!

    • Hi Fabricio, thank you for your message.
      1 Yes, unscrew the hexagonal nut. There may be a little play on the fingerwheel.
      Also, you may want to check if the number ring is positioned correctly. If that does not work, on the back the mechanism is mounted on a metal plate which is fastenend to a plastic disc with 2 bolts. If you undo those you can move the mechanism a little.
      2 is there a diode in the handset? This should prevent loud clicking.
      Regards, Arwin

  8. Hi Arwin,

    I just got a RTT56A, looking like the one one your site (http://www.matilo.eu/wp-content/uploads/2015/11/RTT-56-Copper.jpg). Most of the time the rotary wheel runs ok but sometimes it returns slow or gets stuck when I pull out my finger. I guess it needs some cleaning so I was wondering if I can remove the finger wheel without worrying about unwinding of that spring you mentioned. Is this also the case for this model? I saw the finger wheel is attached with a single screw but don’t want to unscrew it and find myself in trouble.

    Kind regards,
    André

    • Hi Kevin, this could have a number of causes. First make sure you dial mechanism is clean and oiled, as explained in the article. Have you done that yet?
      The particular dial on the W49 is an NrS38, the same as in the article. Make sure the fingerstop is not bent. Often a bent fingerstop inhibits the proper turning of the finger wheel.
      Turn the dial on it’s side and look through the fingerstop. Give the dial a full turn and check if the fingerwheel does not touch the stop.
      Do the 3 and 6 misdial? Always or just sometimes? How do you know it is the 3 and 6?
      Regards,
      Arwin

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