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!
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?
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)
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.
1. Spring housing, in this case it contains a leaf spring
3. Finger wheel
1. Shunt contacts
2. Pulse contacts
3. Pulse wheel
Disassembly, be careful
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.
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.
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.
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.
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.
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
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
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.
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.