1917 Voltmeter fix/rebuild

[70sWagoneers]

Hello, I'm trying to fix a voltmeter for a 1917 Detroit Electric car. I've sent it to one shop so far. After about two months they finally looked at it and said they couldn't fix it. The boss said to try and fix it myself. So... here I go. Maybe you can help?

 

The electric car has 14 six-volt batteries in series for 84volts nominal. The charging voltage tops out at about 113volts. The scale on the meter face reads 0-125 volts. From research, most meters of the day had ratings of 50mV  0-1mA. So we could assume this rating for now.

 

I can obviously see the wire wound resistance coil in the meter is corroded and broken/disconnected. There is also a larger wire wound resistor separate from the meter case that is wired in series with the meter to provide added series resistance. So my total resistance for the meter will be large supplemental resistor, the resistor in the meter, and the resistance of the movement itself. So I understand I'll need to replace that with new NiChrome resistance wire. To guesstimate at the overall resistance I need, I will unwind to original broken resistance wire and measure all the little lengths I pull out. The larger resistor Ive calculated at about 6,840ohms. Ive yet to measure the smaller resistor. Since the accuracy of the gauge will depend on the resistance of my coils, I will experiment with different lengths of wire. This part I'm not totally daunted with. But doing the math I can't seem to wrap my head around. Can anyone calculate about what total resistance I need? I can't

 

So if that wasn't enough, the moving coil looks like it has a burned shorted spot in it too. It also does not have continuity. So what is the wire wrapped around the moving coil made of? It is very fine copper colored wire. Is it just bare wire or is the wire coated/insulated? And is it resistance wire or just plain copper conducting wire? And then do I need to replace it with a certain resistance or a certain mass off wire?

 

[Alfre]

The  copper wire is insulated , you will have to determan  how much Resistance is in a given  length and then how much total length of all the wire is to determan total resistance.

This type of wire was used on the back of older TVe  picture tub sets and some of us ham operators have some. 

Al

[jp928]

I would think that somebody familiar with that style of volt meter would be able to give some guidance on the resistance required for a meter of that range (0-120V).  Your 50mV= 1mA gives a resistance of 50 Ohms from Kirchoff, which is  1000 Ohms per volt, a commonly quoted number. See the formula for extending range here - 

http://www.electrical4u.com/working-principle-of-voltmeter-and-types-of-voltmeter/

New R = ((External V(125) - internal V (.05)) * internal resistance (50))  / Internal Volts - .05

I get 125k Ohms (actually 124950) for a movement with 50 Ohms internal , and FSD at .05V. If your meter's external has shorted internally that would explain the internal coil being fried?

Is it possible to find a replacement movement of the right spec and fit to your face with a new external resistance. Any unit the right size whose internal spec is known would do - just calculate a new external resistance.

 

HTH, hope I didnt stuff it up

jp 26 Rover 9

[37_Roadmaster_C]

The coil of fine wire is simply a magnet coil.  The wire is most likely varnish insulated magnet wire.  The wire is common to motor rewind shops, however the gauge of the wire is critical and very fine gauge wire is not common in rewind shops.  The number of turns is the most critical item as the dimensions of the coil will determine the length required for the number of turns and therefore the resistance. The coil is critical in all specifications: number of turns, gauge of wire, coil dimensions and shape and the location and attachment to the spring pointer.  I do not want to be a spoil sport, but the chance of someone not familiar with the construction and forming methods used in these meters is unlikely to be sucessful with this type of repair.  I know how it was done and I would never attempt the repair.  As for how to get it repaired, I would try to contact people who advertise repair of old gauges and see if any of them can recomend someone to do the repairs.  I do not know of anybody to point you towards. Sorry.  You might try to ask on one of the antique radio/Tv forums on the web.  Also  you could try amature radio forums or possibly a local ham radio club for pointers.

   One caution....  If you attempt to repair be very careful and diligent about detailed documentation of what you have.  Once disassembled and unwound the coil details will be lost.  Remember, a meter of this type is a magnetic device and coils form the electromagnetic portion of the movement.  Caution and care is most important.

 

Good Luck and let us know how it turns out.

 

Robin

[Spinneyhill]

I know a fellow who winds new magneto coils, armitures etc. They have fine lacquered wire like that and each layer is insulated by paper. Doug has a winding machine that counts the turns as it goes and gives the exact value required. He impregnates it with epoxy under vacuum for longevity and insulation with no paper layers. His rewound magnetos (factory new when he received them) have been used in modern Mercedes and BMW racing cars because they are more reliable than factory new examples.

 

So you might try approaching a magneto refurbishing person, although if they are any good they are usually very busy.

[Digger914]

What you have here is a D'Arsonval meter movement. Don't know if these people are still repairing, but if they arent, they probably have a new one thet will fit inside your old case. 

Instrument Meter Specialties (661) 945-5664

339 E. Ave. K8 STE 105 Lancaster, CA 93535