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Miller Ammeter

bodlan

Well Known and Active Forum Website User
VOC Member
Can anyone suggest someone who can recondition these old ammeters?
My No75V has just packed in and since I think it is an original one I would like to get it fixed.

cheers

Dave
 

Ken Tidswell

Well Known and Active Forum Website User
VOC Member
It all depends on the fault. If the insulating varnish on the coil has failed, then it is a little tricky to revarnish the coils, so they produce the correct magnetic field.If the pointer has come out of its mountings , and you have no movement, you have to prise off the metal bezel and disassemble the unit , all this can be done with patience and care and a very small screwdriver.
I have just replaced the glass in mine. i would volunteer but am not prepared to damage someone elses property, only my own, even for cash money. The 3 small tags which are crimped onto the perimeter of the body after several reverses of the metal can break off, but 2 tags will hold one together There speaks a Yorkshireman. I have done 3 so far. If I can do it anybody can.Ken Tidswell
 

bodlan

Well Known and Active Forum Website User
VOC Member
I have got the bezel off. It appears that water has got into it as there is a lot of corrosion in there. The needle is in place but what electrical tests can one do to it?
The varnish does appear to have deteriorated at the ends of the coils though. I tried connecting it briefly to a 1.5v battery but not even a flicker!
 

Ken Tidswell

Well Known and Active Forum Website User
VOC Member
You can see that the pivot is adjustable with a screw slot you can try to unscrew this . Take off the scale carefully with screw driver and see how muchch rust is there , a squirt of Duck oil or WD40 may wash it off. You will have to remove this. tweezers are handy, and small pieces of paper towel
The coil should take 8 amps at 6 volts this is full scale deflection .The deterioration of the varnish may be due to soldering of the wires to the plates at each end. Hope this helps, well done
 

Rob H

Well Known and Active Forum Website User
VOC Member
Can anyone suggest someone who can recondition these old ammeters?
My No75V has just packed in and since I think it is an original one I would like to get it fixed.

cheers

Dave

Hi Dave, these are easily tested to find the problem and disassembled to fix. I have fixed a couple from spare parts from other broken ones. First find out what is wrong, they are basically simple on operation so generally not many things can go wrong, usually the pointer is stuck or the coil is broken

First is to test continuity, making sure the coil is not broken, sometimes the thick copper wire (coil) can break way from the connecting post, this can be re-soldered back on. Remember connecting a battery alone would not show anything as you are measuring current not voltage. To test continuity you can use a multi meter or by connecting a battery and bulb in series with the ammeter. By doing this you can also check the accuracy to see if it is working properly. For example if you use a 12V battery and 36W bulb the ammeter should read 3 amps. Remembering from school, Amps = Watts (bulb rating) / Volts (battery).
Also check that the pointer moves freely and does not stick. Centralising of the pointer on the scale can be achieved by slightly rotating the rear fixed magnet.
More difficult is to assemble different parts from different ammeters, e.g. a coil from one and pointer mechanism from another. You would need a bit of patience with this as the pointer mechanism is quite delicate and will not necessarily move freely without touching in a different coil. I found I had to adjust the shape of the coil a little to make it work.
You should have a go yourself, if you are careful and not heavy handed you can not do much more irreparable damage then if it is not working already.
I would not recommend you remove the actual pointer from its support housing though as if it very delicate.

hope this makes sense and is of help
regs
 

bodlan

Well Known and Active Forum Website User
VOC Member
Thanks everyone for your comments.
I have now established that I have continuity after testing with a bulb as suggested by Rob H. The needle is nicely mounted and swings about freely on its mount but just does not react when the current flows through it. The varnish is definitely broken down though, on the bends, so maybe this is the problem. So is the varnish anything special or can you just paint some ordinary clear varnish on with a tiny brush. Also, what does the varnish actually do to help the ammeter do its job?
 

SteveF

Well Known and Active Forum Website User
VOC Member
The varnish keeps the adjacent turns of wire insulated from each other. If they short together then the magnetic field generated by the current flow through the coil will be too small and no needle movement will happen.
The magnetic field generated is directly proportional to the current flowing and the number of turns in the coil. Shorted coils don't count.

Now that you've cleaned it up and reassembled, my guess is that it should be working. They are simple beasts with not that much to go wrong. How did you test it? A 1.5 volt battery is unlikely to be able to generate enough current to move the needle. A twelve volt battery with a headlamp bulb in series would be a better test as that should require about 8 amps to light, so you would definitely see some movement if it's working.

Cheers - Steve
 

bodlan

Well Known and Active Forum Website User
VOC Member
Yes, I used a 12v battery and a bulb in series and not so much as a quiver when the bulb lit up!
 
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