Moving Coil Repair

[AgX]

Ever done that?

I got me a Leningrad 8 meter. The digit

does not go up beyond 3/4 of the scale
does not go down below 1/10 of the scale, unless I strongly tick the body
at changing sensitivity ranges the results are 2 or 3 stops off

I am not inclined to repair that meter, it makes a good enough show piece. But I am not shure I ever came across any meter with a faulty moving coil.
Have you repaired a moving coil? And how? Aside of applying a micro dot of lubricant to the bearing.

 

[Bill Burk]

If you can check the magnet, it might have flecks of metal shavings that became attracted over the years. Cleaning the magnet, removing bits of metal, may help sticking needle.

 

[AgX]

Dirty magnets are not something I have thought of yet. Thank you.

 

[shutterfinger]

Despite what it does electrically you should be able to physically move it from one extreme to the other. If it will not go to the extremes then something is blocking the path or the pivot is worn.

On VOM's I have found the zero adjustment turned 3 to 5 turns clockwise effectively putting too much pressure on the movement preventing it from moving freely.

 

[E. von Hoegh]

You can remove magnetic debris with sticky tape and a lot of patience.
The zero adjustment is usually an eccentric pin moving the outer fastening of the hairspring, it goes back and forth - try moving the zero screw through a full turn, the needle will swing back and forth. There is usually one pivot jewel mounted on the end of a screw to set the endplay of the pivots; this should be (very carefully) turned in until the needle is constrained, the backed off until the movement has just barely perceptible sideplay - say equal to the diameter of a hair (.002"). Oiling the pivots is neither necessary nor a good idea, there is virtually no load.

 

[AgX]

Have all moving coils of light meters (built into cameras or accessory ones) jewel bearings (pivots)?

 

[E. von Hoegh]

Have all moving coils of light meters (built into cameras or accessory ones) jewel bearings (pivots)?

I don't know. The pivots are hardened steel, the bearings are jewels, sapphire.
All high grade moving coil meters of any application do; I saw one very old Sangamo panel meter that actually had hole and cap jewels, just like a high grade watch.

 

[paul ron]

if you get that spring fixed.. you can start working on my closet full of meters with bad springs. I haven't repaired one yet.

those main springs usually break at their connections and are a real pia to resolder.

the biggest problems with meters are those very thin hair like wires linking the moving coil.

brain surgery! good luck friend.

Paul

 

[DWThomas]

Have all moving coils of light meters (built into cameras or accessory ones) jewel bearings (pivots)?

Don't know about cameras, by the 1960s for electrical measurement instruments there was an alternate construction called a "taut band movement" that eliminated the jeweled pivots in favor of a thin metal ribbon tensioned between two flat springs. Back in the day I worked on "force balance" accelerometers that used a meter movement with a moment arm and position sensor to measure acceleration. Ah youth -- can't imagine working on stuff that delicate at my current age!

There are meters that used more of a cantilevered flat spring too, but I believe they were limited to relatively cheap and crude high current applications -- think automobile ammeter/charge indicators.

 

[AgX]

I got me out of the trash an old multimeter with freshly fractured glass pane of this model:
https://image.jimcdn.com/app/cms/im...neuberger-vielfach-multimeter-typ-unavo-4.jpg
(sample without mirror)

I thought, I just put in a new pane and then I got (another...) meter with a nice mirror-scale.
Well, if you look carefully you will see the scale is not nice at all as it necessitates constantly multiplying (in ones head...)

Anyway, at further inspection it turned out that needle sticks abruptly at about value 11. I could not see any debris between the coil and the enclosing magnet.
First I thought to disassemble the coil mechanism. But in contrast to what is said above, I have no clue at all how to access that coil bearing. But on further inspection it turned out that the needle moves fine when the scale is facing down. So I let the meter face down and moved many times by hand the needle all its way and let it swing back. Finally the coil worked fine even with the scale facing up.
Bllowing out the coil mechanism could have been another approach.


BUT this coil error made me really feel irritated. What about a crucial measurement with a moving coil instrument with such a behaviour? There is a likely chance that one would not realize such error.
My advise: ALWAYS check such meter before actual metering with an appropriate load on full swing!



By the way, the meter circuit got a transformer that already is blackened, even one of its iron sheet is warped! So there was extreme overload once on this meter.
Another reason to be very sceptical with meters of unknown history !!