Sticking D'Arsonval Meter Movements

[jonpaul]

Bonjour a tous!

Have a 1960 400V power supply like Heathkit IP-17/IP-2717, = homemade, with API meter-relays,for  overvolt and overcurrent trip off,  (thyratron).

Meters are 250 and 500 uA movements, and have become "sticky" so as the indicated value is slowly increases, the needle sticks at certain points eg 200 uA and stops movement.

Tapping the face unsticks it. I have exercised the zero ajustement to no effect.

Same issu on some fine GenRad 1863/1864 Megohm Meters.

Perhaps damage of jeweled bearings by overload or vibration, or dust/debris contamating movement spring or jewels.

Disassembly always kills' the delicate movement.

Would a gentle wash with 99% IPA do any benefit (or harm!)?

Any other suggestions  fix sticky meter movements?

Mille mercis a tous,

Bon week-end,

Jon

[Gyro]

It's probably a fleck(s) of varnish that have flaked off the moving coil assembly and fallen into the magnetic gap. If so, gently holding the pointer and gently blowing around the coil might be just sufficient. If the PSU has suffered any impacts then check the hair springs for overlapped turns (unlikely if it is mostly working) or possibly bent counterweights catching, or the pointer catching.

If this treatment doesn't cure the problem then you'll need to remove the meters or at least get them horizontal and put it under a decent magnifier to look for, and remove any debris. The jewelled pivots are probably least likely to be the problem. Very fine copper wire is a good probe of dislodging debris because it isn't attracted by the magnet and can't exert too much force (a moistened straight human hair is remarkably effective too).

I would avoid washing with IPA at all costs - the varnish on the coil and attachments is quite likely to be shellac, which is soluble in IPA. Washing that combination into the pivots would be bad news indeed.

Looking at the meters, you have the added complexity of the limit needles.

P.S. removal of the scale plate should be the limit of disassembly needed.

[guenthert]

     Hmmh, might be worth checking whether the scale (label?  not sure how it's called) warped and the needle is now touching it at some point.  I'd hesitate to prod the coil, but often the case can be opened to change (customize or calibrate) the scale.

    Well, looking at the picture, the shadow is kind of giving away that warping of the scale isn't the issue here.  Never mind then.

[jonpaul]

Gyro and Gunthert, many thanks,

1/ No vibration of abuse, equipment stored on the floor for decades.

2/ Scales are flat, nothing touches the needles and the relay contact is at full scale, sticks happen about 75% of deflection

3/ Indeed the meter scale of righthand V meter has flaking paint but the left meter I scale is perfect and the left meter is sticking.

4/ I will remove them from the PSU, open, remove scale, shake while face down, then use compressed air on the movement.

  By the way the old API meter relays are very good for industrial controls in the epoch before digital.
The contact is just for dry sig 10 mA max so we use a thyratron to crowbar the main supply with a mercury wetted relay.

Will update after

Bon Journee,

Jon

[Gyro]

You're welcome.

4/ I will remove them from the PSU, open, remove scale, shake while face down, then use compressed air on the movement.

Whoa there, I was talking about a gently blowing by mouth. Compressed air will destroy the spiral hair springs! As I mentioned, gently restrain the pointer (a finger close to each side rather than holding it) to avoid it crashing into the end stops and bending.

[jonpaul]

Hello again: Great tip re moving the pointer to the side while doing the proceedures.

Yes I thought about the compressed air danger, instead shall use a heat gun or hair drier for a wider and much slower airflow. Certainly not a compressor and air nozzle.

Hope that just shaking while face down may do it. Need to pull out of the heavy PSU.

Finally found a spare of the exact meter but 500 uA not 250 uA, in mint cond, so could swap out and modify the meter ckts for 2X current.

Many thanks again

Jon

[jonpaul]

Rebonjour a tous:

Removed the meter, removed front cover, shake face down, held pinter, blew gently and shook again.

Much better but still sticks  occasionally, I shall repeat the drill a few time.

Mille mercies pour votre gentil aides!

Jon

[kleiner Rainer]

Jon,

I once had the case that the polystyrene window of the meter had collected a static charge that kept the pointer from moving - wiping it with a moist piece of cloth helped. The charge was on the inside of the polystyrene window. It took me some time to find that out, because I could not imagine that the charge was strong enough to stop the pointer, until I saw that the charge on the (plastic) cover of my record player lifted the arm clear off a record after wiping the dust off that cover during a very dry winter day.

Greetings,

Rainer

[jonpaul]

Ranier: many thanks This problems is always at the same point ~ 80% deflection,

I am aware of static charge effects, they are intermittent erratic and random.

The meter is mounted on a grounded panel,

PS: Anyone have thoughts on the photos of the power supply in operation? I built it ~  1962-1965

Bon Journee,

Jon

[Gyro]

PS: Anyone have thoughts on the photos of the power supply in operation? I built it ~  1962-1965

It is indeed an impressive looking beast, with impressive output capabilities too! The fact that it still lives today is a testament in itself, good old-school engineering withstands the test of time. It's a shame you can't find those meter relays these days, they are an excellent visual indication of the approach of limit.

With regard to the meter, if it's just sticking at 80%, it might be worth gently moving the needle to that position (human hair is good again) and looking under high magnification to see if anything catches at that point. It's not unknown for a tiny fragment of magnet plating to come loose and stick in the gap. More stubborn than a varnish flake but possible to persuading to move to a non troublesome position (if you can't safely remove it completely).

[jonpaul]

Bonjour Gyro:

Mille mercis: Yesterday I fixed it by shaking face down and gently blowing air into the movement. I am sure it was a stuck flake of meter scale paint or debris stuck in the movement.

I built it in the early 1960s, the meter relays are from API, used in industrial controls. Design is based on an ancient RCA tube manual note,

like the Heathkit IM-17. I was improving the Heathkit design with surplus parts from NYC's "radio row".

The massive power transformer is from an old TV set, power and HV switches are telephone style, pass element is three 6080 dual power triodes instead of the smaller two tubes in the Heathkit.

Jon