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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

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Old 10th Jan 2017, 7:43 pm   #1
The Philpott
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Default Avominor 'Z' screw repair

Has anyone attempted repair or replacement of the needle zero screw in an avominor?

The little peg has snapped off the rear of mine, presumably due to lack of caution when replacing the movement in someones previous repair attempt. I managed to find the missing peg floating around inside, which is nominally 1.5mm diam and 2.5mm length.

I believe that the Z screw is screwed and glued (??) into the backing disc, so am going to try repair rather than drilling the whole thing out and possibly making things worse. My latest plan is to drill a little hole into the disc with a 1.5mm bit and glue in a stainless peg using a tiny amount of araldite. Superglue will creep and sieze the whole adjuster hence the araldite idea.

Any thoughts?
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Old 10th Jan 2017, 8:17 pm   #2
pmmunro
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Default Re: Avominor 'Z' screw repair

That sounds like a good plan. I assume that you have chosen stainless steel because it is a grade which is non-magnetic. Another option would be to use brass.

PMM
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Old 10th Jan 2017, 8:35 pm   #3
The Philpott
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Default Re: Avominor 'Z' screw repair

Yes ideally wanted something non-magnetic.. Have now drilled the hole, then deliberately snapped 4mm off the blunt end of the drill to trial fit as a peg. Now arguing with myself as to whether this improvised 4mm x 1.5mm HSS rod is too close to the magnet.

I think perhaps i will buy a small brass hinge and use the rod out of it.
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Old 10th Jan 2017, 8:46 pm   #4
merlinmaxwell
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Default Re: Avominor 'Z' screw repair

I wouldn't worry about the very (vanishingly) small effect the magnetic pin would have. After all it is very small compered to the magnet and not in the field the coil uses.
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Old 11th Jan 2017, 10:30 am   #5
Leon Crampin
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Default Re: Avominor 'Z' screw repair

You don't specify the age of the Avominor. If it's a later one using a plastic screw, snap off a suitable length of darning needle (including the point) and use long nose pliers and a soldering iron to embed your new pin in the plastic screw - pointed end first.

This can usually be done in-situ. If you operate slowly and carefully to embed a decent length of pin, the result will be quite strong enough. Ensure that the axes of the pin and screw are parallel.

Leon.
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Old 11th Jan 2017, 12:52 pm   #6
The Philpott
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Default Re: Avominor 'Z' screw repair

Thankyou chaps.

The gist of it is that i am hopefully making a functional Type E from two defunct units. They are broadly contemporary with each other (1949 and 1944) but the former has an o/c movement and Z screw problem and the latter is giving some low and random readings and is very reluctant to function on K=1 for reason i have yet to fathom. The latter also doesn't want to zero consistently either.

The switch points on both appear to be ok.

The Z screw is nickel over brass, screwed/glued into a plastic disc at the rear. The repair of this is looking good, the other challenges are now under scrutiny!
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Old 11th Jan 2017, 4:50 pm   #7
The Philpott
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Default Re: Avominor 'Z' screw repair

Z repair successful. Drill bit fragment made over-length then clamped in mole grips and ground to around 4.5mm long with dremel and goggles. Damage probably originates because the scale plate clips one of the bobbins as it goes in and may have been forced in rather than slid in at a slight angle.

K=1/K=2/Amps work, Ohms not tried yet. Zeroing of needle somewhat improved possibly following attention to tightness of jewels. Some Intermittency in function remains and by a process of elimination this problem lies within the actual moving coil. We tried.

I can't think of another minor that shares this particular movement.. (1.25mA..?)
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Old 13th Jan 2017, 11:11 pm   #8
The Philpott
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Default Re: Avominor 'Z' screw repair

Thread can be closed at moderator's discretion. Parts in post to re-attempt assembly of a functioning meter- (incorporating the repair which was the subject of the thread)
Thankyou
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