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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 21st Jul 2005, 5:09 pm   #1
Sam
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Default AVO Valve-Tester Movement

I bought an AVO Valve-Tester from Chris C (thanks!) a bit ago, and have finally gotten round to having a look inside. I knew the meter spring was binding due to a bad knock it suffered in the post so I was wondering what is the better option: try and un-bind the spring by bending it; replace the coil with a similar-sized coil and spring, and use resistors as required to correct the FSD current; or just try and find a replacement movement? The spring has to be 'matched' to the coil as the spring provides the balancing torque to the electro-magnetic forces caused by the current.

My plan is just to try and gently bend the spring so the pointer moves freely accross the scale. Small bends at a time!

So, does anyone know the resistance and FSD current of the movement of the AVO Valve-Tester?

Sam
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Old 22nd Jul 2005, 7:50 am   #2
Mike Phelan
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Default Re: AVO Valve-Tester Movement

Sam
I would plump for the spring. You need two good pairs of tweezers, and plenty of care. Straightening it will not alter the FSD, providing the location is set correctly.
Done it many times on clock hairsprings.
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Old 27th Jul 2005, 1:27 pm   #3
pmmunro
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Default Re: AVO Valve-Tester Movement

As the damage occured in transit, if I'm reading your description correctly, it is most likely to be a tangled hairspring rather than bending.

In their service manuals, Avo recommend attempting to untangle a hairspring by tracing the spirals gently, from inside to outside, with a non-magnetic needle. They claim that even apparently very badly distorted springs respond well.

If this is not successful, or you feel it is not appropriate, unsolder the outer end of the hairspring. Once it is free. it should be possible to untangle it with careful manipulation, but be careful not to twist the flat wire of the hairspring.

The core of these movements can be removed from the magnet assembly but I would recommend sliding a steel keeper across the poles to maintain the field strength.

There is an official jig to hold the moving coil during work - a rectangular piece of hardwood with the corners radiused. Two holes drilled at right angles at one end are useful to allow the jig to be supported on an inprovised stand with a post cut from studding attached to the baseboard. This is vey useful and saves the moving coil frame from crushing accidents and give some protection to the pointer.

The inner ends of the hairsprings are soldered to collets consisting of spirally wound metal ribbon. The collets are a light interference fit and can be slipped off the hardened steel pivots using fine bent-nose tweezers. This is the normal method of replacing hairsprings, complete with collets - the inner ends of the hairsprings are difficult to solder. (Note the exact orientation of the hairsprings and the direction of spiral before dismantling).

After repair, the movement will almost certainly need to be rebalanced, which takes time and skill, and ideally a draught-proof box. You may also need to adjust the sensitivity by rotating the magnetic shunt.

If you do not have the facilities or confidence in your skills, a number of repairers (see previous thread) can probably offer an economic service.

For dispatch, the movement should be mounted face-down on a rectangle of hardboard with brass screws and wood block spacers to give clearnce between the pointer and the board. The movement and board can be wrapped in cling film and put in a cardboard box
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