Re: AVO Multimeter survey
General observations on a 1934 vintage Avometer-
Pic 1- Further to comments about the aluminium back-box, being a pressing it's always going to be slightly tapered and a less than perfect fit, but in this case i had to carve out some material from the insulating sheets that i had only just glued in. Fortunately with the help of a blade Araldite gives up sufficiently easily to avoid damaging it's surroundings. It is entirely possible to take a lump out of the bakelite by forcing on a tight box, and this is exactly what a previous owner had done.
Pic 2- The vertical horseshoe magnet has a shunt consisting of two iron bars linked by a screw. In the picture 3/16" of movement towards the concentrator has (on a 10 volt test) reduced deflection of the needle from 85% to just a shade over 83%. A useful amount and more than i expected. The picture shows the shunt as close to the concentrator as it can possibly get on this particular meter- otherwise the (hidden) bar hits the cork blocks that define the min and max needle stops, and pushes them out of alignment. I assume that if one had a magnet that was lacking significant flux that could not be compensated for, removing the shunt altogether would be worth trying. Note of course that disturbing the shunt often liberates (or at least makes visible) ferrous swarf which has to be removed.
In the event of, say, a later Model D with horizontal horseshoe magnet which had too much flux, it might be feasible to fabricate (or salvage) a shunt of this type, as it is more effective than the swivel type fitted to these later models. Care would have to be taken to give clearance to the wirewound shunt chain, and also the radial leg at the rear of the movement that acts as a coarse 'needle zero' adjustment. A magnet that is slightly TOO powerful after 80 years sounds optimistic, but i have at least three instruments where this is the case.
Dave
|