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Old 19th May 2018, 12:20 pm   #1672
The Philpott
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Join Date: Aug 2016
Location: Colchester, Essex, UK.
Posts: 4,081
Default Re: AVO Multimeter survey

Precision Avometer movement calibration/set-up only.

1. Check against known voltage reference to see if it needs attention!
Slide lid from hinges, remove chrome plated facia screws, carefully invert meter and lift box off. Check integrity of box bottom, as it is rather thin plywood and can tend to warp away from the sides, which are much more substantial.
Remove loom clip screw (Pic A) and 4 main screws holding the wooden shunt frame to it's hex pillars. De-solder red and black wires from protection rectifier (Pic B). Note green earth wire solder tag under corner of shunt frame. (Pic C) The tag may also need to be de-soldered from the GN wire to allow removal of the white polythene movement dust cover.

2. Hoist (or get an assistant to hold) the shunt frame up at an angle (Pic D) and remove the screws and shakepruf washers holding the (polythene?) dust cover to the facia. RD GN and BK wires now have to be fed through the circular holes as the dust cover is taken off.

3. You will note at this point (Pic E) the temptation to extend the holes in the dust cover to the edge, to facilitate future removal without use of a soldering iron. The polythene on the subject meter is high quality, but the thickness is variable and it is almost translucent in places.

4.Lower shunt frame down and attach with one screw at either end. The steel 'helmet' covering the rear of the movement should be wiped over thoroughly with blu-tack to remove ferrous/magnetic/magnetised material (Pic F)

5. Carefully remove helmet, (set aside GN wire) and wipe inside with blu-tack. As per (Pic G) the magnetic shunt is now best accessed with a long screwdriver. What is not recommended is a conventional spanner, as access from the side is limited and a mistake could occur.

6. Unfortunately the movement will show some 6% more deflection with the helmet removed, so every time the meter is tested it must be replaced for a correct reading! There is a certain and inevitable amount of non-linearity which means- if you set the meter correct at FSD it will lag a little behind at lower deflections. I set mine up at 66% FSD due to lack of a suitable voltage reference for FSD, but it all worked out quite well.

7. Reassembly is reverse of disassembly, but note that before the meter is finally fitted back into it's box, there is very good access to the rotary switches and leaf switches. if the meter has been locked away in the warm for decades with rubber test leads inside the lid, these leaf switches are likely to be blackened.

-Mods feel free to shuffle this summary (complete with pics which follow) elsewhere if it would make more sense.

Dave
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