Re: AVO 7 voltage reading low.
Things worthy of note-
Battery compartment lids- (concerning meters where the lid is directly under the leather handle)
The distance between the (centreline of the) fixing screw-holes can be either 133.5mm, 134mm or 135mm- and these are just the ones i know about! A small difference but enough to mean that these parts must be of the same type when putting together a meter from parts:-
a) The lid
b) The case
c) battery box
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I fabricated a new lid from an (Argos?) steel bookend which just happened to have a close paint match- silver hammered. I chickened out from doing the keyholes as on the first attempt at an exact replica c/w keyholes, there wasn't enough 'grippage' for one of the screws.
I protected what remained of the Farnell repair sticker with UV resistant 'weatherproof' outdoor tape, the stuff that is supposed to patch up old greenhouses. It's fairly hopeless outdoors but very tough for an indoor job.
The bias for the rectifier on this Model 7 can be seen to be working- a small rise of the needle whenever the DC selector is moved to 'AC' and the AC selector is in ANY OTHER position than 'DC'....even one click to a blank position either side of the DC position is enough. This facility appears to improve sensitivity to a 14vAC input by around 0.5v, compared to a Model 7 without the bias feature.
The cutout continued to glitch and I ended up replacing the sliding contact plate for a spare one with much pointier contacts- instant fix.
The main AC leaf switch stack had an issue with the spacer rod being slightly too short; fixed by aralditing a 20 thou shim to the leaf above it, which enabled all the contacts to move by a healthy amount.
The 'snow' inside the meter was oxide from where the cell had rotted the aluminium case. Always tricky to clean away from fragile windings but i got most of it and greased the inside of the case to catch any future floaters.
I'm quite pleased with the enamel repaint of the front legends- managed to get a reasonable ivory colour from mixing off-white with yellow and khaki- sometimes the incised legends are worn down and difficult to fill on these meters- these were nicely crisp and unworn though.
Dave
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