Re-calibrating a AVO Model 8 Mk5

[George Edmonds]

HI

i have strapped every other switch contacts together and then done an insulation resistance test at 1Kv between the two strapped contacts and the contact in the centre.  Insulation resistance found to be >999M  at 1KV on all occasions.

Looks like your switches are well past their sell by date.

George G6HIG in Dover UK

[Specmaster]

Update on the meter...., all done and working.

Transpired I believe to be entirely my own fault, when I acquire something new I always tear it down for inspection and on this occasion, I took it upon myself to clean the zero ohm pots and switches with contact cleaner while in there with some Servisol Super 10 cleaner, which I have used for years with no problems. This it turned to be a big mistake, I think maybe the formula for it might have changed, but now it seems to have adopted the characteristics of WD40, in as much as, it will penetrate into any microscopic hole or crack what ever, and it will remain there, regardless how many times you scrub, immerse and even ultrasonic clean it, the Servisol remains and is conductive. Hence why my DC volts were not being read correctly because of this conductivity between some switch sections on the contact rings was lowering the effective resistance and because this contact cleaner was reading into the Mega Ohms, it made the DC Volts read progressively higher as I increased the range. This was due to the parallel resistance that was added, lowering the effective range resistance and therefore volts read higher.

The cure was to remove all wiring from the contact rings and pop them into an oven set at 250 degrees on a layer of cooking foil, contacts down, for a few minutes to dry out the cleaner, and now I have resistance of 3,000M ohms or better between all contacts.

Printed plastic ribbon cable replaced with conventional ribbon, switches all rewired and connected up and the ranges tested and  all seems to be within the factory prescribed specs for this meter, all ranges tested except for the 3KV as I don't have anything that high to test it with.

Many thanks to the various forum members, in particular George Edmonds  , for all advice and help given on this project (George Edmonds, I sent you a PM, did you receive it?)

Here are some shots of its progress, I discovered after doing the voltage tests, the needle zero position was a tad to the left of the correct position, so the accuracy of the value shown is actually higher than it looks.