I read the patent document, it is quite interesting - I can see how Avo managed to do away with a grid bias control, it is quite clever, quite simpler and easy to use provided one is only looking to test for 'goodness' - the shorts/leakage test though does look to be an afterthought.
On the subject of the meter - I spent an hour or so on the meter and managed to get the tiny bits of metal stuck between the pole pieces out using a thin strip of masking tape wrapped around a bit of brass shim - the pointer now consistently stops on zero without resorting to tapping the case, the bits of metal look very much like strands of steel wool, I initially tried to coax them to one side where they wouldn't get in the way but they seem to have a mind of their own and work their way back.
Looks like the magnet has lost some power over the years as FSD is now 700uA, resistance is 26 ohms, I played around with an online meter shunt calculator (
https://www.calculatoratoz.com/en/sh...lator/Calc-483) to see the affect of changing the swamp resistor value, if I reduce the swamp resistor from 40 ohms to 17 ohms the effective meter resistance will be 43 ohms (swamp R + meter R), the 90, 9 and 1 ohm shunts will all work ok - however the Avo service/setup document states that the swamp resistor value should be kept as high as possible to minimise temperature variations affecting the calibration, I can't see how this could be, 700uA through a 17ohm resistor is only microwatts of power, am I missing something here ?