28-07-2020, 08:53 PM
(This post was last modified: 28-07-2020, 08:54 PM by Mike Watterson.)
I replaced the innards of AVO meter in one of their Valve testers, I forget which model. Edited scan of old scale to suit the new meter's deflection angle and added an LM358 op-amp to convert from original resistance and 30uA FSD to drive the 50uA meter. I used the 2nd op-amp in the package as a voltage follower / virtual earth to null offset. As offset can be + or -, I used a pair of TO92 regulators, +5 and -5 from the raw DC via 1N4148 diodes and a small clock radio transformer. The offset adjust pot fed by +600mV and -600mV from two diodes, so +/- supply tracking, otherwise the offset pot won't work!
The circuit will allow anything designed for a meter from 15uA to 100uA to drive any meter from 20uA to 1mA. One resistor to simulate old meter, a resistor to set basic gain and a pot on the output to fine tune. Obviously mechanically zero the meter with power off and then null offset with no input connected.
The owner had also two AVO transistor testers as possible donors for meters as it uses the same movement, though the internal ballast may be a little different. Both O/C. No protection circuit. The original was sort of sticky above 1/2 FSD, which gave the illusion of reduced sensitivity. I think it's the worst movement in any AVO instrument, ominous that "servicing" and "faults" of the meter is in the manual.
The circuit will allow anything designed for a meter from 15uA to 100uA to drive any meter from 20uA to 1mA. One resistor to simulate old meter, a resistor to set basic gain and a pot on the output to fine tune. Obviously mechanically zero the meter with power off and then null offset with no input connected.
The owner had also two AVO transistor testers as possible donors for meters as it uses the same movement, though the internal ballast may be a little different. Both O/C. No protection circuit. The original was sort of sticky above 1/2 FSD, which gave the illusion of reduced sensitivity. I think it's the worst movement in any AVO instrument, ominous that "servicing" and "faults" of the meter is in the manual.
