28-05-2024, 04:22 PM
I used a commonly available meter of completely different much higher rating and deflection on an AVO valve tester. I forget which model.
Ironically the owner had two AVO transistor testers that used the same meter. Those two meters sadly also scrap.
I photo-edited the original scales to fit new meter and used an op-amp amplify voltage on a resistor equal to original meter resistance. I didn't have a low enough offset op-amp (expensive instrumentation type needed) so I used a dual with a split supply and split reference and the 2nd op/amp then fed an offset to the amp on the dummy meter load. A preset to electrically zero after meter is mechanically zeroed. Due to amps being on one chip and split supply via two diodes from one secondary winding, the zero preset on 2nd amp worked with no observable drift with time, temperature or mains voltage.
I forget if it was a 20uA or 37 uA meter originally.
I also replaced meters on an AVO VCM163, but in that case the meter spec was close enough and I only had to photo-edit new scales.
The actual AVO test meters with bigger meters seem to mostly to be OK, but they definitely had valve and transitor testers from mid 1950s to 1960s that used meters where the magnets went weak or something. I've seen maybe 4 or 5 duff meters now from AVO gear that "moved" but had reduced sensitivity. Also some sticky.m
Ironically the owner had two AVO transistor testers that used the same meter. Those two meters sadly also scrap.
I photo-edited the original scales to fit new meter and used an op-amp amplify voltage on a resistor equal to original meter resistance. I didn't have a low enough offset op-amp (expensive instrumentation type needed) so I used a dual with a split supply and split reference and the 2nd op/amp then fed an offset to the amp on the dummy meter load. A preset to electrically zero after meter is mechanically zeroed. Due to amps being on one chip and split supply via two diodes from one secondary winding, the zero preset on 2nd amp worked with no observable drift with time, temperature or mains voltage.
I forget if it was a 20uA or 37 uA meter originally.
I also replaced meters on an AVO VCM163, but in that case the meter spec was close enough and I only had to photo-edit new scales.
The actual AVO test meters with bigger meters seem to mostly to be OK, but they definitely had valve and transitor testers from mid 1950s to 1960s that used meters where the magnets went weak or something. I've seen maybe 4 or 5 duff meters now from AVO gear that "moved" but had reduced sensitivity. Also some sticky.m
