I am working on a project to add an Arduino to an old 1960s Hickok 539B tube tester. Unfortunately many of these old testers that have seen frequent service use are now experiencing issues with their analog meters and replacements/repair are either very costly or nigh impossible.
To the point, I have gotten most of the code squared away, however I am running into a problem when trying to measure the bias voltage in the tester. Attached is a schematic of the way the circuit I've prototyped to interface the tester with the Arduino.
From left to right, L1 is the transformer that the bias voltage is derived from in the tester. The unmarked legs of the transformer go off to the remainder of the bias circuitry, which goes through a rectifier, dropping resistors, some other voltage dividers, etc.. The center tap is where the bias voltage eventually ends up connecting to the tube. The voltage divider there is to shift the -40-0V down to -5-0V, then goes to a summing amplifier to convert it to a more ADC friendly 0-5V.
V1 is what I am using to represent the voltage of the main meter of the tester. C1 is to remove the AC component (original meter has a cap across its terminals to deal with the AC) and the resistor in parallel to provide a way for the cap to discharge so the reading doesn't lag due to the capacitance.
Both the bias and main meter circuit are inevitably linked together though the remaining circuitry of the tester and the tube under test. So I am getting some interactions between the two readings when the Arduino. I corrected one issue where I had made a ground loop (doh!) when hooking everything up. But with *just* this circuit hooked up (Arduino isn't even in the picture) I can't adjust the bias below 1v any longer. Obviously I'm getting a current, probably through the voltage divider circuit on the bias measurement, that is causing the bias voltage circuit to offset by a volt. If I remove the circuit in the schematic, everything reverts to proper operation. If I try to measure either the bias or main meter circuit in isolation of each other, it works right.
So doing some reading, it looks like a differential ADC may be the solution to my issue, it would (hopefully) eliminate the interaction I'm getting between the two due to sharing a common ground. Or do I need to try another approach here?
Below link goes to a schematic for a military TV-7 tester, which uses the same basic testing circuit, so someone can get a basic idea of what is going on in the measurement circuits in the tester. Most of the differences between that tester and the one I have are generally immaterial with what I'm trying to accomplish here. Scroll to page 16 for the simplified schematic.
http://www.i1epj.ham-radio-op.net/Manuali/TV7U/TM%2011-6625-274-35%20hi%20res.pdf