thanks eric.
been there done that. That datasheet is from the NLS website, and I have it, but NLS are not responding to emails.
I also have a data sheet for the AY53500 but this is VERY vague.
The AY53500 is a single ramp integration device with supply voltages of -7.5 and -15 Volt. The panel meter has external op-amps to precondition the input and also an internal 7806L voltage regulator. The panel meter has 4000 series cmos logic to handle I/O from the AY53500. That datasheet you supplied is for the PM350 range and is current product, not what I have from the 1980's. You ask why don't I buy a new one and the answer is I need a referrence voltage to give a FSD of only 50 mVolt. The modern stuff is made with unpackaged IC's and these are difficult to modify.
I appreciate your effort; tks.