At 10:00 AM 7/8/2014, Richard A Jones wrote:
>Any ideas or experience or knowledge?  David?  ;-)
>
>Richard

Quick answer, more later.

1) You can test the input with a potentiometer and voltmeter.  Input
lead is biased to B+ with one megohm, electrodes ground out
successive resistors in a voltage divider inside the little black
module by the tank.  Result compared against similar network at the
left-side LM324 chip.

2) The LM324 chips die a lot.

3) Some info and schematics here

Gotta go, back later.

d