Quote:
Originally Posted by mole42uk
I tried a small voltage at the FET end of R114 which gave no indication of its' presence on the display.
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That wont work!!!
You are on the wrong side of the multiplexor!
Q110 to Q113 switch in signals in a controld sequence, such as signal, 0V, reference.
The microprocessor controls the A-D converter to look at and process these levels to get the correct reading. The signal at R114 will be a multiplexed signal of four components, sequenced for the A-D conversion.
As I said in an earlier post you need to connect your test voltage to the signal side of Q110 at R107.
Also check the logic to the four gates is correct before looking at the A-D and processor circuits.
U106 and U109 control the FET's and act as single pole double throw (SPDT) analogue switches.