Oops! the divider ratio of R8/R9 is 2100, not 210. Best regards, Charles >Ken wrote: > >>R6 slider to U1 varies from +15v to -15v but the output of U1 only >>varies by mv's, I put this down to the loading of R9 to earth on the input >>to pin3 of U1 (221 ohm),only allowing a small variation to the input on pin >>3 of U1, maybe this needs further investigation! > >Look at the circuit. The 100k potentiometer (R6) feeds a voltage >divider composed of R8 and R9 (464k and 221 ohms, >respectively). The tap of this divider feeds the reference input of >U1. R8 and R9 attenuate the +/- 15v range of R6 by a factor of >[2100], so the divider tap voltage (i.e., the reference voltage for >U1) only varies +/- 7.14 mV from ground for the full range of R6. > >U1 is an integrator, so it has lots of gain at DC (Q38 switches the >integrating capacitor to give two time constants). If one of these >capacitors is leaky or shorted (or there is another leakage path >from Pin 6 to Pin 2 of U1), it would reduce the gain of the >integrator and could produce the symptom you are having. > >Best regards, > >Charles