[volt-nuts] Fluke 5200A repair

[Charles Steinmetz]

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