[volt-nuts] Fluke 5200A repair

Charles Steinmetz csteinmetz at yandex.com
Wed Jul 9 14:21:13 EDT 2014

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 210, 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,


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