On 20/07/2011 21:11, Carl Denk wrote:
> Yes, this is the external resistor divider in a series, and then the
> rotary range control connects to the appropriate junction. It appears
> the divider is good, with the same voltage displayed (when connected to
> the same source) with different degrees of digits as I switch through
> the ranges. For a 1.5 volt AA cell (measures 1.363 volts with good
> meter): 1.059, 1.05, 1.0, 1 volts. The whole range stays in sync when
> adjusting the pot. Probably if I would adjust the pot now, it would be
> at near the limit of adjustment. It sure points to something in the pot
> circuit, but hard tracing that area.:(   If I thought it was the pot, I
> would replace it, but it just seems so smooth and consistent.
>
> Replaced battery with fresh 9 volt alkaline (voltage slightly above 9
> volts with good meter), no change. The removed battery was 7.4 volts.

7.4 volts seems a little low, I would leave the alkaline in just in case.
I think the pot sounds like it's doing it's job fine, and I can't see=20
how it would cause the symptoms you are seeing anyway. More likely I=20
think is problems with the voltage source feeding the pot. If I=20
understand correctly it sounds like the pot is part of a=20
trim/calibration circuit, so it's likely it either feeds a reference=20
input on the sampling IC directly, or combines with an opamp buffer to=20
do so. Try and find and check both what is feeding the "top" of the pot=20
(I'm assuming it is set up as a standard voltage divider with bottom to=20
ground, although this may be wrong) and also where the divided voltage=20
is going.
I would be suspicious of any polarised caps in this region, and check=20
thoroughly for dry joints or contamination of the PCB (a bit of leakage=20
current with high impedance opamp inputs can cause big problems)
Sometimes when nothing is obviously visible (time is short/exhausted=20
most possibilities/unknown circuit with no info or schematic) I have=20
resorted to running a soldering iron (+ a bit of flux) over as many=20
joints as possible, and changing any polarised caps. Not the most subtle=20
approach, but this has worked for me a few times, although it leaves a=20
niggle if you never know exactly what the problem was.. :-)


--=20
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