>> [...] there migth be excessiveripple on
one of the power lines.
Have you put a 'scope on the power liens to check
for ripple?
Don't have a 'scope yet. [...]
I am tryign to think if
there's any way you could find the fault with
just a multimeter...
Well, if the meter has an AC setting, you can hook it up with a
middling-large series capacitor, er, condenser, to measure the ripple
on the power rail fairly directly. (What counts as "middling-large"
depends on the frequency of the expected ripple and the impedance of
the meter - you want the cap's impedance at the frequencies in question
to be small compared to the meter's. Depending on the meter's design,
you may also need a DC path somewhere.)
I'd thoguht of that, but as you saiod there are some 'unknowns' -- what
size of capcitor to sue, whether you need a shut resisotr across the
meter and if so, what vlaue, etc. You may not know what hte meter readign
realy means, particulalry if you don't ahve much experiecne with this
sort of circuitry.
I would start with a1uF non-polarised (not electrolytic) capacitor and a
1M resistor in parallel with the multimeter. Try it on a known-smooth DC
rail (like a battery) and a low-voltage mains-frequency supply 9e.g. from
a small transformer) to ge some idea of the sort of readings you get and
how it behaves.
-tony
