13 Feb
2009
13 Feb
'09
10:45 a.m.
Josh Dersch wrote:
I have a high voltag insulation tester that would easily measure 39M, but
(a) you'd have to take the resisotrs out of circuit (I don't think the
other bits would like having a few hundred volts across them) and (b) I
am not sure how accurate it is. It would pick up a shorted or open
resistor, sure.
My multimeter doesn't go that high (I really
need to get a good one one
of these days.)
My Fluke 25 won't go that high either -- max range on that is 32
megohms. I've got a Solartron 7150plus bench DMM that can read down
to one milliohm in the
2K-ohm range (6.5 digits with averaging enabled), but still limits at 20M-ohms
max.
Yes, but unless the original resistor has drifted to 40M, then fitting a
40M reisstor in place of it should change the symptoms. It may not be
possible to get to the original spec (strightness of lines, etc) but you
should see some change.
I replaced these with two 20M's in series
(figured it'd
be close enough to at least see if it changes anything, and I can't find
any place that actually stocks 39M resistors that aren't SMD...) No
change in display behavior.
The spec in the parts list was 1%. 1% of 39M is 390k-ohms either way... 40M is
well out of spec. I'm starting to wonder if some normally-irrelevant
parameter of those poly
caps has shifted and fragged the vector generator.
I am wondering if the fault could be further back. What if one or both
of the DACs has 'gone slow', and no longer produces a sharp output change
when you change the digitial inputs?
-tony