Advice for Tek 4051 vector-drawing issue?

Tony Duell ard at p850ug1.demon.co.uk
Fri Feb 13 12:45:01 CST 2009


> 
> Josh Dersch wrote:
> > 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 

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.

> 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.
> 
>  > 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.

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'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



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