[Test-Equipment] Question regarding antique LCR bridge

[Barry]

The chopper and the DC source are synchronized.  They're powered by individual windings on the power transformer.  I've watched the output of the chopper on the scope but I would like to compare the rectified DC against the chopped signal to see the exact relationship between them.

I'm having problems getting a steady NULL on R and L.  C nulls nicely but R and L tend to null and then drift back off the null point.  I've watched the AGC line and the detected signal and on R, after it nulls, there's a small signal that reappears thus causing the meter to slowly rise from the null point.  I'm not sure if it is because the AGC is slowly rising and that's causing any small imbalance on the bridge to be seen as the gain goes up or what.

Not sure where to go from there.  I replaced the AGC cap with an electrolytic and that has helped steady the nulls a bit; however, when I checked the old cap on another bridge, it showed virtually zero leakage at the rated voltage so not sure why replacing it helped.

Thanks for the response.
Barry - N4BUQ

 On Tue 10/04/12 10:59 AM , Brooke Clarke brooke at pacific.net sent:
> Hi Barry:
> 
> Do you have a schematic for the TF1313?  Is the vibrator wired so that
> there's synchronous detection?
> That's strange.  The problem with using DC as a test signal is that you
> then get errors due to dissimilar metals, see:http://www.prc68.com/I/Kelvin.shtmlEven when using a 4-wire Kelvin setup there's a real limit on low Ohms
> measurements because of this problem.
> My TF2700 has provision to use external DC bias but makes the measurements
> using AC signals.http://www.prc68.com/I/TF2700.shtml
> Have Fun,
> 
> Brooke Clarke
> http://www.PRC68.comhttp://www.end2partygovernment.com/Clarke4Congress.html
> 
> Barry wrote:
> > I have a Marconi TF1313 LCR bridge (designed in
> 1960).  When measuring L or C, the bridge is fed with an AC signal (1KHz or
> 10KHz).  The output goest to a couple of amplifiers and the detected signal
> is fed to a meter.  When the bridge approaches balance, the detected signal
> approaches zero and the meter indicates a null.  Standard stuff for a
> bridge.>
> > When measuring R, though, a DC (full-wave
> rectified, unfiltered) signal is fed to the bridge and the output is
> connected to the input of the first amplifier through a chopper (old
> fashioned vibrator).  Amplification and detection is the same process as
> for L and C.>
> > My question is why it is necessery to change the
> configuration of the bridge for R measurement?  Why wouldn't it work with
> an AC input for R the same as for L and C and eliminate the chopper? 
> Wouldn't a balanced bridge made entirely of R produce a minimum signal at
> the output and allow for null detection?>
> > I know the answer is probably simple but I don't
> see why this is done this way.>
> > Thanks,
> > Barry - N4BUQ
> >
> >
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