[Test-Equipment] Question regarding antique LCR bridge

[Dave Brown]

The TF1313 is a 0.25% bridge- so it may well have to do with achieving that 
order of accuracy- less accurate bridges like the TF2700 would perhaps  not 
need to go to such lengths.
DaveB, NZ

----- Original Message ----- 
From: "Barry" <n4buq at knology.net>
To: <n4buq at knology.net>; "Discussion of Electronic Test Equipment" 
<test-equipment at mailman.qth.net>; "Brooke Clarke" <brooke at pacific.net>
Sent: Wednesday, April 11, 2012 5:06 AM
Subject: Re: [Test-Equipment] Question regarding antique LCR bridge


> 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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>
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