[volt-nuts] Unable to zero 3420B on 10**5 sensitivity

Howard Davidson hld42 at att.net
Mon Nov 13 21:52:27 EST 2017


I have worked on H-P instruments that use the photo-choppers. One failure mode is degradation of the neon lamps in the chopper. With use some metal from the electrodes is sputtered onto the inside surface of the glass envelope. This dimms the bulb, and buries some of the neon. I have restored the choppers by replacing the neon lamps.
 hld 

Howard L. Davidson 
hld42 at att.net

      From: Dr. Frank <frank.stellmach at freenet.de>
 To: volt-nuts at febo.com 
 Sent: Monday, November 13, 2017 2:16 PM
 Subject: [volt-nuts] Unable to zero 3420B on 10**5 sensitivity
   
David,

I've thought a bit further about this excessive offset  of at least 8µV.
That could be simply due to excessive offset of the DC amplifier part of 
the meter amplifier, caused by ageing, i.e. drifted resistors or faulty 
elctrolytics.

The chopper has about 5000x ac gain, due to rectification, that's about 
2500x effective dc gain.

So these 8µV translate into 20mV too much of dc offset of the dc 
amplifier. How the 1V zero (R16) affects the DC amplifier offset, I 
don't comprehend yet.

But the next step is to check the DC amplifier.

Frank


_______________________________________________
volt-nuts mailing list -- volt-nuts at febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.


   


More information about the volt-nuts mailing list