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
hl...@att.net
From: Dr. Frank <frank.stellm...@freenet.de>
To: volt-nuts@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@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.
_______________________________________________
volt-nuts mailing list -- volt-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.
