[volt-nuts] 3420B chopper drive problem

[David C. Partridge]

I'm trying to resurrect a 3420B.

I'm looking at the waveform at TP1 on the A2 board.  This is supposed to be
a square(ish) wave, but within a 6mS period, there should be two cycles of
the signal, one with an amplitude of (say) 150mV, and the next cycle only
100mV.   I think that  the larger should be symmetric about 0V, and the
other only somewhat above 0V at the peak.

What I actually observe is a square wave with all cycles the same amplitude
and only -ve going.  I can adjust the amplitude using A2R10 up to about
250mV p-p. 

The chopper neons are part of this circuit, and I'm having problems getting
my head round what could be wrong.

Both neons appear to be firing.

The schematic is page 7-15 in the manual (which for some reason is on page
37 of the scan on the Keysight website):

<http://www.keysight.com/main/redirector.jspx?action=ref&cname=EDITORIAL&cke
y=1866858&lc=eng&cc=GB&nfr=-536900193.536896952.00>

Any help in understanding what's wrong will be most gratefully received.

Thanks
Dave