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