Datron 1281 offset null, bias current adjustment


The offset adjustment R317, is an AC adjustment, not DC as one might
surmise. As I recall I applied a 10X scope probe to the  O/P point at
the junction of R329 and R330 to view the AC component. I probably used
a 10Meg ohm with a cap in parallel, in a well shielded enclosure across
the input jacks. I probably set the 1281 to the 10V range.

As for R222 the input bias, I think I just barely touched up the
adjustment so that there was little drift with the input open. That is,
the input would float around a low displayed value, and climb to a large
number in either polarity.

I paraphrased the adjustment info from the 1081 and applied it as much
as possible to the 1281.

Another thing I learned from owning two, and working on a few more
1281's is that the first thing to do is replace all the aluminum
electrolytic caps on the analog board as a minimum. Especially the small
1uf caps on the 3-terminal regulators die and the noise increases. This
will cause all kinds of problems. I change them all, on all the boards.
Due to the low profile of the larger caps careful selection is necessary.