My B&K 707 sensitivity seems wonky

[tonyptony]

I've had my 707 for a while. I bought it used but completely rebuilt and it has until now calibrated just fine and worked perfectly. Lately I've noticed the meter needle being very unstable when doing the sensitivity readings, and tubes which had been checked in the past and are unused are now reading much lower - and again somewhat unstabl-ey. I went through the calibration procedure and was concerned when I got to the Balance adjustment. I put the Sensitivity dial up to 100 and it looked right from the start like it was okay (meter needle riding right at zero), but when I went to move the Sensitivity dial just to see what would happen the needle hopped around a bit but kept settling at zero, even when I had the dial set to 80. I don't remember this happening the last time it was calibrated. Doesn't seem right.

The actual sensitivity adjustment was fine - 1.504 VAC dead on and that was stable. But the needle still now seems very unstable when doing the Test1 and Test2 readings. Where should I start looking for the problem?