One of my favorite test instruments is an -hp- 339A distortion analyzer (THD meter). The only maintenance I have needed to do is cleaning the rotary switches and the unit seems to meet all its original specifications.
However, it has one troublesome habit: whenever I switch mode, the meter momentarily pegs and takes several seconds to come back on-scale. This is very annoying when doing THD as a function of signal level, since that requires switching back and forth between "input level" and "distortion" for the panel meter, and sometimes it's not clear if a pegged "distortion" reading is really pegged, or if I need to wait for it to recover. This also happens when switching the hpf and the two lpfs in and out.
Is this behavior normal for this instrument, or is there a repair/fix?