Jim and JURB,

Ok, I've checked the value of the offset that appears when using the
"ADD" function and it is a constant one division (5 subdivisions)
regardless of the vertical scale deflection setting. (Of course, both
channels are always set to the same vertical deflection). So does this
suggest that the problem is in the channel preamps or a following
stage?

Also, I poked around the vertical switching circuit on the input side
to U370 (monolithic switch) and found all DC voltages to be consistent
with the schematic, thus suggesting that there's nothing wrong with it.

Suggestions?

Thanks. This is very helpful.

Best,

-Dave