Auato---I had the same "Trace Drift" problem, but only on channel #2.
I wildly and quickly rotated the Vertical Position knob back and forth, making sure to engage the spring loaded detent on each CW and CCw rotation of the position knob.
I did this about 20 times and the Vertical position adjustment stabalized and started working again.
I just discovered this and can't be sure that it is a long term fix, but for now, mine is working again.
I assume(?) the spring loaded detent, on either side of the knob's free movement, enguages a "switch" that allows an internal ckt, to provide pulses for the "Vertical Position"/ data word, for that channel's "Vertical Position".
I suspect the switch had oxidized contacts from setting and not being used regularly?
I hope it works for you too.
Frank Hertel