However both traces are visible (two solid lines) in 'chop' mode, but both disappear when I'm adjusting vertical position pots or any switch.
Is this repeatable?
When you connect the calibrator to the scope in 'chop', does the sweep look right? E.g. is the signal in scale and linear?
> u870 has incorrect voltages on some pins:
pin6 (auto rc) -0.3v (+5.2v in service manual)
pin17 (swp disable) -0.2v (+0.3 in manual)
pin19 (auto) 0v (+2.5v in manual)
The service manual specifies to measure these in NORM mode, with sweep not running. From the zero volts you measure on the auto pin, I gather you've measured these in auto mode?
Can you induce a sweep with the single sweep button?
I don't own one of these old beauties, but I've read tales of repairing the sweep by re-seating socketed transistors <>. Does your scope have socketed transistors?
Depending on your symptoms, she may also be due for switch cleaning.
