Since this scope, like most analog scopes, only produces 1 trace at a time (single trace scope), then the "double trace" is a result of some small shift in vertical position or offset that is either concurring on alternate sweeps, or is bouncing back and forth very quickly.
Slow the sweep speed way down, to something like 100ms/div so that each sweep is 1 second long. If you see a single trace that varies in vertical offset with each repetition, then you know you have some kind of sweep related offset issue. If you still see a double trace, then *carefully* work your way up through the various sweep speeds, *including* slowly adjusting the vernier, to see if you can find the repetition rate that is bouncing the trace up and down. You might then be able to relate that to something else going on in the scope.
Thanks for the reply, great videos by the way! The trace is actually doubled no matter what sweep speed. Even with a 1 second sweep there's two clear dots tracing across the screen. I guess its bouncing back and forth very quickly as you say?
Can you show just channel 1;
1) AC coupled [see if voltage measurement is correct]
2) DC coupled [see if voltage measurement is correct]
3) Ground [again].
...
Edit: Also check C36, C38 and C39 near the IC U10.
Thanks, I've attached the photos below - the two different probes were near identical so i combined the second probe pics into one image. From top to bottom they are - 1st probe with AC coupling, same with GND, then DC coupling, then the same again with probe 2. The final pic is a sweep on channel 2 with cursors for reference. As you can see the sweep seems rather accurate if you consider the lower of the 'double' vertical images.
The cal signal is 0.5v at a khz or so, which seems to be detected fine by the built-in meter. I haven't yet had the chance to take a look at the caps you mentioned, but i intend to do so tomorrow and will report back here. Again thanks for the help!
This scope is otherwise working fine, and there is no real urgency to this problem being solved - although it would be nice to have both main channels working right. I'm prepared to have a play inside, but if it involves changes that would drastically throw out the calibration then i'll probably just put up with it for now. Still though, would be nice to fix!
As i said above i'll take a look at the caps tomorrow and report back with pics.
Cheers again guys,
John