On 15/12/2020 08:22, Grumps wrote:
Hi All
A friend has acquired an old Goldstar OS 7020 dual channel CRT based
'scope, and I offered to check it out to see if it is all working.
So, mostly it is fine except that the timebase wanders randomly,
sometimes speeding up, sometimes slowing down. And it does this to the
same degree in ALL timebase settings except the X-Y mode.
I have the circuits, they come with the manual, and have scoped various
points in the sweep generator circuit. When in X-Y mode the sweep gen is
still active and is stable.
There are at two main timing caps for the timebase depending on speed
selected, but I have ruled these two out as the source of the problem as
they're not used in all settings. There is also a little cap which must
be part of the timing cct, but even if that was duff, why should it have
a huge effect on the varying speed when in a slower setting as its
capacitance would be dwarfed by the main timing cap.

So, I'm a little stuck. It is an easy unit to probe as it uses all
conventional through-hole components and the sweep gen board is on the
top when you remove the cover.

If anyone can give further clues that'd be great. Or if there are more
relevant forums available to post to.
Ta very much.
I don't know that model but the I had a Tektronix scope "back in the
day" that developed dirty timebase switch contacts - might be worth
checking if your scope has the usual stacked wafer switches.
Alternatively (and I don't have the circuit diagram) might the output
stages be wandering, or there be a problem with the power supply. If
it's an old'ish scope, and without full test gear, some useful steps
would be to clean all switches and connectors, look for pcb cracks,
re-flow and dodgy-looking joints and replace any electrolytics.