On 15/12/2020 12:10, Harry Bloomfield wrote:
The Natural Philosopher used his keyboard to write :
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.
Is there a pot involved? Most scopes have a fine tune pot for s3eep
speed...if that is scratchy...

Check the timebase voltage regulation.

I did quickly check it, and it looked OK at 12V.