On 15/12/2020 11:49, Fredxx wrote:
On 15/12/2020 08:22:41, 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 would be searching for the manual. Apart from leaving the 'scope
turned on for a while to see if settles down finding the fix is like
shooting in the dark.

This might help:

https://elektrotanya.com/goldstar_os.../download.html

You will also need DjVuLibre or similar app to view the file.

As others have said it could be a noisy pot, but then resistors can
become noisy with age. Look at the wiper on VR601 on page 56 of the
manual, and check how stable the voltage is.

Good luck.

I have the original printed manual which has the schematics.
VR601 sweeper shows a voltage that varies in amplitude in harmony with
the TB frequency drifting.