On 15/12/2020 15:52, Grumps wrote:
On 15/12/2020 08:37, Martin Brown wrote:
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.

There is often a potentiometer somewhere as well with the timebase
only being strictly calibrated when it is at one end of its range. My
first guess would be a dry joint causing trouble.

That particular pot doesn't have the range that I remember when I last
used a scope like this. It does vary the TB a bit though.

If anyone can give further clues that'd be great. Or if there are
more relevant forums available to post to.
Ta very much.

How much is the timebase shifting around by?

About +/- 25%.

My suggestion would be to check that pot and any trimpots there might be
inside for signs of damage or erratic behaviour. It may work OK when at
the lesser used end of its track and be worn or damaged at the other.

If all else fails use it in X-Y mode and build a new timebase sweep.

--
Regards,
Martin Brown