oscilloscope METRIX OX 8620

[gpascu]

Hello everybody my name is gigi I'm new in your community, and unfortunately I'll start with an issue that i had recently. I bought from internet an oscilloscope METRIX OX 8620 end is in a physically good condition, description of item was " used but almost new in very good condition and working very good", but when I switch on my new tool i was surprised to see that after 10-15 min the traces on display became both blurry and no change to be adjusted with *focus* knob and if I turn up "intensity" the fascicle became more blurry.
what should I do because I really want this tool but I have no much experience to fix it, if someone can give my some advice I'll be grateful. Many thanks.

 

[BradtheRad]

It is probably heat-related, since the problem takes a few minutes to appear.

Try touching components which adjust the focus. See if any get extremely hot. (To find the focussing circuit, either you will need a service manual, or you must examine the wiring, circuit board, etc.)

Do you have freeze spray? Apply it to cool components, and you may be able to find the culprit.

 

[D.A.(Tony)Stewart]

This scope uses average persistence Phosphor and uses a 15kV flyback transformer to scan the beam.
Since the problem is thermal, check that the fan works. Then vacuum/ replace fan or add one externally to the rear. Keep in mind 15kV hurts.

When beam current is high due to some reason, it gets out of focus hurling those electrons at the screen

... splaat.
. >>> ( * )

Use minimum brightness for clarity and phosphor longevity.