Oscilloscope UNAOHM G4020 = BECKMAN 9020 = Voltcraft_2040

[neuron]

Hi,
A friend offered
Oscilloscope UNAOHM G4020 = BECKMAN 9020 = Voltcraft_2040

In first hours worked ok ... I saw the 0,2 V 1000Hz
and other signal in the cinescop.

But now I only see 1 or 2 dots (channel 1 or both or ch 2)

I can move the dots UP and DOWN, LEFT and RIGHT
so the screen - cinesc. is working.


I test the trigger, connect CAL 0,2 to ch 1 or 2
and the dot is in 1 , 2 or more lines up depend on VOLT/DIV
INV - and the dot go down invert.

A sinos. wave, I can rotate TRIG LEVEL and the dot follow
the wave.
But I can't see the wave in display


Some examples of scope:
youtube.com/watch?v=3nBYp1pN40U

youtube.com/watch?v=fPq1KgQ5ggQ

youtube.com/watch?v=5osskamWJHg

Manual and schemat. I have the French and Germany version.

archive.org/details/voltcraft_Voltcraft_2040_Oscilloscope_Service_manual

eserviceinfo.com/downloadsm/63296/Beckman%20Industrial_9020.html

Can you please provide help?

Thank you,

Ruy

[neuron]