Oscilloscope HPS50 with display problem (contrast)

Hello Velleman support team.
I live in Portugal and 1 year ago I bought an HPS50 oscilloscope on a electronic repair shop. I just used it a couple of times but since the first time of use I noticed the following problem:
I connected the 12V/500mA (Valleman) original power supply and when I press the On/Off button the display shows the Valleman logo with high contrast fill in all rectangle display. I can see the logo but all display get very dark (with maximum contrast) and after a few seconds the logo disappears and a normal screen with an horizontal line appear and the contrast is back to normal.
Then suddenly times to times all the rectangle display gets with very high contrast again just for a few seconds and then gets back normal again (normal contrast). This happen randomly and just for a few seconds. All the rest is working well. The big problem is the sudden high contrast that appears randomly fill in all display but just for a few seconds.
I already disconnected the battery and reset the oscilloscope for several times but the issue still remain which is very annoying when you are working with it.
Since the oscilloscope is not in guarantee period anymore, could you help me and tell me if there is anything else I can do myself to solve the problem? I’m electronic technician and I’m able to deal myself with electronic hardware if this is the case.
I would appreciate your kindly help.
Thank you in advance for your attention and I’ll wait for your reply.

Since you have had it so long and not used it I wonder if the battery is causing the problem
Have you tried to power it up without the battery?

Hello Wrong_Way.
I removed the battery and connected only the power supply and the issue remain.
I did a reset by pressing the reset button for several minutes and still the same.
The high contrast fills the entire display when I turn on the oscilloscope and simultaneously with the Velleman logo and seconds later everything returns to normal. I can see the logo in the background of the high contrast. All the rest works fine.
However the high contrast suddenly reappears in a sudden way and fills the entire display. It lasts a few seconds and then everything goes back to normal. It appears to be some bad contact or intermittent component failure.
I await more suggestions.
Thank you.

Unfortunately that was the only idea I had.

Thanks for your kindness. I appreciate it.

I’ll wait for more suggestions.