I have a brand new HPS50, clean out of the box.
I did this:
- Plugged the worldwide adapter into the Netz mains
- Inserted the 12VDC connector into the HPS50.
I saw: - The red charging light came on.
- The device sat for 10+ hours until the red light went out.
Then: - I tried to power On with a short press on the auto power timer. Nothing happened.
- I tried an On without power off timer with a long press. Nothing happened.
- I tried the Display/Setup button and the screen brightness buttons. Nothing happened.
I pressed the reset button for 12 seconds. The screen lit up, uniform bright grey. - I released the reset button. The screen stayed bright for 2 seconds and then went off.
I claim, as a scientist of 45 yrs standing, I did everything right. I once (1980s) even bought a Hewlett Packard sampling 'scope for £20,000 (also HP) and made it work.
I notice that the manual suggests that the batteries be removed for a moment or two if there are reset problems, but I am reluctant to remove the 6 screws to open a device that is new and under warranty. I would only do this if I got ‘authorisation’ to do it with an assurance that the problem will be solved with 99% probability. This is a scientific approach. After all, as the Velleman expert has said elsewhere, “electronics do not go wrong overnight.”
Oh, and the big news? The Uni bought the scope from Maplin Manchester for me and I have taken it (by Eurostar & TGV) to the South of France where I was intending to do field tests on land mine detection using ultrasound
robinmarshall.eu/betterway.html
So my project has been halted! Stalled. Stopped. Funding sponsors (Man U and Aon) asking daily how I am getting on. “Sorry Sir Bobby, kit isn’t working”. 
I really do not wish to spend €30 of my cash, mailing the 'scope plus functional chargers and cables, manual and boxes back to Manchester. Then another one gets mailed out and it might not work.
There must be a better way to get my scope working. 
Robin