after assembling my EDU08 kit, for the first few minutes, everything worked fine. I could measure some DC voltages and the AC signal from the EDU06 kit. I was even able to enter the menu of the osci. But then the screen turned into weird ascii and non-ascii codes, vanished, returned, and stopped with a horizontal bar.
After switching the device off and on, it worked again for a while, but shorter, before the same issue repeats.
It is reproducable. The longer I wait before switching it on again, the longer it may run before it crashes again.
Can you give me a hint where to look for the root cause?
Yes, that was my first suspect, too.
Batteries are fine, even replaced them - no change.
Checked the voltages at VR1. In: 6.1 V, Out: 3.3V
Checked the voltage at ZD1 (Anode): -5,2V
So far, everything is ok.
Then I noticed that the out voltages of VR1 (and the -5.2V at ZD1) were not stable, breaking down to nearly zero after a while.
I could not find any cause for that, so I re-soldered all joints in the surroundings of VR1. As a result, the voltages are stable now. Seems as if this was a problem with the junctions?! Whatsoever - I was in good hope that the issue with the oscilloscope was resolved therewith.
However: Tried again - issue was still reproducible!
Meanwhile I re-soldered nearly ALL solder spots from the base PCB - no change.
Then I re-soldered all solder spots of the LCD. This revealed an intense effect! I was able to operate the osci for a longer time before the failure reoccurred. But it still reoccurred…
Then I took an old toothbrush and cleaned the solder junctions of the LCD with lighter fuel on top and bottom side of the PCB.
–> wow!
As far as I can say from now: that’s it! It works now.
Must have been coming from the flux in the tin-solder. Is that possible?!
Allow me one more question: Is there any Logic in the LCD element? Something that can get out of sync? I mean, is there an own processor in the LCD unit that can hang?
Glad to see that you got the EDU08 working.
Indeed, the symptoms and the resolution (cleaning) indicate that there was some conductive residual on the PCB.
[quote]Must have been coming from the flux in the tin-solder. Is that possible?! [/quote]If you used pure rosin flux-core solder, it shouldn’t be conductive.
[quote]Allow me one more question: Is there any Logic in the LCD element? Something that can get out of sync? I mean, is there an own processor in the LCD unit that can hang?[/quote]Yes, there is a display controller (processor) in the LCD unit.