Lowering the temperature of 5012H did not help. It still locked up or reset after putting it in a ridge for several hours.
My hot air gun finally arrived today, 36 days after the 2nd order. Heating the 5012H up seemed making it more stable for a while. I amost thought the issue was fixed, but it's not.
Now it was consistent in locking up or reset when we put the input at the lowest gain 2 settings i.e. 5V and 10V (x1) at timebase at 50ns or shorter. If it did not lockup or reset, the 10V range sometimes picked up a few cycles of sinewave of around 62MHz at 6ns timebase.
With no probe connected or with probe connected and grounded at x1 or x10 did not make any difference. The amptitude of the sinewave was way too high to be coming from the probe.
If we used a higher gain setting, 2V or above, including 100mV and 50mV, 5012H survived all timebase setting including 6ns and did not pick up the 62MHz noise. This is the reverse of what we expected if the noise is coming from the input.
It suggests there may be a noisy grounding in the front end since in 5V/10V settings the input to the amplier sitting in front of the ADCs would be closer to ground Or there may be something wrong with switches themselves. The input switches seem OK at DC with lower speed timebase. Then with this "wetting" of -8V on the probe, I found that the 5V and 10V gain setting could survive the 6ns timebase. The 5012H uses optical couplers, would a photo transister work correctly as a simple high frequency switch before saturation, i.e. without any "wetting"? In the good old rotary selectors, we don't have to worry about that. We have more relays in the older version, now we cut it down to one....
Anyway the 5012H front end sucks. Even at DC I can tell that the probe isn't giving me 1/10 reading on 10x.
I remember seeing a circuit drawing of the input attenuators/selectors. Do you know which page is it on? I'll troubleshoot the 5V and 10V switches. Don't have another scope capable at 6ns range, but I can do some bypassing.
Lockup/reset on my 5012H may be a firmware bug or hardware related. It seems to happen on handling more complex and higher frequency waveform. The 62MHz noise picked up is not a continuous sinewave, its harmonics may be out of range. I guess that noise is driving the firmware/hardware nuts more often than showing up on the display.