Rigol DS1074Z - strange reading, help needed

[Riker]

Hello everyone,
I have a big request for help. Yesterday I probing simple circuit based on Arduino and TIP122 for 12V solenoids valves. I sending a short 20ms pulse to open the valve and trigger at the scope. Everything was fine until I set the scope scale to 500mV/div. Readings has unexpectedly changed and I have no idea why. I figured that the "transition" occurs between 750-760mV/div. Higher scale is ok (800mV, 1V, 2V...), but the "750" and below causes bad readings.

I know it sounds strange but please take a look at pictures and tell me what you think. Why a simple change of scale effect on the results? Is it something I'm doing badly?

Setup:
DS1074Z @ default settings, sw 00.04.01.SP2, hw 0.1.1
http://www.ecs.umass.edu/ece/m5/images/TIP122_transistor_schematic_S.jpg
Measured between GND and tip122 collector, pulse duration is 20ms
Screen captures:

pic_01 (5V/div)
pic_02 (1V/div)
pic_03 (760mV/div)
pic_04 (750mV/div)
pic_05 (500mV/div)
pic_06 (300mV/div)

Problems do not occur in other cases (at least, I haven't yet noticed ...). It seems that this particular example is somehow strange ...
For example, if I measure the base of a TIP122 everything is ok. The duration and all the rest are correct.

I'll be grateful for your help. Thanks in advance.
Best regards,
Krzysztof

[David_AVD]

Ask yourself what the voltage range of the signal you're trying to look at is.  Then work out the full scale capability of each range you chose.

[Riker]

In fact, that's right. If I decrease the the voltage on the valve, "the problem" disappears. I appreciate your help. This is my first oscilloscope and I still learning