Gerhard and all others owning a SNA-20/23/30/33 Series A..E:
I had the same problem with the zebra stripes with a C-Model after the Dallas RTC with SRAM and Li-Battery was completely empty. I did not yet modify the dallas chip with well-known external battery mod, but pressed DEL (or CLR on internal keyboard) on my external keyboard during bootup to enter the BIOS. Then I modified the Standard BIOS setting for HDD C: to „Disabled“ (from the default setting „8“). Hit ESC, and then ALSO enter the Advanced BIOS. Here I set the Shadowing to NONE or Disabled (default value BOTH is wrong!)
Hit ESC, selected „Save to CMOS and exit“, confirmed with „Y“ and now it reboots.....
DO NOT SWITCH OFF THE SNA!
My SNA-23 now boots from C: (my orange Renata Battery is still working), starts the measurement SW, AND....
TADA...
shows the spectrum!
NO ZEBRA ANYMORE
Looks like the Zebra-Effect comes from a memory conflict of BSK-3 with enabled BIOS shadowing.
The service manual even mentions that shadowing must be disabled, otherwise conflicts will occur.
So as a Conclusion:
Whenever your Dallas Battery is empty and CMOS writes will not be stored anymore, remember to set HDD C: to disabled in Standard BIOS and Shadowing to disabled in Advanced BIOS, after installing a fresh battery.
Hope this helps to get rid of lots of zebra cases out there.