After some time I found a solution for LOAD but not for SAVE. During LOAD sync pulses are generated every 30us instead of 64us. So I added a filter to suppress sync pulses coming to fast. During high period the capacitors of the standby sync generators are unloaded. Unfortunately this does not work during SAVE because pulses are very different. But I think a picture only during LOAD would be acceptable as SAVE isn't used very often. People are not programming or entering information on ZX81 - more load programs from a tape or PC.
Now the picture during LOAD is nearly the same as known very well from the past when using analog monitors. Could see this scratching screen very well and looks amazing - same as in the past. Today many modern LCD or Plasma TV's are not displaying a picture during LOAD / SAVE as the sync signal are out of specification. Some TV's just display nothing, some switch off or switch to another source automatically after a timeout of 30 or 60 seconds.
Today I loaded all new 1k programs from Dr: Beep and tested them with the video mod. Display is 100% okay, just one program (RETURN) has some mysterious disturbing. I am not sure if this belongs to the program or maybe some bad timing in the HIRES program. The other 9 are 100% okay. And the shown screen during LOAD is very realistic. Great. Loaded all games with my IDE via the sound card without any problem (first try).
Changed schematic (forgot to block the unloading of the second capacitor for hsync pulses):
- ZX81SCP_sch.gif (21.12 KiB) Viewed 7936 times
- ZX81SCP_board.png (10.84 KiB) Viewed 7936 times
Looking forward to my boards!
I second that! 
