Initial testing was done with the 5.21 firmware, which has the PX-8 driver programs.
This firmware uses the EF9345 Display Processor pin 5 (HVS/HS) for the sync signal.
When the 7.11 firmware was used for testing, the vertical sync was missing, resulting in a vertical rolling of the screen. The scope confirmed that the vertical sync is present on pin 6 (PC/VS). This led to the fix described below, which ANDs the two signals to a combined sync. But then the 5.21 stopped producing a screen, as pin 6 is low continuously. 

The fix for 7.11 broke the 5.21 firmware. Looking at photo's of the original board, it seems pin 6 is not connected at all to the video circuit (it goes to a strip of test points, but stops there).

My solution to this problem is adding an extra jumper to be set or un-set when you switch firmware version. This would mean a new version of the board, which I don't like. An other option would be to patch either ROM to match the sync configuration of the other, but I haven't looked seriously at a disassembly yet.

The problem only exists when you want to switch firmware. For either firmware is a
working circuit, but they are not the same.

Fix for firmware version 7.11

This is the description of the fix for the 7.11 firmware but will break the 5.21 firmware. An unmodified 0.10 board will work with the 5.21 firmware. With the fix
in place, disconnecting the diode from pin 6 will restore the original situation.

The cathode of the diodes is where the band is (usually black or white). The diodes
I used were BAT85, but most generic small signal schottky diode will probably work.

• Cut the trace between U5-5 and U8-2 (old HS)
• Add resistor of 5.6k between U8-14 and U8-1 (pullup resistor)
• Add cathode of 1st schottky diode to U5-5 (new HS, horizontal sync)
• Add cathode of 2nd schottky diode to U5-6 (new VS, vertical sync)
• Connect the anodes of the two schottky diodes (new CS, combined sync)
• Add a wire between the two schottky diode anodes to U8-2