NMS8250 upgrade to MSX2+ and flickering

the heatsinks is not present on real MSX2+ with V9958 !!

IMHO, the heatsink might be needed since there are no bus termination series resistors on the VDP address bus in NMS8250.

Worst case - VDP99x8 in Yamaha YIS604 driving simultaneously 2 banks of 8x4164 DRAM chips. There it is really getting hot.

Hello,

Let's put your "flickering" definition aside. If the screen moves from left to the right 1mm, you have a sync problem. And since V9938 and V9958 are different in a way how the CSYNC drive is implemented, here is the question:

Have you increased the value of R357 from 4.7K to 10K? And have you connected the 100pF capacitor in parallel to it?

If you didn't, then your CSYNC signal has quite a bad slew rate, which will affect the TV sync and then your raster will wobble.

Don't use 100uF, since it is 1000000 times bigger capacitance than 100pF! You will kill your VDP.

Try to look for a disk ceramic capacitor from the old radio or so. It will be marked as 101 underlined. You can use any capacitance between 100pF to 560pF. Check it with multimeter. If it shows infinite resistance, it will do the job.

Both V9938 and V9958 are pretty power-hungry for a passive cooled plastic part. So a heatsink makes sense in any case.

A reason for 'wobbly sync' might be if the +5V on pins 21 or 58 is poorly filtered. In which case it might help to insert some filtering components in the +5V line going to those pins. Like a miniature coil of a few dozen µH followed by a few hundred µF capacitor (or something along those lines - check some MSX2 -> MSX2+ descriptions online).