To conclude this in case someone else stumbles on it:
- the NEO-244 at position C2 ( Buffer for P Bus) had half of it buffers shot - this caused the graphics problems
- the NEO-244 at position E2 (Buffers for ADPCM) had 4 input/8 output shot, this caused missing sound and probably instability, MS4 would hang in the demo mode at the same spot
- for the NEO-244 at position A2 (Buffers for control lines) you really want a perfectly working NEO-244, this one deals with the clocks, including the master clock with 24MHz, I wouldn't risk running it through patch wires with my routing skills
Lots of defektive outputs, sometimes just a single one of the two, I still replaced the buffer as such. Most of times both outputs would be defective, in one case the input turned into a an output..
FWIW, I wasn't able to check high frequency signals on A2 with my low quality logic probe, an old analog scope was much better for that.
I've used 74HCT244 SMD replacements on breakout boards, can't say for sure but I'm assuming the NEO-244 used LS, but HCT should be compatible.
two 244 for C2:
Each input is going to two outputs, each output is assigned to two slots. (1&2 and 3&4).
I've tried using a single 244, one input goes to one output and just wire that output to all slots, this would save half of the 244 needed to replace a NEO-244 (which has 5), but the risk is that the noise from 4 instead of 2 slots might cause instability.
I've stacked two breakout boards with an 244 each, connected the inputs (and GND, +5V, /OE) via pins, and routed each output separately to keep them separated.
(ignore the 3 long red wires, thats a spare buffer, I'll keep the wires until I'm certain I won't need it)
The bling-bling gold is a failed attempt to distract from the wiring mess.. but using same colors for each input helps to keep an overview.
another 244 for E2:
only needed to replace 4 input/8 outputs, a single 244 would do, also connected two inputs here (jumpers)
Graphics are fine now, sound works mostly now (still a minor issue on slots 3&4, I'll fix that later).
Replacing the NEO-244 with discrete logic ICs is a spacing/routing problem, replacing a single NEO- 244 means stacking 5 breakout boards with SMD ICs is possible but fugly, there would be nearly 60 wires coming out of that..
TBH, right now I'm not that confident about this machine, kind of surprised by the amount of failed 244s, I'll keep testing it until I'm more confident or it fails.
I'm not aware of any drop-in replacements for NEO-244, only way to get the original replacement parts to cannibalise other MV4F/MV4FS.