1040 ste - no Med-Res display - multiplexer ?

[Sbn31]

Hi, I'am experiencing a display issue with my 1040 ste, it won't display Medium Resolution on a sc1435 monitor, I'm getting a black screen. (In fact it could switch to Med-Res like 1 time on 10 try. )
- No problem to display Lo-Res or Hi-Res on a Sm124 monitor.
- Cable checked, solder on 13pin video connector, checked,
- STE psu has been recaped few month ago, voltages are ok
- I tried with original ram simm and other kind
- i tried tos 2.06, tos 1.62
- the motherboard is a CA4003290 with 1712 shifter

I'am suspecting the multiplexer 74F257AN and i should try to replace it.
I tested continuity on the multiplexer pin connections (with no power) it looks normal.
I ordered 10 chip on aliexpress but none are working I get white screen on boot.

Now at Jemeco i found a Motorola MC74F257N and a generic 74LS257 as replacement, at Mouser there is a Ti version 74F257N, can't see any difference on datasheet, they are all suitable référence ?

Many thanks !

[exxos]

The multiplexer controls colour / mono. It shouldn't matter for low / medium resolution.