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.

[AtariPope]

had similar problem and it was because of MC1377 U403 which produces the sync signals for the low/med res for colour monitors. Remove R439 (exactly left to the modulator and towards the back of the motherboard in front of Q404) and solder at the RIGHT hole of it and underneath a 470ohm resistor and drive a wire to U211 pin6. This way you wont see the cable from above.

For a quick test, lift the LEFT leg of the R439 resistor and drive a wire from the top on pin6 of u211. no risk as you keep the resistor in series. so, pin6 of u211 to the right leg of the resistor on the air not soldered and not touching anything, left leg of the resistor as it is soldered. It should work. Also pin 2 of MC1377 is same with pin6 of u211 and it is closer