Is that still an an address failure. ('Testing RAM - Address') ?
No, it stops at Check first 2 KB of RAM now
Which type of POST card:
ISA slot, or
parallel/LPT ?
ISA card. Sometimes it shows 00 FF, another time 00 04, 00 F4, 00 89 but maybe it doesn't matter
So things have gotten worse.
The motherboard now behaves unpredictably.
Sometimes it displays an image on the screen and then shows a first 2 KB error.
But sometimes it doesn't turn on and then the POST card only displays "bA"
Most often it does not turn on when I move the POST card to another ISA slot or when I remove and insert the U14 chip.
(The motherboard behaves exactly the same when I remove the U14 chip...)
When the motherboard does not display anything on the screen then there is no pulsation on pin 13 in the 8253 chip
I haven't found a reason why the motherboard sometimes doesn't turn on. Perhaps there is something wrong with the U14 socket, but I'm not sure. When I remove and insert the U14 chip several times or when I remove the POST card and insert it into another slot, the board starts working.
I think it's not a problem with the U14 chip because I have 3 different ones and each of them sometimes works and sometimes doesn't.
The socket is probably soldered correctly. I measured each pin on the top and bottom of the motherboard with a mult
imeter.
When it works, the readings on the memory chips are as follows:
A0 – High
A2 – High
A1 - Pulse
A7 - Pulse
A5 - Pulse
A4 - High
A3 - Pulse
A6 - High
And on chips U39 and U 40 the readings are as follows:
U39 | | U40 | |
1 nothing | 16 HIGH | 1 nothing | 16 HIGH |
2 pulse | 15 nothing | 2 pulse | 15 LOW |
3 nothing | 14 pulse | 3 pulse | 14 pulse |
4 HIGH | 13 pulse | 4 pulse | 13 pulse |
5 pulse | 12 HIGH | 5 pulse | 12 pulse |
6 pulse | 11 pulse | 6 pulse | 11 pulse |
7 HIGH | 10 pulse | 7 pulse | 10 pulse |
8 LOW | 9 HIGH | 8 LOW | 9 pulse |