Hi guys,

I'm new to Vogons (kind of) and have started looking at repairing some old 486 motherboards and am a bit stuck. I am not sure if anyone here is an electronics guru for these kinds of boards

I have a post card that shows the reset LED always on and all reset pins on all chips are high across the entire board

I have checked the clocks and they are all OK.

I have checked all the voltages and they are OK too (voltage regulator also changes based on jumper settings).

My understanding of most of the boards around this era is that they use the powergood signal for reset and that reset should go high for a second and then go low so it can then initialise the BIOS code

I have traced what I can and found the following:

powergood signal (5.1v) goes to pin 5 (input) of the 4069 Hex Inverter

pin 6 (low inverted output) of the 4069 appears to go nowhere unless it's broken somewhere inside the board as I cannot see any traces coming to/from that pin. (I'd expect this to go to a schmitt trigger buffer / hex inverter somewhere or 2 before getting to the chipset)

powergood signal splits off before getting to pin 5 of the 4069 and goes to pin 8 (input) of an SN7407N schmitt trigger buffer

pin 9 (output) of the SN7407N then goes to a diode which goes to ground

positive diode pin splits off to a 0.04 nf capacitor which goes to ground

positive pin of capacitor splits off and goes to the chipset (W83C491) - I am unsure what pin this is as I can't find the datasheet for this chipset, all I know is that it's a renamed Winbond / Symphony Labs

I have also traced back the RESETDRV from the ISA Slots

this goes to pin 10 (output) of SN74F04 Hex Inverter

then pin 11 (input) goes to pin 3 (input) of the 4069 Hex inverter (very odd)

pin 4 (output) of the 4069 Hex Inverter goes to the chip select of the RTC clock chip

I traced the reset switch jumper pins and they go straight back to the chipset (W83C491) - It doesn't seem to go through a buffer at all which is odd

I think I traced some of the other pins on the Hex buffers and inverters which also go back to the chipset but without the datasheet it's really hard to know what they are doing

I assume there is a RST In pin and Powergood / PWR OK pin on the chipset but don't know

Any help would be appreciated

Links to the various chips etc

https://theretroweb.com/motherboards/s/dataexpert-exp4045

https://pdf1.alldatasheet.com/datasheet-pdf/v … TI/SN7407N.html

https://www.ti.com/lit/ds/symlink/sn74f04.pdf … uct%252FSN74F04

https://pdf1.alldatasheet.com/datasheet-pdf/v … C14069UBCP.html

Thanks,