First post, by Archon
Hi, I've just joined, but was visiting the site quite often. It really helped me while working with some retro stuff, but now I have a "serious" problem.
I would ask you for your help with 286 motherboard that completly refuses to work. I found a very similar one here https://theretroweb.com/motherboards/s/auva-c … -,-sam-12-m2040, but also I've attached a picture of it.
So, what's actually happening? Motherboard seems to be dead, gives no RDY signal - hangs on RESET with no beeps and no video. I've spend couple of days trying to figure out what's wrong and here's what I know:
- there's no direct short on the board, caps seems to be ok;
- no signs of batery leakage. I've bought the board with no installed one, hovewer I've checked all the nearest traces for continuity - everything seems to be fine;
- tried to switch n80l286-12/c2h with another, that I'm pretty sure is the working one - no difference;
- tried to trace RESET signal over the board, done some measurements with the scope and it seems like it does not react to manual reset whatsoever;
- measured all the oscillators and they gives expected frequencies (i.e. 24MHz for the CPU);
- PowerGood signal is present, `mc146818p` seems to work, however I've socketed it and bought a new one - no difference;
- I've also switched keyboard controller with different one but from the same era let's say (I've read somewhere that they are interchangeable) - no difference;
- placing the rechargeable battery gives no difference;
- switched the BIOS ROMs to different ones that should actually work - no difference;
- ISA slot pins are not connected with each other (they're not shorted). Also I've checked the back side of the board and it looks all good;
- removed all RAM chips, swapped them, placed only 256 KB in the first bank, etc. - no difference;
- tested the board using two working power supplies - no difference;
Currently there are no caps near the RAM modules - I've desoldered them and now I'm waiting for the new ones, but afaik these are only "filters". There was also a ceramic cap near the CPU - I've desoldered it as well just to check if the cap is bad - still, no luck.
What I've missed? Is this motherboard completly dead or maybe there's something obvious that I'm not aware of? Please give me some advice, I'm desperatedly want to make it working again 🙁.
Thanks!