Here's a picture of the whole mainboard, I believe it's in the 2155 family of IBM PS/1 motheboards.
https://theretroweb.com/motherboards/s/ibm-ps … t2155a,-mt2168a

You could check the reset signal c16 from what I understand reset should be present on boot then go down, I actually have this problem on one of my boards where reset is stuck on constant 5v which means it won't let the cpu progress further in boot

Have you tried different PS/2 keyb + mouse? Their solder joints?

Did you check all voltages? The LEDs may light up despite the voltages being out of spec. Out of spec voltages may prevent the Power Good signal from being generated causing the PC to do nothing.
It would be unusual for reset to be issued for just a moment if Power Good is missing though.🤔

Edit: How/why is the +3.3V LED lit on your POST card if you are using it in what looks like an ISA slot?

I went ahead and checked that and I'm not seeing any voltage so I'm guessing that's not the problem.

treeman wrote on 2023-02-27, 01:28:

You could check the reset signal c16 from what I understand reset should be present on boot then go down, I actually have this problem on one of my boards where reset is stuck on constant 5v which means it won't let the cpu progress further in boot

I've tried with and without them plugged in and had no luck. In the past, if they weren't plugged in I would get an error code on the monitor for the keyboard.

Nexxen wrote on 2023-02-27, 01:48:

Have you tried different PS/2 keyb + mouse? Their solder joints?

Good catch, it never struck me that the 3.3 light wasn't supposed to lit. I went ahead and tested the 5 -/+v and 12 -/+v on one of the isa slots and it looked ok.
To figure where the 3.3v led was coming from I swapped out the AT psu I was testing with for a modern one that doesn't provide the -5v. Once on the new psu the 3.3v light went out and when I switch back to the old AT psu it lit back up. I'm guessing even though it isn't documented that the 3.3v led will light if the -5 rail is hot?

Doornkaat wrote on 2023-02-27, 07:26:

Did you check all voltages? The LEDs may light up despite the voltages being out of spec. Out of spec voltages may prevent the Power Good signal from being generated causing the PC to do nothing.
It would be unusual for reset to be issued for just a moment if Power Good is missing though.🤔

Edit: How/why is the +3.3V LED lit on your POST card if you are using it in what looks like an ISA slot?

You will need to disable the 486SX via the B14 pin which is on this upgrade socket, via a small resistor to ground when using the 5x86 CPU. A quote: "Pull the socket pin B14 low via ground this connects to the UP# pin on the 486SX. Then you can use normal 486DX or DX2 CPU without need to buy a overdrive ..."

Cheers,

That shouldn't be needed on this board unless something else has gone wrong. If I move jumper j25 to the topmost two pins that tells the board to use the overdrive socket. This behavior was working just fine a few weeks ago but now it doesn't matter what position it is in. If I remove the AM586 cpu and move the j25 jumper back down there's no change.

pentiumspeed wrote on 2023-02-27, 19:40:

You will need to disable the 486SX via the B14 pin which is on this upgrade socket, via a small resistor to ground when using the 5x86 CPU. A quote: "Pull the socket pin B14 low via ground this connects to the UP# pin on the 486SX. Then you can use normal 486DX or DX2 CPU without need to buy a overdrive ..."

Cheers,

failuresuccess wrote on 2023-02-27, 19:56:

That shouldn't be needed on this board unless something else has gone wrong. If I move jumper j25 to the topmost two pins that tells the board to use the overdrive socket. This behavior was working just fine a few weeks ago but now it doesn't matter what position it is in. If I remove the AM586 cpu and move the j25 jumper back down there's no change.

pentiumspeed wrote on 2023-02-27, 19:40:

You will need to disable the 486SX via the B14 pin which is on this upgrade socket, via a small resistor to ground when using the 5x86 CPU. A quote: "Pull the socket pin B14 low via ground this connects to the UP# pin on the 486SX. Then you can use normal 486DX or DX2 CPU without need to buy a overdrive ..."

Cheers,

Maybe it killed a resistor? Check if any R connected (same line - ok, you get what I mean) to the pins.

I'll have to see if I can figure out the trace, the pin for the surface mount chip has that trace on the underside of the board. That said perhaps I'm not being very clear, I can't even get the board to start with just the onboard 25MHz cpu with proper jumpering.

failuresuccess wrote on 2023-02-27, 22:10:

I'll have to see if I can figure out the trace, the pin for the surface mount chip has that trace on the underside of the board. That said perhaps I'm not being very clear, I can't even get the board to start with just the onboard 25MHz cpu with proper jumpering.

To me, in my ignorance, it is a dead smd. Have yo checked the mosfets?
Necroware has many interesting videos about it.

One fast test is to probe V on the socket to check if the cpu gets the right voltage.

failuresuccess wrote on 2023-02-27, 01:18:

I've tried testing with one of those PC Analyzer boards but the code readout never displays anything.

I'm not sure original IBM BIOSes actually output debug codes to port 0x80 for the POST card. I saw some dongle for parallel port for original IBM machines but I'm not sure which generation uses that method - never had original IBM PC.

A "dead" mobo can be anything really, from bad chips to microcracks in traces and vias. Before you start messing with the BIOS chip, especially since you want to preserve the sticker, borrow or buy a cheap scope and test the BIOS pins for activity. This will at least tell you if the CPU is running or not. And perhaps look into that printer port dongle thing, if your machine uses that kind of debug output you will need it.

failuresuccess wrote on 2023-02-27, 18:06:

Good catch, it never struck me that the 3.3 light wasn't supposed to lit. I went ahead and tested the 5 -/+v and 12 -/+v on one of the isa slots and it looked ok.
To figure where the 3.3v led was coming from I swapped out the AT psu I was testing with for a modern one that doesn't provide the -5v. Once on the new psu the 3.3v light went out and when I switch back to the old AT psu it lit back up. I'm guessing even though it isn't documented that the 3.3v led will light if the -5 rail is hot?

Yeah, sounds plausible. It would make sense to use the same LED to indicate -5V on ISA (since there's no +3.3V on ISA) and +3.3V on PCI (since that doesn't have -5V). Pretty good idea imho.👍