you can ignore -5V, not needed for boot
cold 5V cpu is weird tho, it should be getting 5V right away and at least getting warm even if held in reset?
do you have a multimeter?

rasz_pl wrote on 2026-07-12, 15:27:

you can ignore -5V, not needed for boot
cold 5V cpu is weird tho, it should be getting 5V right away and at least getting warm even if held in reset?
do you have a multimeter?

Yes, I have a multimeter, but I'm not very experienced with it. You'd need to tell me step-by-step what to check and how to do it... thanks.

just for sanity lets measure CPU supply, no idea how cpu could not be getting power, but hey you say its cold so who knows:
Multimeter in Voltage mode range <10V.
remove everything from motherboard, no cards no ram no cpu, just psu connected
put a steel needle in any of the Vcc holes in CPU socket, pinout on page 6 https://theretroweb.com/misc/documentation/24 … 22046088602.pdf for example C1
stick black multimeter probe into molex connector (the one for hard drives) middle pins (ones going to black wires)
red probe on the needle
turn on PSU and read the voltage, should be 5V

if that passes
connect a switch to Reset header, or have screwdriver in third hand to short it
needle in L18
multimeter in voltage mode
power on
press/short Reset while looking at multimeter to see if reading changes from 0 to ~5V and back to 0 after letting reset go

if that also happens then hmm it might be the case of one of those motherboards that act dead if Dallas RTC battery is dead. I dont know how to check it externally, afaik its encapsulated shut in plastic so you would have to pull one out from another motherboard or drill into this one _gently_ to get to battery terminals . This shows where battery terminals are inside plastic
https://www.classic-computers.org.nz/blog/200 … attery-chip.htm
https://www.dreamcast.nu/en/dallas-rtc-battery-replacement/
You dont have to desolder the module to get to them, but this will be quite the operation to expose those. First remove BIOS from socket, then either dremel gently or use sacrificial soldering iron to melt plastic away. Then measure battery voltage, if <2V solder new battery.

Another easy thing you can attempt is to flash BIOS into fresh chip if you have access to eproms and programmer.

$10 post card, usually sold under stupid names like "PCI ISA Diagnostic 4Digits Card Motherboard Post Test Analyzer Checker" on Amazon/aliexpress, would be great here.

Although this should not explain the no-POST, you aren't really testing with RAM in, because you need a matching pair of SIMMs. The Pentium memory bus is 64 bits wide, and one SIMM is 32 bits wide, and I don't believe your 430LX chipset is one that can cope with only one SIMM in a kludgey way like some can. Also, searching the HY5117404C chip on your SIMM, that is EDO and the 430LX only supports FPM.

rasz_pl wrote on 2026-07-12, 19:54:

just for sanity lets measure CPU supply, no idea how cpu could not be getting power, but hey you say its cold so who knows: Mult […]
Show full quote

just for sanity lets measure CPU supply, no idea how cpu could not be getting power, but hey you say its cold so who knows:
Multimeter in Voltage mode range <10V.
remove everything from motherboard, no cards no ram no cpu, just psu connected
put a steel needle in any of the Vcc holes in CPU socket, pinout on page 6 https://theretroweb.com/misc/documentation/24 … 22046088602.pdf for example C1
stick black multimeter probe into molex connector (the one for hard drives) middle pins (ones going to black wires)
red probe on the needle
turn on PSU and read the voltage, should be 5V

if that passes
connect a switch to Reset header, or have screwdriver in third hand to short it
needle in L18
multimeter in voltage mode
power on
press/short Reset while looking at multimeter to see if reading changes from 0 to ~5V and back to 0 after letting reset go

if that also happens then hmm it might be the case of one of those motherboards that act dead if Dallas RTC battery is dead. I dont know how to check it externally, afaik its encapsulated shut in plastic so you would have to pull one out from another motherboard or drill into this one _gently_ to get to battery terminals . This shows where battery terminals are inside plastic
https://www.classic-computers.org.nz/blog/200 … attery-chip.htm
https://www.dreamcast.nu/en/dallas-rtc-battery-replacement/
You dont have to desolder the module to get to them, but this will be quite the operation to expose those. First remove BIOS from socket, then either dremel gently or use sacrificial soldering iron to melt plastic away. Then measure battery voltage, if <2V solder new battery.

Another easy thing you can attempt is to flash BIOS into fresh chip if you have access to eproms and programmer.

$10 post card, usually sold under stupid names like "PCI ISA Diagnostic 4Digits Card Motherboard Post Test Analyzer Checker" on Amazon/aliexpress, would be great here.

Okay. I'll give it a try and let you know. Thanks for now.

jakethompson1 wrote on 2026-07-12, 20:02:

Although this should not explain the no-POST, you aren't really testing with RAM in, because you need a matching pair of SIMMs. The Pentium memory bus is 64 bits wide, and one SIMM is 32 bits wide, and I don't believe your 430LX chipset is one that can cope with only one SIMM in a kludgey way like some can. Also, searching the HY5117404C chip on your SIMM, that is EDO and the 430LX only supports FPM.

Do you think I picked the wrong RAM module? I was actually wondering about that myself... Can I post the ones I have available so you can tell me which ones would work for my motherboard?

A pair of 4MB, 70ns FPM SIMMs would be pretty standard when this was new.

Badly matched ram might be one of the factors, but wouldnt explain no beeping without ram " I tried booting without RAM"

rasz_pl wrote on 2026-07-12, 21:14:

Badly matched ram might be one of the factors, but wouldnt explain no beeping without ram " I tried booting without RAM"

Agreed, just pointing out the OP has not actually done a valid "with RAM" test as of yet.

Hey guys, I’ve taken better photos. The power supply starts up, and the processor fan does too. But there’s no beep and no sign of life. The CPU stays cold. Could this capacitor be the cause?

you still didnt measure CPU supply
blown C12 cap is on 12V line so not a cause
C13 on the other hand, there is a burn mark discoloration visible under it, maybe even inside PCB material
its theoretically possible 5V is not sunk into solid power plane but first feeds thru just a track somewhere under C13 - in that case hard shorted tantalum down the line could have fried that track inside the PCB cutting 5V from whole board, but its just speculation until you MEASURE

something easier to measure
AT power connector pinout https://fs3.kel.si/instructions/atpwr-pinout.htm
measure resistance between RED pins (5V ones) and last pin (28) of BIOS chip (chip with a sticker, next to CPU) and pin 40 of keybaord controller (40 pin chip next 2 chips over from BIOS). Pin 28 and Pin 40 are the pins with small surfaece mounted component right next to them, those are small capacitors.

rasz_pl wrote on 2026-07-14, 11:53:

you still didnt measure CPU supply blown C12 cap is on 12V line so not a cause C13 on the other hand, there is a burn mark disco […]
Show full quote

you still didnt measure CPU supply
blown C12 cap is on 12V line so not a cause
C13 on the other hand, there is a burn mark discoloration visible under it, maybe even inside PCB material
its theoretically possible 5V is not sunk into solid power plane but first feeds thru just a track somewhere under C13 - in that case hard shorted tantalum down the line could have fried that track inside the PCB cutting 5V from whole board, but its just speculation until you MEASURE

something easier to measure
AT power connector pinout https://fs3.kel.si/instructions/atpwr-pinout.htm
measure resistance between RED pins (5V ones) and last pin (28) of BIOS chip (chip with a sticker, next to CPU) and pin 40 of keybaord controller (40 pin chip next 2 chips over from BIOS). Pin 28 and Pin 40 are the pins with small surfaece mounted component right next to them, those are small capacitors.

Here are some new photos, including the back. It doesn't look like there are any damaged traces.

this