If it were me, I would pull the BIOS chip and read/reflash it with my chip programmer.

Repo Man11 wrote on 2025-03-11, 16:06:

If it were me, I would pull the BIOS chip and read/reflash it with my chip programmer.

I do not have the hardware to do that, and I also haven't updated the BIOS in a very long time

That's the only suggestion that I had as you've already covered everything else I would have tried (with the exception of trying another CPU). But have a bump in case someone else has an idea.

Booting without the CPU, expecting beeps but nothing

The CPU executes code from the BIOS, which is what contains the programming for it to beep. So if the CPU is unplugged you definitely won't get a beep.
When you attempted to boot with no RAM installed, a beep code would be expected. Since it didn't beep, it suggests that the CPU or BIOS isn't running or at least not getting far enough to reach that point.

The fans spinning means there's some kind of power on the +12V rail, but that's a long way from a working system.

I find I always have to reseat slot-1 CPUs (and sometimes the RAM) before they start working, but once things are working they usually stay working and don't need to be reseated again just because it sat idle.

Do you have a card that displays POST codes? This would at least show if the CPU/BIOS are doing anything. Makes it easier to know if the CPU is making contact and basically running, but beyond that the POST codes are sometimes informative.

Do you have a multimeter? If so I'd check voltages. If you have one and want more detail, I can go into it.

I find I always have to reseat slot-1 CPUs (and sometimes the RAM) before they start working, but once things are working they usually stay working and don't need to be reseated again just because it sat idle.

It did sat idle, but I can try again in the future. Perhaps grabbing another S1 CPU would be worth trying too

Do you have a card that displays POST codes? This would at least show if the CPU/BIOS are doing anything. Makes it easier to know if the CPU is making contact and basically running, but beyond that the POST codes are sometimes informative.

I have no image, and no hardware POST code display

Do you have a multimeter? If so I'd check voltages. If you have one and want more detail, I can go into it.

Go ahead, as I plan on grabbing one when I'll have the occasion!

TeknoRider wrote on 2025-03-12, 09:28:

Do you have a multimeter? If so I'd check voltages. If you have one and want more detail, I can go into it.

Go ahead, as I plan on grabbing one when I'll have the occasion!

You can put the black probe on any convenient ground point, like up against the case of the PSU or plugged into the black wire on one of the drive connectors. Then you just move the red probe around to whatever you want to measure.
The easiest voltage rails to check are +5V and +12V - you can test those from an unused hard drive power connector (red=5V, yellow=12V).
You can check 3.3V by backprobing an orange wire on the ATX connector that's plugged into the motherboard.
+5Vstandby is purple on the ATX connector.
The spec allows +/- 5% on the main PSU voltages (not sure about Standby power) but in practice if you're towards the low end of that range you're likely to see stability issues.
These voltages should be stable, they shouldn't jump around or drop appreciably under load.

On a Slot-1 board there's probably a pair of MOSFETs near the CPU which are part of the Vcore circuit. The easiest spot to measure on those without shorting anything is the large metal tab on the back. If you measure that tab, one of them will normally read the +5V supply going into the VRM, and the other will read the Vcore coming out of it which powers the CPU.

On most Slot-1 boards the SDRAM (and probably the 3.3V power pins of the AGP slot) are powered directly from the +3.3V rail that you can measure at the ATX power connector, but some boards generate that onboard.
Some Slot-1 boards don't actually use the PSU's 3.3V rail at all, and instead have an onboard regulator to generate it (and this component could fail).
The only way to confirm how the board powers 3.3V devices is to check continuity between a few places. For example, if you find a pinout of the 168-pin SDRAM DIMM sockets you can check for continuity between them and the ATX connector.