Oh wow, I never seen anything fall so quickly on second page. Huh, why are my questions always unanswered? Maybe because my questions have no answer all the time 😁

A constant beep on an IBM clone BIOS means that there's a short somewhere. It can also indicate that some of the voltages are missing.

Do you hear the beep also without CPU installed?

Read the ROM as 27C512.

Predator99 wrote:

Do you hear the beep also without CPU installed?

Read the ROM as 27C512.

Oh, I didn't think about that. Actually without CPU the board does the exact same beep. And even without the ROM chip it does that.

I read the ROM as you said, but the code was present two times and with lot of margin space, first from 0x6000, second from 0xE000. It is saved though so I can burn it back if something goes wrong.
Also there is some super duper old PROM chip 74S287, but I have no idea how to read that.

I am using known good PSU by the way if you ask.

Then its actually a 27C256, half the size. The image you read is correct, but includes 2 copies.

74S287 is most likely a logic IC (74LS287), you can also check it with the TL866.

Indeed, than the beep indicates a short. Leave CPU+ROM uninstalled until fixed. Check the pins on the back if some are touching each other. Check the power lines on the ISA bus. Buy a POST card 😉

Supposing it works like an 5150/5160 motherboard...does it still beep with P8 removed?
I also have an 5160 board that shorts but PSU works when removing P8 from the PSU connector on the board.

Of course the beep stops when P8253 is removed, but that is because the P8253 itself is connected to the oscillator and generates square wave in its counter which is modulated by the UMC82C55A. Both of those goes into NAND gate, then through NOT gate and straight to the speaker. If one of those are not present, the NAND gate will never activate and never beep.

The motherboard looks OK, no shorts visible. If it was, I would already notice it. And all the voltages seems OK to me.

Failing tantalum caps are not always visible. Learned this the hard way.

Umm.. I guess there are some, but not very much. And tantalum caps has really bad habit of showing they are indeed bad by exploding and horrible smell... Probably will change them in future, yeah, but thay have to arrive first 😀

Today I made myself an reduction for my logic probe, so I hope some action happens now. Sadly, I start to point at the UMC chip, seems to be messed up.

Good with your trouble shooting! Would be sad to throw such a nice board away. Hope you will find the defect!