Ok, does it do the same with math-co removed and jumpered no-math ? Odd you do not get any good readings off the Diag card but not all early boards posted to port 80 iirc....and sometimes you needed to check a few diff slots.
Also do not rely on an ATX psu on an old 286/386 board, and do not use a cr2032 coin on one = my rules because they just have issues working proper in many situations or attempt to recharge it (bad thing).
Glad you got the board to post even if it takes a few tries. Probably have some leaky tantalum caps causing issues...

I would start with replacing the cmos battery. I've had many many boards over the years either play dead or act really strangely with a dead,low, or missing bios battery. Once the system posts can you get into your OS?

Cmon guys! hpxca clearly stated he used not-a-varta and its even visible in the picture 😀

Problem starting can be due to bad Reset generating circuit, early on it was something super basic like capacitor to +5V.
Does it start from Glitched state after shorting Reset pins?

@Horun @chrismeyer6
I've thought about replacing all the tantalums on the board but I have not done it. I may yet give that a shot - it's fairly cheap to try and easy to do and there are not that many. None of them look visibly damaged or burnt and in my experience bad tantalums usually either short, explode or darken/burn.

As for the battery, it is a not-a-varta and it has the OR current switch on the bottom. That IC acts like a diode with a very low forward bias voltage and a high enough reverse breakdown voltage that there's no risk to the battery from the boards charge current but the output voltage is still over 3V (with a good battery). With the CR2032 installed in the not-a-varta the CMOS saves work as expected.

@rasz_pl interesting thought, I actually never made myself a reset button with a few leads attached but it might be worth doing and looking at the reset circuit.

Aside from the first boot attempt it behaves so much like it can't find anything in the ISA bus and the way the video card misbehaves has made me think about replacing the some of the 74LS244 line drivers. Actually one of them is an Hitachi 74LS244 and the others are national 74F244, I don't know if there is a reason they used the two different parts but i'm a little worried there is. Some are between the ISA slots and difficult to get to as well.

Also worth noting is that this board is actually rated at 33/40 MHz bus speeds and I am running it at 25MHz since at 25MHz CPU is all I have. I don't think that should be a problem though but if someone knows for sure otherwise please let me know!

I'm actually starting to suspect the 82C206 on this board. It seems to have everything in common with the unusual behavior:

-It handles the power good signal from the power supply, which could explain the strange behavior and difference between power supplies. When the power good is low, all address, data,data strobe and R/W pins are disconnected from the processor. This could explain the boot failures and no POST codes on the initial boot. Even though the power good is not low, all bets are off if the chip is faulty.
-It handles DMA and IRQ directly related to the ISA bus which could explain the strange boot behavior and video card issues on the 2nd boot attempt where it acts like can't find a video card or the video card puts out the purple screen.
-It handles the 32kHz square wave generated by the RTC oscillator and contains the actual RTC implementation, which could explain the odd RTC "ticks" in the CMOS setup after it finally boots.

Its going to be a bitch to get off of there without damaging those ISA slots though, and I don't have an vaccum de-soldering station 🙁

how about you try resetting it first? or putting scope on reset line in single shot mode.

>Aside from the first boot attempt it behaves so much like it can't find anything in the ISA bus

yes, any card can do this without seeing reset signal

Yes, i'll give that a try before I try to remove the 82C206. Rather then make one I just ordered a https://www.amazon.ca/gp/product/B09W3XFSR2 off amazon to make that a little simpler. I'll also take a good look at the pins and traces around the 82C206.

So I checked the ISA reset line (pin 2 of the ISA slot on the bottom row), it looks normal. It stays low the entire time after the system is powered on, going high only for a millisecond or so as the system powers down. This was the case on attempts when the system both did and did not POST including attempts when it gave the "no video card beep code" when there was one installed. When the reset button is pressed the reset line pulses high for about 100ms before dropping back low again and the system resets as it should. The problem is the same whether I reset it via the button or by power cycling the entire system.

You said it did work after power cycling, so does it start working after manual reset? or do you need keep resetting to get it to work? or it doesnt help at all?

Nice board really, hope it will recover.

@raz_pl It's the same cycle whether I use the reset switch or the power switch, meaning I need to either power cycle it 3-4 times or reset it 3-4 times (or any combination there of) to get it to POST.

After messing with it for more hours tonight I figured in for a penny in for a pound. An eBay seller had new old stock of that UMC82C206F for a fairly reasonable price so I bought one. I removed the old one (thank the can of perrier I had just drank and some kapton tape for the heat shield). I think the ISA slot came out of the deal intact. Will be several weeks until I get the replacement though.

Really hope this is it, I think we're beyond economical at this point anyway.