You may find that being because of worn slots and the position to be assistance by gravity.

I had a strange one once, where a machine wouldn't boot when I screwed in the card brackets. Checked all round the card bracket, no shorts, wasn't a grounding thing... turned out the card was going "too far" into the slot, which shouldn't really have happened, some specs stacked negative or something. Anyway, there were traces above the edge connector that the socket was able to make contact with and short across.

So, what I'm suggesting might be happening, is that when the cards skew in the slots under weight, some trace or solder spot on the backside near the edge connector might make unwanted contact.... so try masking any areas where it looks like that could happen with tape or a masking compound.

Good thought BitWrangler ! Have also seen where mounting horizontal works then flipping vertical causes the back side of the cards come out slightly due to your same theory of mis-aligned back panel to motherboard heights.

Could also be cold solder joints under the slots or possibly the pads actually cracked off the board around the pins opening a gap.

Edit: Also check for anything loose in the power supply, stray screw or something. I swear if they're sitting around fan grill upwards any dropped screw heads straight for them.

I sold a 486 Pc Chips 912 board that with a specific Cirrus Logic card wouldnt display in a case, but would post no problem on a test bench. Ended up beating on the bracket and cutting a bit off the bottom because it was a hair long and would cause the back part of the card to just slightly lose contact. Then it worked no problem

ODwilly wrote on 2021-08-08, 02:26:

Ended up beating on the bracket and cutting a bit off the bottom because it was a hair long and would cause the back part of the card to just slightly lose contact. Then it worked no problem

Sometimes for that you can slacken the nuts on D connectors and screws on card and shift it a tad.

Hi everyone thank you for your many replies although none of them appear to be my situation directly it definitely gave me some thoughts in the right direction
I've decided to take it slowly and disassemble the entire build in order to eliminate any shorts etc. What gave me the idea is the system not even reaching 01 POST state which I found really weird.
What I've narrowed this down to is this reproducible situation, when I apply a moderate pressure with my hand to the CPU it does indeed POST, boot etc.
Disclaimer warning: do not fiddle with your hand inside a running computer, shocking hazard!

Now trying to eliminate the causes I've went through the following:

1. Checked for any bent, missing PGA pins.
2. Tested with exactly the same type of CPU, just the different specimen (AMD 5x86-133)
3. Tested both the CPUs in a different motherboard (V4P895P3), booted ok with both
4. Shot some air in the CPU Socket
5. Used alcohol on CPU PGA pins, did insert and lock cycle a few times

But all of those to no avail. Please kindly suggest your ideas regarding the socket as I do realize replacing it is not an easy task.
One more thing of note, I've noticed that regardless of applied pressure the pins sort of not go all the way in, leaving some 1mm air space which is equal at all sides. I don't see a way of pushing it "all the way" without dangerously excessive force. Please see attached photos.

Hmmm, I'd want to pop the slider off the top of the socket and have a look inside there, maybe some random debris blocking the CPU from sitting all the way down.

BitWrangler wrote on 2021-08-11, 21:14:

Hmmm, I'd want to pop the slider off the top of the socket and have a look inside there, maybe some random debris blocking the CPU from sitting all the way down.

Good advice, that CPU should be flush in the socket. Apart from mystery crud, one of the pins inside the socket might be mangled causing this.

As I can see, the CPU is not inserted in the socket all the way - it seems that the gap between the socket and the edge is the CPU is bigger than it should be. Can't you really push deeper into the socket?

If the CPU doesn't go all the way in you may have to open the socket to see what's blocking it.

I'm not sure the problem is that the CPU does not go all the way into the socket. 486 CPUs have 4 pins with some kind of "bulge" (one on each corner in the innermost row) which on certain sockets prevents the CPU to sit flush, and it leaves a small gap just as you show in the picture. I think it depends on the socket manufacturer, but I have definitely noticed that in certain cases and it works just fine regardless.

If the problem goes away when you apply pressure, I think it's more likely that some of the contacts inside of the socket are worn out, dirty or damaged. Ideally you could remove the top part of the socket and check that yourself, however there's always the risk of breaking the plastic tabs of the socket if you do that.

Using a small drill bit on the holes first 0.5mm where the pin keep the cpu up off the socket 7 should seat it nicely.

If he has this board (Asus VL/I-486SVGOX4 rev. 1.5 from the first post), it has a socket 3 and it seems to me that it is completely flat and the CPU should go lower.

As already said, 486 processors have a small stop on one of the pins that prevents full insertion into some sockets. Although it is not okay to review it.
I would try to remove the bracket from the expansion cards to make sure the insertion is correct.
I know what

I see what you mean, and it's true, but still, the gap between the CPU and the socket seems (for me) larger than the ratio to the outlet inside the pins. But maybe it just looks that way because of the angle of the shot?

If you don't trust that the socket is okay, you can always disassemble it But since the plastic is old, it may break. I have never personally done it in a socket 3, done it in a socket 370 and in an AM2, the latter had a broken pin inside that prevented inserting the processor.
But if the connection between the processor and the socket fails it will not be because it is not inserted to the bottom, in fact you could leave it a little slowed down a bit and it would work the same. If you disassemble it, you will understand what I mean. Look for photographs of disassembled ZIF CPU sockets for reference.
In this post, a member disassembles a socket 7.
Replacing a broken socket 7 cpu latch/lever
It seems more logic that it has a cold solder joint if it works applying pressure.
But sometimes there are goblins inside computers. 😀

Tips for potentially brittle plastic, get it warm, put a hairdryer on it for a bit, but don't go warmer than you can bear to touch. If possible, apply gentle leverage at several points or along a line if possible so that one single point is not overstressed.

In general, if your socket looks like a unicorn, proceed with extreme caution, or put up with it how it is. Otherwise, determine if in worst case scenario, you can buy a scrap, burned or parts only board with the same socket cheap enough to steal the socket parts off if bad things happen.

Hoping wrote on 2021-08-13, 13:05:

As already said, 486 processors have a small stop on one of the pins that prevents full insertion into some sockets.

My bad, I took a better look, and only now do I understand what You and TheMobRules meant - pin stop, right? I'm sorry, my English is really desperate and I didn't get it right away.
This could be the reason for the gap between the socket and the CPU, although the holes for these pins are deeper in the socket than for the smooth ones. I don't have better photos, but perhaps it's noticeable compared to the other holes...
Do these stops have all 486 CPUs? I never noticed it (I have an Am486DX2 66).