You could try taping pin b21 and see what happens.

Pretty sure my old memory is right that that's the fsb 66/100 pin.

I saw that about on this forum but with no details. Googling brings stuff up but I wouldn't want to break anything - can you please link to a reliable source with more info on that procedure?

myne wrote on 2025-07-18, 11:13:

You could try taping pin b21 and see what happens.

Pretty sure my old memory is right that that's the fsb 66/100 pin.

I have done that many years before with my celeron 300a, worked well @450mhz, i just used tape with clue.

Cut really thin piece of sticky tape.
Place carefully on the edge finger you've identified as b21.
Triple check you can count and didn't cover the ones next to it
Insert into socket.
Make sure it looks like it didn't slide off.
That's pretty much it.

At least 4 pins should be labelled. A1, A121, B1, b121

It's tricky to get right, but even harder to screw up so bad something dies.

Ah, so just masking the 21st pin counting from B1 on the edge connector, got it. I'll give it a shot

Bingo.
Good luck!

Board / chipset appears to be 100MHz rated, even if it's the budget ZX (unless they did the dirty and put a ZX-66 on).
So if it isn't a ZX66 chipset (for which a max oc of 83 sounds feasible) then the other potential is that without detecting a 100MHz CPU, it sets straps and dividers that won't work with 100.
So it might work with the B21 hack, it might work, it might not, or if it selects 83 then that would be pretty conclusive that they downgraded the chipset
Looks like the chipset heatsink is glued, so removing it to check the model & stepping is not an option

It worked brilliantly, thank you.

If someone with the same issue stumbles upon this post, here's what I did. It's the pin marked on the screenshot. Tape was too fiddly for me so I just painted it over with nail polish. If you 'go over the lines' it's easy to delicately scrape the polish off of the neighboring pins before it completely solidifies.

Once connection was blocked on B21 the PC immediately recognized the CPU as 450MHZ (100x4.5)

Nice one