Back to live again! One more board for the retro-computing colletion. Thanks for your help.
Out of interest: how did you confirm that the cache was working?
I just checked mine and it claimed cache was enabled, doesn't give any POST errors for bad cache - but cachechk doesn't find any (L1 or L2), and speedsys shows very poor results, indicating that cachechk is right about it not working...
I have 256kB 25ns Mosel SRAM L2 - and I've tried everything from the original, supported i486DX-33 (8k L1) to an AMD 5x86-PR75 (133MHz with 16k L1 - via a voltage interposer), but absolutely nothing gets cached.
Well, I don't really know if cache is working, I assumed it is just because POST prints the following:
Internal (486) cache controller enabled
External cache controller enabled
External cache size 256KB
With other BIOS images I tried before, POST just said Cache: 0KB. And everything was REALLY, REALLY slow. For example, in the BIOS setup, I could see the characters appearing on the screen almost one by one. But maybe this is not related with cache, and it is another side-effect of using the wrong BIOS.
Next week I'll do some deeper testing, to see if I can figure out something. One guess: maybe the turbo header has something to do in the behaviour of the cache? With it shorted is much faster than with it opened. Maybe this board disables cache when the trubo header is open?