2nd hand A4000 does not show Fast Mem. How come ?

[Ilwrath]

Yep.  The CHIP ram needs to be a 2MB simm in Bank 0.  The Fast RAM should be in banks 1 and 3 if you are using double-sided simms, or in banks 1,2,3,4, if you are using single-sided simms.  And I do believe there is a jumper to set for the 1/2 or 4/8 SIMM sizing.  And, most likely, single-sided 8MB SIMMs, or double-sided 4MB SIMMs aren't compatible, and will be detected wrong.

I think that was the lessons I learned on A4000 RAM setup.  ;-)

Warren Block's A4000 hardware page is an excellent source of knowledge.

[Ilwrath]

Yes, I believe you should use exactly a 2MB simm for your chip RAM.  Others may work, but return unexpected results.  I have heard that this has to do with various sized SIMMs using different internal addressing systems.  The Amiga is expecting a single-sided 2MB SIMM in bank 0.  Anything else is a crapshoot as to what will happen.

-jumper 852 : set to 256Kx32 and not to 1x32

This should be set to 1x32.  That allows 4MB and larger simms.
(After all a 4MB SIMMs is 1MBx32bit.)  ;-)

[Ilwrath]

so it could be that the not-2Mb-chip in the chip mem bank causes the other memory banks not to work

I don't know.  I've never tried anything other than a 2MB SIMM.  I don't *THINK* it should interfere with other RAM, but I'm not sure.  More likely, I'd think it would just result in mis-detected CHIP RAM size (which you've had) or corruption and errors in CHIP RAM, causing gurus, unbootable, etc.

But, for sure, the 1x32 jumper should be set, to use any SIMM larger than a 2MB in fast RAM.  Also, the SIMMs used in fast RAM should all be the same size and style.  (in other words, don't use a 4MB and a 8MB together, and don't mix single- and double-sided SIMMs.)  I always keep the same speed and manufacturer on all SIMMs, as well, but probably not necessary.