A500 mobo 8a1 ram question

[Deling]

I have 8a1 mobo A500+ with 512kb , I want upgrade this machine to 1mb chip or 2 mb chip direct to mobo
there is 8375 agnus , what I have to do ? looking for good solution

 

[BLTCON0]

For 1 MB chip RAM onboard, you simply convert the board to A500+ specs (alter some jumpers, add an IC, install missing RAM chips). Then it can be upgraded to 2 MB chip RAM via a 1 MB trapdoor expansion.

For 2 MB chip RAM onboard, forget it. Only the "good" revision, rev6A (and its lost identical twin, the rev7), can be "cleanly" upgraded to 2 MB chip RAM onboard. You can of course do it on the rev8 as well, but it requires stuff like piggybacking another set of 8 RAM chips on top of the existing 8 ones and routing the 8375 Agnus' secondary (trapdoor) RAS lines there. Not a clean solution, and the added elevation caused by piggybacking the chips may well interfere with the shielding and/or the keyboard (which is known to bend downwards at the center).

 

[Deling]

add an IC , what is this ?

 

[Sardine]

IC = Integrated Circuit.. or chip.

 

[roy_bates]

i think the missing ic is a MC74F139