Ok. Heres what is kewl:
According to the service manuals, the A3000 (8372B 318069-03) AGNUS is pin compatable with the
rev 6a A500! All of the signals are in the right place. Even the one (pin 41)that controls PAL/NTSC on bootup. According to the information in the Rev 6a a500 schematics, and the A3000 schematics, you should be able to drop an 8372B 318069-03 AGNUS into an A500, add a 74LS139 DEMUX chip to create the 4 separaet CAS signals, add the extra DRAM, and it should work!
SO heres what you do to put 2megs chipram on the motherboard in a rev 6a A500:First, replace the 8372A agnus with an 8372B 318069-03 AGNUS.
Next, get out your soldering iron and the following materials:
Eight 256k x 4 DRAMs
One 74LS139 demux
Some insulated hookup wire (I use 40ga Kynar)
Add A20 Address line:1)On the back of the motherboard, Hook a wire from pin 35 of the agnus socket to pin 48 of the 68000 scoket.
Demux original CAS signals with DRA9:2)bend up pins 7 and 12 of U35.
3)Take the 74LS139 and bend all the pins out horizontal and clip them to about 2mm (1/8" length).
4)hook a wire from pin 8 of U35 to pin 3 of the 74LS139.
5)hook a wire from pin 13 of U35 to pin 13 of the 74LS139.
6)hook a wire from pin 56 of your AGNUS socket to pins 2 and 14 of the 74LS139.
four separate CAS signals:7)hook a wire from pin 4 of the 74LS139 to the pad on the motherboard where pin 12 of U35 used to be connected.
8)hook a wire from pin 12 of the 74LS139 to the pad on the motherboard where pin 7 of U35 used to be connected.
9)Piggyback 8 more 256k x4 DRAMS on top of the existing 8, bend up pin 17 on all 8 of the added chips. Solder all other pins to the pins on the chips below them.
10.)hook all the pin 17s together on the left 4 (U18,U19,U22, and U23)and also to pin 11 of the 74LS139.
11.)hook all the pin 17s together on the right 4 (U16,U17,U20,and U21) and also to pin 5 of the 74LS139.
clock enable:12.)hook pins 1 and 15 of the 74LS139 to pin 3 of U33.
Hot, ground, & mounting the extra IC:13.)hook pin 8 of the 74LS139 to pin 10 of U35.
14.)hook pin 16 of the 74LS139 to pin 20 of U35.
15.)Use hot-glue or double sided tape to secure the 74LS139 on top of some existing IC..
Getting A19, A20, XCLK, and _XCLKEN straight:16.)Make sure that JP2 and JP5 are set as follows:
DOUBLE CHECK EVERYTHING and make sure nothing is shorted/grounded etc..
NOTES:Ok.. its possible that DRD9 and CCK might each need about a 10ns additional propogation delay.. I doubt the timing is THAT critical, but I have not built this mod, so I dont know. I will try it as soon as I can get my hands on an A3000 Agnus.
Also, these instructions are assuming that you are starting off with a rev. 6a A500 motherboard that has already been fully populated with 1 meg (eight 256k x 4 DRAMs) of Chipram.