Use of Additional Ram on the A500:
First thing you need to do is pop the case back apart and look at the board on the extreme right side, just in front of the disk drive, and find out for sure what revision motherboard you do in fact have.
If you have rev 6a A500 motherboard:
Those "empty spaces" where the RAM chips are are for the option of installing a full 1 meg of chipram on the motherboard. If you do populate these "empty spaces" with 44256 (or equivelant 256k x 4 DRAM chips) and make the appropriate jumper changes, your machine will have 1 meg of chipram on the motherboard, and will no longer look for the upper 512k of chipram on the trapdoor expansion. With this configuration, any RAM you do plug into the trap-door expansion slot will show up as "fast ram" but will in fact be what some people refer to as "slow ram".. That is, it is not directly adressable by the custom chips, and access by the CPU to this ram will suffer from bus cotention with the agnus chip, since it is on the chipram bus. So really, if you already have 512k of ram in the trap door expansion slot, and it is showing up as chip ram, its best to leave it alone and look for a ram expansion that goes on the side-car expansion or an accelerator board for additional fast ram solutions.
If you have a Rev 8a (A500+) Motherboard:
Some later model A500s were actually built with the A500+ motherboard. Commodore did this to reduce production costs in later years by only producing one board for both machines. This is good news because you can easily upgrade this to a full blown A500+ and even add up to 2 megs of chip ram. The first step involves installing the 8375 AGNUS chip, moving several motherboard jumpers, adding U32 (74F139 demux IC which was left off of 8a boards built in "a500 configuration"), and adding four more 44256 (or equivelant 256k x 4 DRAM chips) to the "empty spaces" in between the existing DRAM chips. This will get you to a state where there is 1 meg of chip ram on the motherboard. To get the second meg of chip ram, you can either find an A501+ (or third party manufactured equivelant) which is an additional 1 meg chip ram expansion that goes in the trap door expansion slot, or you can "piggyback" another eight 44256 DRAM chips on top of the existing ones, bend up pin 17 on each chip, and connect all the "pin 17s" of the left 4 chips to pin 11 of U32, and all the "pin 17s" of the right 4 chips to pin 5 of U32. [[This looks/sounds easy, but be advised that the DRAM chips are mounted rather close together when you have all 8 populated, and soldering another 8 on top of them can take some serious skill with a soldering iron. It shouldnt be a problem for experienced techs, but the average hobbyist should think twice.. If you put too much heat on the DRAMs, you can fry them, and if you create any solder bridges between pins, you've got a serious mess to straighten out.. also if you fail to make a good solder connection on any pins, you can cause operational flakiness/reliability issues.]] Once this is done, you will have 2 megs of chip ram. The other thing you can do is populate the circuitry to add the RTC (realtime clock/calendar) to the motherboard. I wont go into this in detail, but the components are exactly the same as the ones used on the RTC section of a standard A501.
Here is the A500+ service manual which has full schematics, component values, and board layout diagrams. The last thing I'll say about this is that the battery used by commodore here was an old NiMH or Zinc "barrel" type battery.. These are famous for leaking and destroying AMIGA motherboards. I use a standard CR2032 replaceable "coin" type Lithium battery socket from an old PC motherboard here. It has worked fine for me even though the CR2032 is only 3.0v and the orginal batteries were 3.6v...
Have fun....
