Problem with memory expansion?

[Zac67]

For the schematics, take a look at http://amiga-manuals.xiik.net/.

A NiMH barrel battery can be had for as little as 2€ at a decent shop. Alternatively, you can mod the circuit for a non-rechargeable coin cell, just make sure you include a diode to prevent charging. The A500 also has enough space beyond the trapdoor to use standard AA or AAA batteries (don't forget the diode for non-rechargeables though).

[Zac67]

Yes, with a "non-1MB" Agnus the RAM is located at $C00000-$C7FFFF. Possibly the solder points on the chips are damaged as well or even the RAM chips themselves, unrelated to acid damage. You should run a RAM test and then focus on the bits in question.

[Zac67]

If the gurus with both expansions are the same (or very similar) the mainboard and/or the Agnus socket seems to be the problem.

You'll need to open the case and take a look at the trapdoor connector - possibly some of the acid damage has spread over there and some cleaning/resoldering is required. Additionally, the Agnus socket can become unreliable with old age and improper storage. I may have to pull out the Agnus chip to inspect/clean the contacts - make sure you use a PLCC puller (~3€). Levering with a screwdriver can very quickly break the socket, especially with age.

Ah yes: the RAM address lines come from Agnus (it's the RAM controller for chip RAM). The data lines run through some latches (U10-U13) to the CPU side. (The latch is the point where the busses can be deconnected to run independently: chipset side and CPU/fast side.)