So I have two Amiga 500s. One is a rev 5 motherboard one from my childhood. Another, I bought recently having a rev 6 motherboard. Both have an extra 512K so a total of 1MB of RAM which is pretty much ‘what you need’ to do most stuff on an A500. My rev 5 one was piggyback upgraded by soldering another 16 256Kx1 DRAMs on top of the the ones on the motherboard. The rev 6 one actually came with a proper A501 expansion.
But rev 6 motherboards have four 256Kx4 DRAMs (for 512K) plus another four empty places for another four 256Kx4 chips. ie. The motherboard was designed to accommodate 1MB of RAM. I had some spare 256Kx4 chips and was keen to try out a new combo solder sucker/soldering iron (ebay is full of them. They are very cheap. They basically have the atypical blue solder sucker integrated into the soldering iron … and the tip of the soldering iron has a hole in it … for sucking).
I did some googling to try and find some notes about using the empty chip placements on rev 6 boards. I found a few references to ; http://www.savel.org/old/amiga/upgrade.html which sadly has disappeared off the internet … and I ended up reading it via archive.org; https://web.archive.org/web/20060212075245/http://www.savel.org/old/amiga/upgrade.html . I’ll repaste the content of the page here as its really short and tells you what you need to know
Memory upgrade
You need to have:
A500 with 0.5MB RAM(mainboard rev. ~5,6). with 4 memory chips (generic xxx4256-xx).
4 memory chips (generic xx4256-xx) from ISA video board, old 286. (Chips maybe called MN414256-10). Chips must be in DIP packacge.
Solder iron (20..40W), some soldering stuff.
Screwdriver. In some Amigas cross type in some Amiga is some weird screws…
Some instrument to cut wires on printed circuit board (PCB).Now you must:
Disconnect all cables from your A500
Open your Amiga, remove keyboard (do not forget how the keyboard is coonected), remove shield under the keyboard.
Locate memory chips. They are same chips in DIP package. If you find 16 chips, you are unlucky and your Amiga is very old.
Find free space near memory chips. You must find 4 empty places. In some Amiga PCB the holes are unfilled with solder in other – filled.
Solder new chips in empty space. Check if you didn’t short the pins and the first pin number is the right place.
Connect cables to Amiga. You don’t need to connect keyboard. Turn on the computer. If it starts, everything is ok. Don’t check the memory – it is still NOT ENABLED, we are checking if our Amiga is still alive 🙂 . Turn off your Amiga. If it is not working check:
If memory chips are not hot, if hot -they are bad.
for bad solder, shortened PCB track of chip pins.
Locate jumper JP7A. It will be connecting 1-2 pads or not connected. Shorten 2-3.
Turn on Amiga. If it starts, insert WB disk. Now you must have 0.5 CHIP and 0.5 other memory.
Turn off you Amiga. Locate JP2 jumper (It must be near CPU, the biggest chip on PCB) Open JP2 1-2 connection and shorten 2-3. AND open JP7A. IT MUST BE OPEN, all 3 pads must be not connected.
Turn on Amiga. Start WB. Now you must have 1MB of chip ram. NOTE: I tested it only with 8375 agnus, but I may work with 8372. I am in doubt about 8370/71…
If it is working, now you are lucky owner of A500 with 1MB chip ram (A500+). If it is not working, you always can switch back all the jumpers and return to old configuration.
I think that if you have 8375 AGNUS and if you plug A501 memory expansion you’ll get 1.5MB or 2MB A500. I didn’t tested it because I sold my a500 after this and CPU upgrade hacks and I didn’t have A501…That’s all about easy memory expansion…
So basically you solder the four DRAMs in and short JP7A (or if you find the EXRAM pin on the expansion connector you just use an atypical motherboard jumper to short it to ground). That is basically all you need to do to get a normal 512K chip ram plus 512k slow/fast ram (ie. same as having an A501). The other notes from the savel.org link are to do with if you want to try out having 1MB of chip ram.
So, me attempting to be smart decided I would solder suck the solder out of the blank DRAM spaces and then put sockets in them, then plug the DRAMs in. So I did that , soldered in some ceramic caps in the spots at then ends of the sockets and put my DRAMs in (I used 100ns ones), put in the EXRAM jumper and powered it up on the bench it and detected the extra ram ok.
All good?
Maybe. Maybe not.
So then I started to put the A500 back together and then noticed the big FAIL I had done. By putting the chips in sockets they stood much higher than the original chips. More importantly the top RF shield in an A500 leans low against this part of the motherboard … and because these chips were higher (due to the sockets) the RF shield would not fit anymore. Because the RF shield couldn’t be screwed down it meant the keyboard would sit higher which meant that basically you could not screw the case back together. Doh!
So out comes the solder sucker … and I sucked out the solder from the four sockets. And then soldered the chips directly in. And it still works!