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macintosh IIsi 4MB onboard RAM upgrade

alectrona6400

Well-known member
so i know a few people have talked about this, and i am also one of the 2 people who tried this recently. any IIsi owner knows these things: the power supplies LOVE to crap their guts out with ELNA caps if not recapped, the speakers drop occasionally because of a strange design, and... the computer has 1MB of onboard RAM that is shared with the computer and also used as VRAM. i'm not going to get into the whole "unused RAM is wasted RAM" thing because i really only did this out of curiosity, and as a test of the equipment i had, but also because i actually legitimately wanted to do this because the other sources don't talk much about what kind of RAM chips to use.

so... first you'll need a macintosh IIsi. you don't have to be me and overpay for an untested one in good condition from a local computer shop for $140 that i ended up recapping entirely (though it works great now), but they're fairly easy to find because its one of the less desirable macs out there, and they're fairly common anyway. speaking of recapping, if your IIsi hasn't been recapped, i would do it ASAP. the main filter caps (the big ones) aren't usually the ones that fail, but the smaller ones do, and ESPECIALLY the -(-- vent style because those are the infamous ELNA "LongLife" caps... pssh, maybe when they were new, but not now!! as for motherboard caps all the SMD ones are 47uf 16v and can be replaced with tantalums of that value (or electrolytic if thats your kind of thing... i don't judge). the axial caps on these can actually be subbed for SMD or radial (with bent legs) and are both 220uf 16v; those i would replace with good electrolytics, i pulled my 2 replacement caps from a dead optiplex 330 motherboard and those are both rubycons that are a fraction of the size of the original axials. i usually pull caps from salvage boards because i'm a bit cheap with my older equipment, but also its a good form of reuse if the caps are known good brands and will last long.

next, you'll need 4x1Mbit RAM chips (4x being "4-bit" since you have 8 chips to install adding to 32-bit for the IIsi's bank requirement, and each RAM chip is 512KB), which are thankfully easy to find new. https://www.ebay.com/itm/122895150641
this is for 30 chips, they have a 70ns speed, and they're new old stock. you could pull them from a double-sided, 16 chip 8MB 72-pin SIMM, but you should always make sure that works before doing that, or just buy the new old stock chips. tried this initially and i only got death chimes because some of the chips failed as i pulled them seemingly... or maybe its just me.

as for equipment... you could solder these by hand but you will get more consistent results with good solder paste and a hot air gun thats specifically for working on electronics. (do NOT use one of those paint stripping hot air guns, you WILL destroy your motherboard!!!!!). first you want to remove all of your RAM chips using hot air (which are 4bitx256Kbit, each chip is 128KB) and then clean up the solder with good quality solder wick. be sure you don't set your iron's temp too high (leaded solder melting point is usually ~183*C but the old solder could be troublesome for sure), because if you do, you will risk pulling off the solder pads on the board and then you'll have to deal with any vintage computing enthusiast's worth nightmare... trace repair. apply a reasonable amount of paste across the areas where the pads are (so every 5 pads just add a good line of paste to it), orient and align the new chips, and then apply hot air. if there are any bridges or "dry spots" you can use the iron to touch up the pads with drag soldering. clean up the board using flux remover or what have you... and you're done!!

soooo.... what do you get from this? nothing aside from an extra 3MB of RAM to spare, but depending on what chips your board had installed you have also just improved the speed of the onboard RAM to 70ns from 80/90/whatever... at least thats how i think it works. theres really not a noticeable speed improvement, and since the onboard memory now has more space for applications, your system could probably run slower since it still shares that with VRAM... but it has more headroom for sure. if you really wanna get more speed out of a crusty IIsi, you can swap the 40MHz oscillator with a 50MHz one and track down a nubus riser to get an FPU. you could also find a nubus video card which would definitely help video performance on the IIsi since that's easily the biggest weak point of that system.

i'll post more pics soon when i get around to it, but i will give you this picture of a "cost reduced IIsi that completely omits onboard ram and comes straight from the factory without working video", aka a IIsi's motherboard stripped of its 8 128K ram chips. if only newer macs were this simple to upgrade onboard ram with... but yeah, tl;dr: its possible with the right chips, i can confirm it works. definitely not a practical thing to do for sure but its definitely possible. and if you bought 30 chips, you still have an extra 22 chips to use for making 2-chip 1MB SIMMs if thats your kind of thing! i know i'll be doing that since i have dead 1MB SIMMs i've had for yeaaars...
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alectrona6400

Well-known member
IMG_1076.jpegIMG_1080.jpeg
the area around the ram chips is kinda discolored because of the crappy flux i used the first time i tried doing this... its a pain to get off but i don't wanna mess with it again. also yes, i did try thinkin things 2 on this outta boredom... it does not run well on a 25mhz 68030 even with the FPU. obviously that came out 5 years after the IIsi... lol
but it works! obviously not much of a reason to do this, but it works and i can prove it. no other modifications were made, just swap the RAM chips out.
 

JC8080

Well-known member
Nice write-up. I did not upgrade my board to 4mb, but I did replace the onboard RAM due to a faulty chip(s). I also used solder paste and would recommend it as a starting point. The reason I replaced the RAM was due to a suspected RAM issue resulting in a death chime. I replaced the ALS245s and RAM chips and it did not fix the issue. That was 1.5 years ago, it just sat in a closet. Just today I got it out and started poking around and found that two RAM chip legs did not have continuity with the pads. I went in and reflowed every leg of every chip, and that fixed my problem. The point of the story is do not trust the solder paste, on this type of chip where the legs curl underneath it is easy to miss a leg and not realize it. I do think the solder paste is a good starting point, I just recommend following up with an iron, and then carefully checking for bridged legs and continuity between each leg and pad.
 

alectrona6400

Well-known member
yeah, its definitely not the only thing to use. thats why i said you'd want to touch up the joints to be sure that everything is making contact and that no bridges are made. you also wanna make sure any stray solder is not present because that can really mess up your day if left unchecked...
 

JC8080

Well-known member
yeah, its definitely not the only thing to use. thats why i said you'd want to touch up the joints to be sure that everything is making contact and that no bridges are made. you also wanna make sure any stray solder is not present because that can really mess up your day if left unchecked...
Yeah, when I was working on my board today I found a few errant solder balls from the paste that had not melted. It would not be good if those migrated to the wrong spot.
 

alectrona6400

Well-known member
yeah, you really wanna make sure nothings left unchecked, especially if you're using cheaper equipment thats not exactly rohs certified.. lol. checked my SE/30 reloaded board that doesn't work with my microscope today and it's a HOT MESS! you really gotta be careful using solder paste, but its the quickest way to get this done... just be sure youve got good equipment, though you don't need much.
 

JC8080

Well-known member
yeah, you really wanna make sure nothings left unchecked, especially if you're using cheaper equipment thats not exactly rohs certified.. lol. checked my SE/30 reloaded board that doesn't work with my microscope today and it's a HOT MESS! you really gotta be careful using solder paste, but its the quickest way to get this done... just be sure youve got good equipment, though you don't need much.
What's going on with your SE/30 board?
 

alectrona6400

Well-known member
i'm doing a reloaded build and i think i may have screwed something up kinda badly since i don't get video anymore. i'll have to fix it when i have the time to, but now that i have the microscope i can easily see what i'm doing, whereas i wasn't really able to see everything earlier.
 

Melkhior

Well-known member
you have also just improved the speed of the onboard RAM to 70ns from 80/90/whatever... at least thats how i think it works.
The speed of chips will not change the performance of the memory subsystem. The IIsi uses synchronous cycle, the timings are based on the CPU clock, not the speed on the chips. The number of cycles required is hardwired in the memory controller (or could be configurable via a hardware register therein, but I dont think there is one in the IIsi) and the memory just need to be 'fast enough' that they answer within the timeframe required by the memory controller. Faster chips will not help, except they enable more overclock (shorter cycle time means the memory chips must answer faster).

Older Macs use asynchronous cycle, but on those the faster chips won't help either, as the asynchronous acknowledge is generated by the memory controller as well; there's no signal from the memory chips saying 'the data is ready'. So the timings is also baked into the memory controller. I don't know what happens to overclock here; if the timings are independent of the CPU clock, then faster chips won't change anything.
 
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