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I have this system with 2MB onboard RAM, but I noticed there are additional soldering pads to the right from memory chips (NEC 424400-80L, but there are also TOSHIBAs TC511664JL-10 on daughter board above?). I read in computer magazines from 1991/1992 similar model of Commodore C386SX-LT (there were at least two different builds / manufacturers):

https://www.scuzzscink.com/amiga/amiga_scuzz373.htm

was upgradable up to 5MB, this may be because the other C386SX-LT had RAM SIMMs (modular)?

Either way, with some old computers, not sure which one you could solder in RAM to add more of it. How to check if this is an option without potentially ruining this fine, working piece of technology from early 90s.

C386SX-LT Motherboard

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The daughterboard is the graphics card. The GD610/GD620 is a quite common chipset for LCD/VGA graphics in laptops. It uses two 64k x 16Bit RAM chips to obtain 64k x 32Bit, which is the usual VGA memory (256 kBytes, but the VGA needs 32-Bit access to get the data fast enough to the screen). Those RAM chips have an access time of 100ns.

The chips on your mainboard are extremely common chips: They are 1M x 4Bit chips (with a total capacity of 4 MBit), and four of them make a bank of 1M x 16 Bit (as this is a 386SX system, the memory needs to be 16 Bits wide). It is very likely that soldering a second set of these chips (which can be stolen from any 2-chip or 3-chip 1MB SIMM) will expand the RAM to 4MB without any further need of customization. You should also add the decoupling caps (the big one is an electrolytic 1µF cap, labelled 105 on the other chips, the small one is most likely a multilayer ceramic cap of 100nF, which is the typical value used for them) next to the four RAM chips. The RAM chips already on the board have an access time of 80ns, so the chips you add should not be slower. They don't have to be by a specific manufacturer, though. All FPM (fast page mode) 1M x 4Bit chips with the correct package are compatible.

Thanks to Alex Hajnal for noticing that there is an electrolytic cap per RAM chip. It feels a bit over-the-top though. One electrolytic per bank and a ceramic cap per chip ought be really good enough.

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    There appear to be two decoupling capacitors for each RAM chip: a ceramic one (beige with metal ends) and a (polarized) tantalum one (1 µF; black with white polarization mark, labeled C105).
    – Alex Hajnal
    Jun 8, 2020 at 9:55
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Since there is no documentation available, all ways I can think of to check ahead of soldering would involve a certain level of measurement devices - like logic analyzers or at least a digital oscilloscope - which I assume are not present.

So I'd simply go ahead and pick four 424400-80 (or compatible), solder them on (not forgetting to add appropriate support capacitors) and see if the system recognize them. These chips are quite common and available from a wide variety of manufacturers. It should be easy to source them from old motherboards or PS-2 style RAM modules.

The 42 4400 are, as the name implies 4 megabit FPM RAMS organised as 1 Mi by 4. Adding four of them should turn the actual 2 MiB into whopping 4 MiB.

The mentioned 51 16 64 are a completely different type (1 Mi bit organized as 64 Ki by 16 bit) and will be no good.

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