If you think RAM is expensive now, try putting yourselves in the shoes of a Commodore engineer, circa 1981. RAM was eye-wateringly expensive by modern standards, and Jack Tramiel wanted 64K of the stuff for the next computer– hence the name, Commodore 64– but he didn’t want to pay for it. The solution was to use cheaper dynamic RAM over the more expensive static RAM that later took over the market in the kilobyte range. That’s a small problem for retrocomputer hobbiests, because while we’re complaining about the price of gigabytes of the stuff, you can’t buy new DRAM chips that fit a Commodore at any price. That’s why [Fabio Battaglia] aka [hkzlab] came up with an adapter board to fit easily-available SRAM chips onto aging C64s.
Nothing lasts forever– not cold September rain, and not DRAM chips. Heat damage? Internal corrosion? There are probably multiple failure modes, but someday the old stock of chips will run out and the retrocomputer community is going to be ready for it. [Keith Olson] sent us a tip on a video by [The Retro Shack]– embedded below, and thanks for the tip, [Keith]!–about this very problem, that serves as a good demo of what you get when you put SRAM into a C64. That said, the adapter board on offer is only good for C64s with the 250407 motherboard. If yours is different, you may have to modify the board– but hey, it’s open source, so go ye and do that thing. Let us know via the tips line if you do.
It seems like it should be rather trivial to emulate a C64 DRAM chip with a small 8 bit mcu and a small 8KB external SPI RAM chip.
I recently restored my childhood Commodore 64, which was an epic journey of debugging, struggle, and ultimately success.
I had a completely dead system, so the first step was buying modern power supply and replacing the PLA with a modern recreation (“it is always the power supply, PLA, and usually both”), which was enough to get some activity, but it took a complete probing of the circuit board to find some broken traces, which was enough to get the Dead Test and especially the DesTestMAX cartridges to show some activity. The last step was ripping out all of the accursed MT DRAM chips and replacing it with a similar SRAM replacement and fortunately none of the truly valuable chips were damaged.
If you are restoring a Commodore 64, just start by snipping off all of the DRAM chip leads, tossing the chips in the trash, installing quality sockets, and using a modern memory replacement. Don’t try desoldering all of those chips – just snip them off and clean up the remaining leads. Then replace the PLA. And THEN start troubleshooting.
This project is interesting and valuable, but I can’t recommend the DIYChris store enough – not only for the SRAM replacement, but they have a plethora of modern replacement components for the C64. For example, $30 for a pre-assembled SRAM module and they have both the 250407 and 250425 board compatible boards.
I think the article is overstating the “penny-pinching” aspect just a bit. There was no reason to use more expensive static RAM for main memory in many home computers back then since dynamic RAMs were cheaper and denser and the more complicated access and refresh cycles were easily handled by the support chips of that era. The Z80 (like used in the TRS-80) even directly handled DRAM refresh itself. As semiconductor manufacturing processes progressed and transistors shrank by orders of magnitudes, SRAM came way up in density and way down in price and it wasn’t necessary to use DRAM chips for low-capacity needs anymore.
I don’t know about “can’t buy new DRAM chips that fit a Commodore at any price” though. Jameco seems to have 4164 and 4464 DRAMs in stock for ~$5 each