Modern replacements for old DRAM memory chips

Question

I have an old video card for a computer from the 1980s era, but unfortunately the memory chips on it have gone bad. As it's quite a rare device today, I would like to repair it.

Unfortunately I can't find anywhere that sells pin-compatible replacements for the old 4164/4464 and 41256/44256 ICs. These were apparently quite widely used ICs - the 41xx is 1-bit memory, and the 44xx is 4-bit memory, with the xx64 having 64k addresses and the xx256 having 256k addresses. All are 5V TTL logic.

Is there an easy way to replace these failing chips? If there are no compatible replacement chips being manufactured anymore, is it possible to find a chip that's similar enough that it could be substituted with minor modification? What would I be looking for? These are DRAM chips, so I'm not sure if they could be substituted with SRAM chips or newer types of memory ICs, since the DRAM refresh may be too slow for newer chips to maintain the data.

The chips on this card appear to be arranged in two independent banks of four 4464 chips each, but I have other failed cards with different arrangements so it would be nice to aim for the more general case of 1-on-1 chip replacement rather than trying to replace a group of them with a modern larger capacity chip - only since that solution may not work elsewhere.

Answer 1

In some cases it is possible to replace DRAM with SRAM, by adding some logic to latch the row address. Whether this will work in a particular circuit depends on the timing of the RAS, CAS, and /WR signals and how the SRAM responds to them. 1 bit DRAMs have separate data input and output pins. If these are not tied together in the circuit then you may also need a Tri-State buffer to bridge between them. Getting the timing right could be tricky.

Here are two examples that work:-

SRAM replacement for lower 16K ← ZX Spectrum

SRAM replacement for TMS99x8 VRAM

You may be able to use higher capacity DRAMs provided that they are compatible - compare their data datasheets to make sure. The main things to watch out for are RAS/CAS timing (Fast Page Mode, EDO etc.) and refresh requirements.

However replacing the the faulty DRAMs with correct equivalents would be much easier and more likely to succeed. Although these older DRAMs are no longer manufactured, many are still available on the second hand market - usually selling at much cheaper prices than when new.

You may not have found what you want because you haven't searched all the equivalent part numbers. eg. '41464' is equivalent to '4464'. Here are some selling on eBay right now:-

D41464C-10 NEC DIP18 10NS

5PCS KM41464AP-12

Just be aware that these chips may be refurbished and possibly remarked with different parts numbers. Usually they are equivalent, but sometimes not (I bought some 'MCM2114P20 1kx4 SRAM' chips that are actually BU2114F 8 bit shift registers!).