Help finding data sheet on mystery ram ICs

[br44]

Hey guys
So I'm working on restoring this C64 (250407 Rev A) and I'm working through all the problems it's having. I replaced a bad kernal rom (was getting hot as hell), and I dropped in a new GAL PLA. I was going around with the oscilloscope looking at the data bus and address lines, and I saw some screwy looking signals. So I ended up down by the ram chips, and this is what I found....

So what the heck are these things? I can't seem to find any data sheets on them, and the whole area is heavily reworked. And when I say reworked, I mean butchered worse than that guy in The Patriot in the tomahawk scene. Seriously, this thing should be in one of those sad music PSA's for abused Commodores.

Does anyone know if Commodore ever used those NMC 3764N chips? Does anyone know what they are at all? I thought the C64 was supposed to use 4164 ram (it's what my schematic says). I'm very suspect of these chips. I wonder if they're correct. All eight chips are the exact same thing, but also all eight have sockets and have been reworked (very old crusty flux everywhere).

Opinions?

 

[HoJoPo]

Googling NMC3764N found the info right away, these are National Semiconductor 64kx1 chips, pin equivalent to 4164/4264 chips. More info and data sheet here: https://www.silicon-ark.co.uk/nmc3764n-20-sram-by-national-semiconductor

 

[br44]

Googling NMC3764N found the info right away, these are National Semiconductor 64kx1 chips, pin equivalent to 4164/4264 chips. More info and data sheet here: https://www.silicon-ark.co.uk/nmc3764n-20-sram-by-national-semiconductor

Ah... it's a .uk website. Maybe that's why it didn't come up for me? Thanks for the link. Honest to goodness when I punch in NMC3764N into my google, I get all sorts of shady chinese websites.

 

[br44]

So after studying the schematic and the data sheet, I have concluded they are the correct ram chips. Weird that after two hours of searching through two different search engines, I couldn't find that dang silly data sheet.

Oh well, back to searching for the problem.

 

[HoJoPo]

So after studying the schematic and the data sheet, I have concluded they are the correct ram chips. Weird that after two hours of searching through two different search engines, I couldn't find that dang silly data sheet.

Oh well, back to searching for the problem.

Glad I could help. If you don't have a C64 dead test / diagnostic ROM cartridge, it's worth spending the $20 or so they go for, like this one:

Commodore C64 / 128 Diagnostic / Dead Test Cartridge v1.1 | DIY Chris

Looking for the Dead Test 781220 or the Diagnostic 586220? Look no further. This cartridge comes with a 512Kbit EEPROM that has both the 781220 Dead Test and the 586220 Diagnostic images on it.

diychris.com

 

[Chuck(G)]

Don't know if it matters in your case, but these are 200 nsec. chips--i.e. pretty slow. I don't know if that's the problem; just adding a bit of information.

 

[br44]

Don't know if it matters in your case, but these are 200 nsec. chips--i.e. pretty slow. I don't know if that's the problem; just adding a bit of information.

Well that's a pretty valid point. Just what would be the best speed for the C64? Would it be better to upgrade to 150n? Would that screw up the timing of the chips?

 

[Chuck(G)]

Upgrading to a faster DRAM never hurts. However, I'm not the C64 effort, so hopefully someone will chime in here and tell you if 200 nsec is fast enough.

 

[powerlot]

Glad I could help. If you don't have a C64 dead test / diagnostic ROM cartridge, it's worth spending the $20 or so they go for, like this one:

Commodore C64 / 128 Diagnostic / Dead Test Cartridge v1.1 | DIY Chris

Looking for the Dead Test 781220 or the Diagnostic 586220? Look no further. This cartridge comes with a 512Kbit EEPROM that has both the 781220 Dead Test and the 586220 Diagnostic images on it.

diychris.com

If you like soldering and have an eprom burner I would recommend assembling a Versa64 cart. You can have several diagnostic ROMs on one cart and reprogram/swap it if you don't need it anymore.

GitHub - bwack/Versa64Cart: C64 Single EPROM cartridge pcb

C64 Single EPROM cartridge pcb. Contribute to bwack/Versa64Cart development by creating an account on GitHub.

github.com

 

[br44]

If you like soldering and have an eprom burner I would recommend assembling a Versa64 cart. You can have several diagnostic ROMs on one cart and reprogram/swap it if you don't need it anymore.

GitHub - bwack/Versa64Cart: C64 Single EPROM cartridge pcb

C64 Single EPROM cartridge pcb. Contribute to bwack/Versa64Cart development by creating an account on GitHub.

github.com

That's a cool project, but unfortunately I don't have a burner or any eproms. I would have to go buy all that stuff first before I could even start the project.

That's why I just went out and bought the GAL PLA replacement instead of making one.

 

[gslick]

So what the heck are these things? I can't seem to find any data sheets on them

One good place to look if you recognize an IC logo, in this example being National Semiconductor, is the collection of component documents on Bitsavers: http://www.bitsavers.org/components/

The 1984 date code on the ICs in the photo quickly points to this databook as a good place to look:
http://www.bitsavers.org/components/national/_dataBooks/1984_National_MOS_Memory_Databook.pdf

That databook on page 3-3 (page 24 of the PDF) does have the datasheet for the NMC3764 65,536 x 1-Bit Dynamic RAM.

It also has a handy MOS Memory Cross Reference Guide chart on page 2-4 (page 20 of the PDF).