31 Jan
2012
31 Jan
'12
10:59 a.m.
The Color SRAM is a 2114. Very ordinary and, in my experience, prone
to failure.
If you have another C-64, you could try swapping the PLA. You will
need an 18-pin socket for the SRAM. The chips are available from a
number of surplus places like Unicorn Electronics, BG Micro, and more.
Speed ranges for the 2114 are, IIRC, 250ms-450ms. I don't remember
what speed the C-64 requires, but you can typically sub a faster chip
than the circuit requires.
-ethan
I have already purchased a replacement sram for the 2114 (acutally 2 of the units, one
P2114 and one MM2114N-2) and intend to replace the IC sometime today. I do have another
C64, but no chips are socketed, and I don't want to ruin it, thus I'll just try
replacing it in the bad unit. My understanding is that I need a chip that has a speed
faster than half a machine cycle for the system to work properly.
The spec sheet on the P2114 states that it has a 450ns access time. For the MM2114N-2 it
looks like its 200ns.
I understand that the access speed requirement is .5*(cpu freq/1)/machine-cycle. I
don't know the machine cycle for this operation, but assuming the worst with an IC
having an access time of 450ns, the machine-cycle couldn't be over 2.25 for things to
be ok. If I use the 200ns IC, I should be good with a machine-cycle of just over 5. I
could probably solve this by opening up the unit and just pulling the IC scribe marks and
looking up the data sheet, but the location with the ICs is 50 miles from the house, and I
happened to be nearby yesterday...so I just winged it. Can anyone checksum my thoughts
here? Is my algorithm and its implementation ok?
I also bought a replacement for the PLA, however, I had misunderstood the requirement. I
thought it was just a plain ole N82S100, but now I understand that it is a specially
programmed N82S100. Although I have found a ROM file I could perhaps burn on the chip, I
simply don't have the gear. I'm hoping this isn't the problem, as I have to
mail order the part, but I understand that this is the most common IC to fail on the C64
board. Since I'm getting only character color palette cycling, and have a border and
don't have a problem with a blank screen, I am encouraged that this IC is probably
ok.
Just to be safe, I bought a replacement for U29, as the description in the c64 chip and
common problems guide has content that really looks similar to my problem, moreso than
even the description for problems with U6 (the 2114).
Thanks for your commentary though. I'm likely to buy sockets for these chips as well,
as they aren't that expensive, and it may save me issues should the selection I made
be suboptimal.
Kevin