• Please review our updated Terms and Rules here

RAM repair for a C64

CommodoreZ

CommodoreZ

Experienced Member
Joined
May 18, 2007
Messages
181
Location
Z Labs.
Awhile back, after I KO'd one of my C64's (when I only had two), I attempted repair on the other one which had scrambled graphics at the time. I transplanted the graphics chip from the fried one into the scrambled one (mmmmm.... Commodore breakfast) So, in the process, I was able to get working graphics on the scrambled one, making it no longer scrambled, but revealing a new problem.

It would spit a message back at me about how it was "out of memory in 0". I could type stuff on screen, but it would only spit more errors at me or lock up completely. From there I looked up the problem online, it seems that I have an issue with one of two things: the RAM chips (mT4264-20 chips) or the RAM logic chips (7708 chips or something else... can't remember)

So anyway, I wanted to know if y'alls agree that this is the problem, and that the RAM or surrounding chips needs replacing. If so, any similar stories, how to fix this, or where to get parts would be appreciated --anything will help. Thanks.
 
You have a URL for that upgrade? Google likes to return all links to the Commodore 128. Grrrrrr....

And on the memory problem, you have bad RAM or a bad PLA chip. Swap the PLA chip (906114-01 chip) between the 2 computers as a quick test.

I doubt it's a problem with the 7708/74LS257 chips, but it could be on an outside chance. RAM or PLA are far more likely.
 
RAM repair

RAM repair

NathanAllan said:
While you're at it, may as well upgrae it to 128K if you can fidn the chips to do it with.
Click to expand...
Um, if you know what I need to do that, let me know. It sounds like what my friend told me his father did back in the day to his C64

channelmaniac said:
And on the memory problem, you have bad RAM or a bad PLA chip. Swap the PLA chip (906114-01 chip) between the 2 computers as a quick test.
Click to expand...
Hmmm... My spare board doesnt have one of those easily accessable (its not in an easy-pry socket like the one Im fixing). I'll crank open my working one to borrow one of those... I will let y'alls know if that works or not.
 
Ram repair...

Ram repair...

Well, guess what... my good C64 also has its 906114-01 soldered in place... damn it all. So, that basically means that I either have to de-solder the one out of the scrapped one, or purchase one (fat chance!). So, Ill bust out the de-soldering braid, and get to work.

Something tells me that I will have to put it on some kinda platform, based on my track record with de-soldering larger IC's. Oh boy, will THIS be fun...

I'll keep y'alls posted.
 
Esd

Esd

Take care not to zap your chips with static electricity (electrostatic discharge). When a printed circuit board is removed from the system, it is no longer connected to earth ground via the green wire on the power plug. However the chips on the board are generally protected in that they are connected to the ground plane of the board which presents a large surface area to dissapate static. The problem is handling the chips when they are not connected to the board.

If possible discharge static from yourself by touching something such as a metal chassis of something plugged into the wall, before handling the chips. Keep the chips in pink ESD foam or pink bags if possible when stored.

Things that create thousands of volts of static are shoes on carpet, and high speed air (heat guns and vacuum solder suckers).

The part you are removing is a bipolar IC which is less sensitive to ESD than NMOS and CMOS parts of the 80's, but it would pay to be careful.
 
CommodoreKid said:
Well, guess what... my good C64 also has its 906114-01 soldered in place... damn it all. So, that basically means that I either have to de-solder the one out of the scrapped one, or purchase one (fat chance!). So, Ill bust out the de-soldering braid, and get to work.

Something tells me that I will have to put it on some kinda platform, based on my track record with de-soldering larger IC's. Oh boy, will THIS be fun...
Click to expand...

I've had to do a similar thing myself. i think these were probably soldered into the board from the factory. If you come across a socketed one in a unit, it's probably because someone's replaced it in the past. :)

It's a chore but take it slow, don't hurry and you'll be fine.

Of course it might be the RAM so don't be too disappointed if the repair doesn't work. C64s are very cheap. How about just getting a scruffy beaten up one that works and just replace the board?..or maybe you're like me and just enjoy the challenge. :)

Tez
 
Last edited:
I was at the Sally yesterday, and they have(?) a C= 64 for fifteen buck$. It was just the breadbox, no P/S or any other goodies, and it had one key busted off, so I passed on it, but if someone would like it for parts, I'll go back and see if it's still there.

--T
 
Not too bad, but I pay less than that for my parts machines. I'm about to clean out my source with a trade. Two Amiga CPUs (2000 and 4000) nicely loaded with accelerators and boards for the remaining C64s in his stash.

He keeps telling me it's more than will fit in the back of my F-150. :D

Now if this rain would stop so he can get them from the storage truck in his pasture. Sigh.
 
Well, I tried that little trick of replacing the 906114-01 chip with one from my scrappedC64, and no luck. I spent nearly an hour desoldering the chip, only to plug it in with the same exact message, (even the random ! marks were in the same locations!). So, I switched back to the previous one, because its just in better condition overall.

Oh, and in the process of desoldering, I dropped the iron, and burnt a small spec into the carpet in my den/geek room. I feel really smart now.

So, based on a tip I read over in that PET 30XX repair thread, apparently "shorted" RAM chips get really hot, really fast. So, I got an idea! Let my busted C64 sit for 5 mins or so, and feel all the RAM chips. If one is hotter than the rest, I will know to replace it, right?

So, I sat it on for a short time, and the only ones that got hot were one of the kernals ROM's, the 6510, and the graphics chip...

...except for something else. BOTH of the 7708's leading up to the RAM were getting very hot, very fast. All of the RAM chips were pretty cool. The thing is, in the spot where the one being repaired has 7708's, the spare board has 74LS257A chips... Im assuming that means I cant interchange them, can I? Anyway, do you guys think that I should go ahead and replace these?
 
Last edited:
CommodoreKid said:
smart now.

So, based on a tip I read over in that PET 30XX repair thread, apparently "shorted" RAM chips get really hot, really fast. So, I got an idea! Let my busted C64 sit for 5 mins or so, and feel all the RAM chips. If one is hotter than the rest, I will know to replace it, right?

Let me know what y'alls think about this plan of action!
.
Click to expand...

Defintely. A warm chip is normal but if there is a RAM chip much hotter than all the rest, that's probably the culprit.

A RAM IC can still be damaged and not become hot though. It all depends on how its damaged.

Tez
 
tezza said:
Defintely. A warm chip is normal but if there is a RAM chip much hotter than all the rest, that's probably the culprit.

A RAM IC can still be damaged and not become hot though. It all depends on how its damaged.

Tez
Click to expand...

Exactly!

You can have a bad cell in the RAM and it'll run at a normal temp. The only time they get roasting hot is if they have an internal short.

The 7708/74LS257 chips are interchangeable. I save and test all the odd duck labeled MOS chips like the 7708 just in case someone really wants to replace a chip with the MOS original version.

It's normal for those to get pretty warm, even uncomfortably warm. If they are finger blistering hot then that's a bad sign.

RJ
 
I've looked and looked but can't find where I read about the ram upgrading. I sent an email to Bo Zimmers asking if he knows of a page or article. Bo is the MAN when it comes to anything Commodore related.

Nathan
 
RAM search...

RAM search...

After my brief hiatus (of trips, work, and otherwise shenanigans) I have returned to try and resurrect my poor broken C64's RAM

Well, I flipped through all the electronics catalogs that I have, as well as their respective websites, and I cannot seem to find a place that sells 4264-20 memory chips to make the repair...

Therefore, if anyone knows of a good reputable place to purchase said RAM, I would love to hear it. I would like to try and avoid eBay. Anyway, any suggestions of where something like this would be appreciated, as I continue looking for some on my own.

PS: thanks for the feedback about my diagnosis- it helped.
 
There are a lot of different part #s on the 64k x 1 DRAM chips used in the Commodore 64.

If there are 8 of them then it's a 4164 or compatible chip

If there are 2 of them then it's a 4464 or compatible chip

By the way, I have just a small (ahem) quantity of Commodore chips.

3639673989_9e569cb536.jpg


RJ
 
Last edited:
Well, I finally got back in town, and the first thing I did was install the RAM I ordered. Finished de-soldering those chips with the 40-watt, and then put the sockets in with the 30-watt. Two traces got torn up/interupted, so I had to run some wires to fix it.
3744776364_e52c0e34ee.jpg

Doesnt that look nice? Im sure it does. But you know what DOESN'T look nice? The screen that stared back at me, upon plugging in the board for a test run. Crap.
3744776466_8cff8fbdc4.jpg

So, any ideas of what happened, or whats wrong now would be greatly appreciated. I hope its nothing big, considering the graphics looked fine before, when the old RAM was still present. *sigh* Its always something, right?
 
You must log in or register to reply here.
Back
Top