Commodore 64: RAM (RANDOM ACCESS MEMORY) PROBLEMS IN THE C64/128 COMPUTER
RAM (RANDOM ACCESS MEMORY) PROBLEMS IN THE C64/128 COMPUTER
latest updates and/or corrections 6-9-2011
If you get an "out of memory" error or less than the normal number of bytes free (should be 38911 for the C64) when you turn your computer on, that's a RAM problem and usually points to a partial failure in one of the RAM IC chips. Blank screen can be a RAM problem but can also be caused by many other failures such as a bad PLA or power supply which are actually more common. Bad RAM can produce a "garbage" screen which is a symptom where the screen fills up with random characters when the computer is turned on, or it can show up as a few random characters anywhere on the screen.
There are two memory control chips that should also be suspected when you have an apparent RAM problem: U13 and U25 in the C64, and U14 and U15 in a C128 and 128DCR. Those are generic 74LS257 logic chips. The C64C integrates all memory control inside its PLA and so doesn't use the 74LS257 chips.
Early C64 computers have eight 4164 TTL RAM chips installed.
Some interim brown case and all later OEM C64C (white case) computers have only two 4464 RAM ICs (41464 is the same IC) as those chips have four times the memory "density" of the 4164. They are not interchangeable with the earlier RAM. Early C128 computers have sixteen 4164 chips and a total memory count at startup of 122365 bytes free in 128 mode. The USA metal case C128DCR has four 4464 (41464) RAM chips and the same bytes free count as the C128 mode startup screen. The 64 mode in all C128 computers shows the same memory count at startup as a stock C64, namely 38911.
One common symptom of bad RAM is the error message "OUT OF MEMORY IN 0" at startup. That means the computer failed its RAM self test and therefore can't load programs or perform even simple tasks. Because it can produce the opening screen, this means one RAM IC has an open element or "stuck" bit. There are eight RAM ICs in there, so how do you find the bad one without just replacing them all one at a time? Fortunately, there are several methods.
It's possible to calculate which RAM bank has failed the self test using the bytes free number, but I could never find exact information regarding that procedure, so I've never used it. Since the computer partially works (opening screen comes up), I use a method that often works: to "piggyback" a known good 4164 RAM chip on top of each computer RAM IC and then power up the computer and observe the number of bytes free on the screen. If the value changes (even if it's still not correct), change that board RAM IC and try the computer again. Never leave the piggyback chip in place as that may cause timing problems or may eventually stop working. The board chip can continue to deteriorate and could eventually short out. It's best to remove and replace it quickly. Some people just replace all the RAM at one time. Those chips are cheap and still available as of this writing.
The piggyback method requires that you install the test chip over a board RAM chip so all pins make good contact and no pins short together. The test chip must be oriented correctly as well. There is a notch or dot (or both) on one end of those ICs. Those markings must line up because if the test chip is installed backwards, it will likely be damaged when power is applied. You can press the test chip pins against a tabletop to push all pins inward slightly, and that will provide some spring tension when the test chip is inserted over the board chip and can hold it in place. It's OK if you want to hold the chip with your fingers and then power up the computer. There is only five volts present and therefore absolutely no danger of electrical shock. Again, if the bytes free number changes, that board IC is probably bad and should be changed. If there is no change when all RAM is checked, it can still be a RAM problem (stuck bit) that keeps the computer locked up. Chip swaps is the next step in diagnosis.
Note that shorted RAM will always produce a blank screen and will often get very hot to the touch. RAM normally runs barely warm to the touch after several minutes of operation. Any that get hot quickly or even noticeably warmer than the other RAM chips are bad!
One or more shorted RAM chips usually point to a failing power supply. That MUST be checked before you turn the computer on again after repairs are done. One easy way is to measure the voltage at the appropriate pins of the four pin DIN plug of the power supply while it's plugged in to AC power but disconnected from the computer. Don't short any pins together while testing! That test should be done while the PS is still fully warmed up.
Two pins will measure 9 to 11 volts AC, which is normal. The other two pins should measure 5 volts DC. If that regulated source is high or low by more than a few tenths of a volt, the supply is failing and should be replaced. All black or white original CBM 64 "brick" power supplies are potted with epoxy inside and therefore cannot be repaired. Note that some supply failures will not show up until the supply has been allowed to warm up for awhile. Therefore it's best to run the power supply with an artificial "load" (not a computer!) connected to it such as a small 12v light bulb. See the PS tester article for more information about that.
Other chips may be damaged by overvoltage from a failing C64 power supply but RAM is particularly sensitive to that problem and will usually be the first to fail by shorting out. Obviously the piggyback method of chip testing will not work with shorted chips that produce a blank screen. It's interesting to note that shorted RAM chips in the C128 are almost unheard of. The 128 switching supply is more reliable and rarely harms the computer even when it does fail. Voltage "sag" (lower than 5 volts output) when loaded usually points to failing filter capacitors in the supply. The C128 power supply is repairable unlike the old 64 epoxy brick.
CHIP REPLACEMENT
There are several ways to replace ICs on a computer board.
One way that doesn't require a lot of work is to snip all the pins off close to the body of the IC with a pair of small cutters and just solder the new IC to the remaining pin stubs. It doesn't look very neat, but it works fine as long as no pins are shorted together. If you get the stubs too hot during soldering, they can loosen up in the PC board holes and start moving around... so you should solder quickly. Examine your work carefully when you're done. Look for shorts between pins and solder splashes or bad connections (sometimes called "cold solder" joints).
The next method is a variation of the first, namely to snip all the pins of the bad chip, but in this case to desolder them one at a time so you can install a socket. Once the stubs are removed, you need to clean the residue of solder from the holes. That's easily done with your iron and a solder sucker, a vacuum device that slurps up molten solder when the trigger is pressed. If a hole is open but a tiny bit of solder remains, you can poke a straight pin through and wiggle it around a bit... gently!
Solder is made of tin & lead and is relatively soft, but if you get too aggressive, you may damage a board trace (the copper wiring in and around the holes) which MUST be repaired before you install a socket. If you install an IC or socket over an unrepaired trace, you've just created and covered up another problem that is -very- difficult to find, even for a tech. Damaged plated-through holes can be repaired by installing a wire jumper through the board from top to bottom, then soldering the wire to good copper on both sides.
With regards sockets, I always use them when replacing chips.
4164 RAM chips usually cost under two US dollars each so just replacing suspected chips by snipping the pins is OK. I always desolder larger proprietary (expensive as well as hard to get) ICs such as the PLA or SID intact. That way, if the suspected IC happens to still be good, I have not ruined it by cutting the pins off. Removing any chip intact requires a lot more care and the proper tools to prevent chip and/or board damage. Once even a small IC is removed, it's still a good idea to install a socket. Then that area of the board will never be subjected to the heat of soldering again. Each board rework with a soldering iron results in weakening of board/trace bonds. The less of that the better.
Ray Carlsen
Carlsen Electronics... a leader in trailing-edge technology
