Commodore 64 RAM Replacement

Main RAM on a 250407 Rev. B board: U9-U12 in the upper row and U21-U24 in the lower row, all OKI M3764-15RS here. The 74LS257 multiplexers U13 and U25 sit to the right of each row.

The Commodore 64 long boards (assemblies 326298, 250407, 250425 and 250441) hold their 64 KB of main memory in eight 4164 dynamic RAM chips, U9-U12 and U21-U24, each storing one data bit of every byte. Ray Carlsen describes these chips as especially vulnerable to over-voltage from a failing power supply and usually the first parts it destroys. The Pictorial C64 Fault Guide adds that chips made by Micron Technology (marked "MT") fail more often than the other makes Commodore fitted, and that long periods without power may contribute. The B-3 (250466) and E (250469) boards use two 64K × 4 chips instead; see Commodore 64C RAM Replacement.

Symptoms

• "OUT OF MEMORY IN 0" on the start-up screen, or fewer than the normal 38911 BASIC bytes free. Carlsen reads this as a partial failure of one chip, usually an open element or a stuck bit.
• A screen full of random characters at switch-on, or the start-up screen with stray characters on it.
• A blank screen. A shorted RAM chip always gives a blank screen and often gets very hot.
• Commodore's troubleshooting table lists U9-U12 and U21-U24 for an out-of-memory error at power-up, RAM for "different characters are displayed and cursor locked when turned on and off", and RAM or U3 (the BASIC ROM) for a SYNTAX ERROR that appears after the machine warms up.

Other faults look the same. The two 74LS257 address multiplexers U13 and U25 should be suspected along with the RAM whenever a RAM fault appears. On a run of 250407 boards repaired by Retro Repairs and Refurbs, failed U13 and U25 multiplexers (MOS 7708 parts) gave a black screen and a one-flash Dead Test result that pointed at U12. The writer lists failed multiplexers or PLAs and stuck ROM outputs among the causes of a stuck data bit, found a failing KERNAL ROM behind a six-flash code whose RAM chip tested good, and traced flash counts that changed at every power-up to poor sockets. A blank screen is more often caused by the PLA or the power supply than by RAM. Work through the Commodore 64 Troubleshooting Guide if the symptom does not match.

Safety

• Check the power supply before powering the board again. Commodore's diagnostic manual warns that several RAM chips found defective may indicate a bad power supply, and its Dead Test manual adds the on/off switch. Carlsen says that one or more shorted RAM chips usually point to a failing supply, which must be checked before the repaired computer is switched on.
• The chips are static sensitive. The board itself carries only the supply's +5 V DC and 9 V AC.

Tools and parts

• Replacement RAM: a 4164-type 64K × 1 DRAM in a 16-pin DIP. Commodore's parts lists for the 326298, 250407 and 250425 give U9-U12 and U21-U24 as "4164 (200 nS)", part 901505-01. Retroleum, a UK parts seller, lists 150 ns or faster parts for the breadbin boards and treats the 4264 marking as functionally identical.
• Good-quality 16-pin sockets. Retro Repairs and Refurbs uses double-sided sockets.
• A Dead Test cartridge (Commodore's C64/C128/C128D Dead Test, rev. 781220); see Commodore Test Cartridge Index.
• A multimeter for the supply checks; an oscilloscope for Commodore's signal checks, which are all specified as oscilloscope readings.
• Small side cutters, a soldering iron, a solder sucker or desoldering station, and desoldering braid. See Recommended Tools.

Procedure

Check the power supply

1. Unplug the supply from the computer and measure its outputs with it plugged into the mains and warmed up. Two pins carry 9 to 11 V AC; the +5 V DC output should not be high or low by more than a few tenths of a volt. Carlsen recommends testing under a dummy load such as a small 12 V lamp, because some failures appear only when the supply is warm. The supply's checks are in the Commodore 64 General Maintenance guide.
2. Do not connect a supply that reads high. Replace it, or fit a power-supply protector.
3. The full test and repair procedure is in Commodore 64 Power Supply Testing and Repair.

Open the case

1. Remove the three Phillips screws along the underside of the front edge.
2. Lift the top half-shell and unplug the keyboard connector and the power LED connector from the board. Rock the keyboard plug gently while pulling it straight up; do not pull on the wires. The keyboard plug has a blanked pin at one end; the LED plug works either way round.
3. Separate the two halves. The rear fasteners are sometimes tight and "pop" when the top is folded back.
4. Lift off the board's RF shield. Breadbin machines used several cardboard-and-foil shields, and some 250425 boards a stamped metal shield that doubles as a heatsink.
5. To desolder chips, remove the seven screws that hold the board to the lower case and lift the board out.

Find the failed chip

Use whichever of these suits the symptom. On a machine that shows the start-up screen, the piggyback test is quick. On a dead screen, use the Dead Test cartridge, the heat check and Commodore's signal checks.

Dead Test cartridge

Commodore's RAM error flash code chart from the Dead Test manual, page 2-2. The long boards are the "Rev A/B" column.

The Dead Test cartridge occupies $E000-$ECA8 and the processor starts it through the reset vector at $FFFC, so it runs without the machine's own ROMs. It is meant for machines that show no picture at power-up and is almost entirely a RAM test.

1. With the power off, plug the cartridge into the expansion port, label up, and switch on.
2. Wait. The power-up RAM test takes about ten seconds with a black screen.
3. If the screen flashes in repeated groups with a pause between them, count the flashes in one group and read the chip from the table below.
4. If nothing flashes and the diagnostic screen has not appeared after 20 seconds, Commodore calls it an initial start-up failure and lists the power supply, the reset or clock circuits, and the 6510, PLA or VIC-II with their control logic.
5. If the diagnostic screen appears (normally 10 to 15 seconds after power-up), watch the RAM tests. A chip named in the red box has failed at least once, even if a later pass shows OK, and should be replaced.

Dead Test flash codes, long boards

Flashes	Data bit	Chip
1	7	U12
2	6	U24
3	5	U11
4	4	U23
5	3	U10
6	2	U22
7	1	U9
8	0	U21

Commodore's chart labels this column "C64 Rev A/B". The 326298 schematic connects its chips to the data bus in the same order, U12 on D7 down to U21 on D0.

Schematic 326106 sheet 2 (326298 board): the eight 4164s from U12 (D7) to U21 (D0), with multiplexers U13 and U25 on the left.

Read the result with care:

• With more than one bad chip, the Dead Test flashes the code for the first one it finds. Replace that chip and run the test again for the next.
• A single flash (bit 7) is the most common false result. If replacing U12 changes nothing, the real fault is normally the chip on bit 0 or bit 1 (U21 or U9).
• A flash code means a data line is held high or low. That is usually a RAM chip, but any IC on the same data line can do it. Carlsen notes that other bad chips can give a false one-flash result and that shorted chips stop the cartridge running at all.
• The Dead Test runs with every socketed IC removed except the 6510, the PLA and the VIC-II, which lets socketed chips be ruled out.

Heat

With power applied, feel U9-U12 and U21-U24. Commodore's first RAM step is to replace any that feels extremely hot. Carlsen says RAM normally runs barely warm after several minutes, and any chip that gets hot quickly, or noticeably warmer than the others, is bad; he also notes that a bad chip does not always get hot.

Piggybacking

On a machine that reaches the start-up screen with an error or a wrong bytes-free count:

1. Press the pins of a known-good 4164 against a flat surface to bend them in slightly, so that it grips the chip below.
2. With the power off, sit it on top of the first board chip, notch over notch, with every pin touching its partner and none touching its neighbour. A test chip fitted backwards will probably be damaged at power-up.
3. Switch on and note the bytes-free figure. If it changes, even without becoming correct, that board chip is the suspect.
4. Switch off, move the test chip to the next board chip and repeat for all eight.

Never leave a piggybacked chip in place; replace the suspect chip. The Pictorial C64 Fault Guide notes that piggybacking does not always reveal a failed chip and works best on "soft" failures that still give a start-up screen.

Commodore's signal checks

Commodore's diagnostic manual checks the RAM with an oscilloscope, the diagnostic cartridge fitted, and all readings within 10 %:

1. Data, pin 2 of each chip: must go to a strong +5 V immediately at power-up. A slow or wrong level means that chip.
2. CASRAM, pin 15: a +5 V pulse. If it is wrong, cut pin 18 of the PLA (U17) and measure the cut pin: a good signal there means a RAM chip is loading the line, a bad one means U17.
3. R/W, pin 3: +5 V with one negative pulse at reset. If wrong: U9-U12, U21-U24, U17, U7, U1 or U2.
4. RAS, pin 4: a +2 V pulse. If wrong: U9-U12, U21-U24, U19 or U26.
5. Outputs of the multiplexers U13 and U25: +5 V pulses. Where one is wrong, Commodore cuts pin 8 (+5 V) of each RAM chip in turn to find a chip loading the line before condemning the multiplexer. The manual prints U13's outputs as pins 1, 7, 9 and 12 and U25's as 4, 7, 9 and 12; on schematic 326106 both chips' outputs are pins 4, 7, 9 and 12, and pin 1 is the select input.

Remove the chip

4164s are cheap, so cutting the old chip out is the safest way to remove it.

1. Cut every pin of the failed chip close to its body with small side cutters.
2. Desolder the pin stubs one at a time and clear each hole with the iron and a solder sucker. A hole with a trace of solder left can be opened by heating it briefly or gently working a pin through it.
3. Hold the board up to a light and check that every hole is clear. Examine both sides with a magnifier for lifted pads, broken traces and solder splashes.
4. The board is double-sided with plated-through holes. If a hole's plating has come away on a pin, the top-side connection is broken: pass a single strand of hook-up wire through the hole and solder it on both sides.

Carlsen's quicker alternative is to solder the new chip onto the cut stubs. It works if no pins are bridged, but the stubs loosen if overheated.

Fit the socket and the new chip

1. Solder a 16-pin socket into the cleared position. Never solder a socket with a chip in it.
2. Check every joint for bridges and dry joints.
3. Fit the new 4164 the same way round as the other chips in the row. Its notch or dot must line up; a chip fitted backwards will probably be damaged at power-up.
4. Retro Repairs and Refurbs found it quicker on one board to socket all eight positions at once and test the originals in a RAM tester.

Reassemble

Refit the board with its seven screws, the shield, the keyboard and LED plugs and the top case in the reverse order. Squeeze the case together at the rear to seat the fasteners before refitting the three screws.

Testing

1. Run the Dead Test again. A further flash code points to the next failed chip.
2. Without the cartridge, the machine should start with 38911 BASIC bytes free.
3. Let the machine run for several minutes and feel the RAM again. Every chip should be barely warm.
4. For a full check, run Commodore's diagnostic cartridge with its loop-back harness.
5. If more than one chip had failed, check the power supply again before returning the machine to use.

Board revisions

• 326298, 250407 and 250425: eight 4164s at U9-U12 and U21-U24 in all three parts lists. Commodore gives no separate list for the 250441 (B-2); it says the B-2 circuit is the B's, schematic 251469.
• 250466 (B-3) and 250469 (E): two 64K × 4 chips. Retroleum notes that these boards use two 41464s, and Carlsen that the 41464 and 4164 are not interchangeable. See Commodore 64C RAM Replacement.

Related pages

• Commodore 64 Power Supply Testing and Repair
• Commodore 64
• Commodore 64 Troubleshooting Guide
• Commodore 64 General Maintenance
• Commodore 64C RAM Replacement
• PLA Replacement Options
• Commodore 64 Power-Supply Protector (C64 Saver)
• Commodore 64 Diagnostic Instruction and Troubleshooting Manual (326070-01)
• C64 Dead Test Diagnostic Manual
• Commodore Test Cartridge Index