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Commodore 64C RAM Replacement

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Commodore 64C 250469 board RAM U10 U11 and memory controller U8
A 250469 board (PCB 252311 Rev. 4): the two 64K × 4 DRAMs at U10 (front) and U11 (rear), with the 251715-01 memory controller at U8 and the 8500 CPU at U6.

The Commodore 64C holds its 64 KB of main memory in two 64K × 4 dynamic RAM chips: U9 and U10 on the B-3 board (assembly 250466), U10 and U11 on the E board (assembly 250469).[1][2] Each chip stores four of the eight data bits of every byte.[3] Ray Carlsen lists the two chips' failures as a blank screen with no border, or a garbage screen, a low bytes-free count or "OUT OF MEMORY IN 0" when the failure is partial.[4] Carlsen describes RAM as particularly sensitive to over-voltage from a failing power supply and usually the first part to fail, by shorting out.[5] The eight-chip long boards are covered in Commodore 64 RAM Replacement.

Symptoms

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  • A blank screen with no border. A shorted chip may run warmer than its partner, sometimes very hot.[4]
  • A garbage screen, fewer than 38911 BASIC bytes free, or "OUT OF MEMORY IN 0" at start-up.[4]

Faults that look the same:

  • E board: the memory control is inside the U8 memory controller, so there are no 74LS257 multiplexers to suspect.[5][2] Carlsen describes U8 as running cool and failing very rarely; it also gives a blank screen with no border, as do the CPU U6, the ROM U4 and the 74LS08 U3.[4]
  • B-3 board: it keeps the long boards' 74LS257 multiplexers at U13 and U25 and the 82S100 PLA at U17, and Carlsen advises suspecting the multiplexers along with the RAM.[1][5]
  • The power supply produces many of the same symptoms and should be checked first.[4]

See the Commodore 64C Troubleshooting Guide for the other chips.

Safety

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  • Check the power supply before switching the repaired machine on. Commodore's Dead Test manual warns that several defective RAM chips may indicate a defective power supply or on/off switch.[3]
  • The chips are static sensitive.[6]

Tools and parts

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  • Replacement RAM: a 41464 (or 4464) 64K × 4 DRAM in an 18-pin DIP.[7] Commodore lists part 390083-02 (150 ns) for the B-3 and 390083-07 for the E board.[1][2] The 4164 used on the long boards is not interchangeable with it.[5]
  • Two 18-pin DIP sockets.[7]
  • A Dead Test cartridge (Commodore's C64/C128/C128D Dead Test, rev. 781220); see Commodore Test Cartridge Index.[8]
  • A Phillips screwdriver; some machines have Torx screws, which need a T10 driver.[9][4]
  • Small side cutters, a soldering iron, a solder sucker and desoldering braid.[10] See Recommended Tools.

Procedure

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Check the power supply

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  1. Unplug the supply from the computer and measure it plugged into the mains and warmed up, preferably with a dummy load such as a small 12 V lamp. The 9 V AC pins read 9 to 11 V AC; the +5 V DC should not be high or low by more than a few tenths of a volt.[5]
  2. Replace a supply that reads high, or fit a power-supply protector.[11]
  3. The full test and repair procedure is in Commodore 64 Power Supply Testing and Repair.

Open the case

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CMD's JiffyDOS installation guide gives the 64C procedure:[9]

  1. Switch off, unplug the supply and remove every cable and anything in the cartridge, cassette and user ports.
  2. Turn the computer upside down and remove the three screws holding the two halves together.
  3. Turn it back over, separate the halves at the front seam and lift the front edge of the upper case.
  4. Unplug the keyboard cable from the board, lifting each end of the connector in turn.
  5. Detach the keyboard from the lower case by removing its retaining screws and sliding it out from under the plastic tabs at the front of the case. Put it to one side.
  6. Unplug the power LED cable from the right side of the board and lift off the upper case, minding the plastic tabs at the rear.
  7. Remove the cardboard-and-foil or metal shield, held by self-tapping screws, if one is fitted.
  8. To desolder the RAM, remove the screws holding the board to the lower case and lift it out. Retroleum, a UK parts seller, gives five on cases with metal keyboard brackets, the two at the bottom left and right passing through the brackets; cases that simply clip together use the smaller breadbin-type screws.[12]

Find the failed chip

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C64 Dead Test system RAM chart (314139-02 p. 2-3)
Commodore's System RAM Chart, Dead Test manual page 2-3: the chip named on the Dead Test screen, translated to the B-3 and E boards.
  1. With the power off, plug the Dead Test cartridge into the expansion port, label up, and switch on.[13]
  2. The power-up RAM test takes about ten seconds with a black screen. If the screen then flashes in repeated groups, count the flashes in one group.[14][3]
  3. Read the chip from the table below. The Dead Test flashes the code for the first bad chip only, so run it again after each replacement.[3]
  4. If the diagnostic screen appears, it names RAM chips by their long-board positions. Translate them with Commodore's System RAM Chart.[15] A chip named in the red box has failed at least once and should be replaced, even if a later pass shows OK.[8]
  5. Feel both chips after a few minutes of running. Carlsen reports that a shorted chip may get warmer than the other, sometimes very hot.[4]
Dead Test flash codes on the 64C boards[3]
Flashes Data bit B-3 (250466) E (250469)
1 7 U9 U11
2 6 U9 U11
3 5 U9 U11
4 4 U9 U11
5 3 U10 U10
6 2 U10 U10
7 1 U10 U10
8 0 U10 U10
Dead Test screen names, translated[15]
Shown on the Dead Test screen B-3 (250466) E (250469)
U21, U9, U22, U10 U10 U10
U23, U11, U24, U12 U9 U11

Read the result with care. A single flash (bit 7) is the most common false result, and the real failure is then normally on bit 0 or 1, which on both boards is U10.[3] A flash code means a data line held high or low; usually a RAM chip, but any IC on that data line can do it.[3] Shorted chips can stop the Dead Test running at all.[6]

Remove the chip

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  1. Cut every pin of the failed chip close to its body with small side cutters. RAM is cheap; save careful whole-chip desoldering for expensive custom chips.[10]
  2. Desolder the stubs one at a time and clear each hole with the iron and a solder sucker.[10]
  3. Check every hole is clear against a light, and inspect both sides under a magnifier for lifted pads and torn through-hole plating. Bridge a damaged hole with a single strand of wire soldered on both sides.[10]

Fit the socket and the new chip

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  1. Solder an 18-pin socket into the position. Never solder a socket with a chip in it.[10]
  2. Fit the new 41464 the same way round as the other RAM chip, its notch or dot lined up. A chip fitted backwards will probably be damaged at power-up.[5]

Reassemble

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Refit the board, the shield, the keyboard (under the front tabs, with its retaining screws), the keyboard and LED cables and the upper case, then the three case screws.[9]

Testing

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  1. Run the Dead Test again until it shows no flash code and passes its RAM tests.[3]
  2. Start the machine without the cartridge. It should report 38911 BASIC bytes free.[4]
  3. Feel the two RAM chips after several minutes; neither should be hotter than the other.[4]
  4. If more than one chip had failed, check the power supply again.[3]

Board revisions

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  • B-3 (250466): RAM at U9 and U10, 390083-02. The board keeps the 6510, the 82S100 PLA and the 74LS257 multiplexers.[1]
  • E (250469): RAM at U10 and U11, 390083-07. Memory control is in U8.[2][5] Commodore's identification table also lists only two RAM chips for the B-3, so check for the separate BASIC and KERNAL ROMs (B-3) or the single combined ROM at U4 (E) before using the table above.[16]
  • Carlsen notes that the earliest 64Cs used the long 250425 board; those machines have eight 4164s and are covered in Commodore 64 RAM Replacement.[4]
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References

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  1. ↑ 1.0 1.1 1.2 1.3 Commodore Business Machines, Service Manual C64/C64C, March 1992, PN-314001-03, hosted at Commodore 64 / 64C Service Manual. "Parts List, PCB Assembly #250466-01 C64-B3", p. 34: U9 and U10 "MEM DRAM 64KX4 Bit", 390083-02, 150 ns; U13 and U25 74LS257A; U17 82S100 PLA.
  2. ↑ 2.0 2.1 2.2 2.3 Service Manual C64/C64C (1992), "Parts List, PCB Assembly #250469-01 C64-E", p. 40: U10 and U11 64K × 4 DRAM, 390083-07; U8 memory controller 251715-01.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 Diagnostic Manual, C-64 Dead Test (1988), RAM Error Flash Code Chart and notes, p. 2-2.
  4. ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 Ray Carlsen, "C64C (true C short board) - chips vs common symptoms", updated 6 January 2020. Repair notes for the 250469 board, with a note on the earlier 64C boards.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 Ray Carlsen, "RAM (Random Access Memory) problems in the C64/128 computer", updated 9 June 2011. Repair notes: 4464/41464 against 4164, the C64C memory control, the piggyback test, power supply checks.
  6. ↑ 6.0 6.1 Ray Carlsen, "C64 blank screen", updated 27 November 2021. Repair notes.
  7. ↑ 7.0 7.1 Retroleum, "C64 chips", product note for the 41464/4464 RAM: 64K × 4, 18-pin DIL; the 4464 is the same chip under another part number. Parts seller. Retrieved 2026-10-25.
  8. ↑ 8.0 8.1 Commodore Business Machines, Diagnostic Manual, C-64 Dead Test, PN-314139-02, January 1988, hosted as File:C64 Dead Test Diagnostic Manual (314139-02, 1988 Jan).pdf. "Dead Test Diagnostic Overview" and "Diagnostic Startup", p. 1-1.
  9. ↑ 9.0 9.1 9.2 Creative Micro Designs, JiffyDOS C-64 Installation Guide, pp. 1-4: tools (Phillips, T10 Torx on some machines) and disassembly of the C-64 and 64C. PDF, linked from the RETRO Innovations JiffyDOS page.
  10. ↑ 10.0 10.1 10.2 10.3 10.4 Ray Carlsen, "Replacing IC chips in Commodore equipment", updated 19 December 2012. Repair notes.
  11. ↑ Ray Carlsen, "All about the Commodore power supply", updated 5 February 2014. Repair notes.
  12. ↑ Retroleum, "Commodore 64 fixings", product notes on C64C case and motherboard screws. Parts seller. Retrieved 2026-10-25.
  13. ↑ Diagnostic Manual, C-64 Dead Test (1988), "Installation", p. 1-2.
  14. ↑ Diagnostic Manual, C-64 Dead Test (1988), "Power-Up RAM Test", p. 2-1.
  15. ↑ 15.0 15.1 Diagnostic Manual, C-64 Dead Test (1988), System RAM Chart, p. 2-3.
  16. ↑ Service Manual C64/C64C (1992), "C64C Board Identification", p. 17.
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