C64C - CHIPS vs COMMON SYMPTOMS 09-28-07.
C64C - CHIPS vs COMMON SYMPTOMS last update and/or revision: 09-28-07 There were several revisions to this model that featured a slimline white case, white keys and an internal heat sink shield. The earliest versions used the original 64 motherboard (250425), the same board as in the brown case C64. Interim boards (250466) used two 4464 RAM chips instead of eight 4164, used either version SID or CIAs, but retained the old MPU, PLA and VIC-II. The latest "cost reduced" motherboards (250469) were narrower and used fewer, more integrated chips. Most of those ICs are not interchangeable with earlier models, and chip ID numbers are different than earlier boards. For that reason, this article deals with the latest board only, although basic troubleshooting is similar for all versions of the C64. U1 906108-02 (6526/ A or 8521) CIA Startup screen normal but no cursor, or blank screen if chip is shorted (remove to test). No keyboard or joyport access. Partial failure: some keys or joystick positions don't work, "stuck" line may print a character at startup. Cartridge works. U2 906108-02 (6526/ A or 8521) CIA Lines or blocks instead of startup screen, but blank screen if chip is shorted (remove to test). Partial failure: marginal or no serial / user port access, keyboard and cartridges still work. U3 74LS08 LOGIC Blank screen, no border. U4 251913-01 BASIC and KERNAL ROM (same as C128) Blank screen, no border. Cartridges don't work except the few that bypass the Kernal ROM such as CBM Kickman and Jupiter Lander. Those can be used as a diagnostic for a bad Kernal. U5 901225-01 CHARACTER ROM Blank screen with border or screen full of shimmering lines or characters. Partial failure: "garbage" characters or blocks where startup page info should be. Cartridge works. U6 318012-01 (8500) MPU aka CPU (6510 works as sub, see text) Blank screen, no border. Cartridge doesn't work. U7 318014-01 (8562 or 8565) VIC-II Blank -white- screen, no border. Partial failure may produce garbage or "checkerboard" screen, or smeary screen that lacks contrast. If screen is blank or garbled from bad VIC, "blind" disk commands from keyboard may still work. NOTE: earlier 6567 will -not- sub for this chip. U8 251715-01 (LH5062B OR YM3535) PLA, MEMORY CONTROL Blank screen, no border. Runs cool, rare failure. U9 318013-01 (8580) or 906112-01 (6581) SID (SOUND INTERFACE DEVICE) Blank or "garbage" screen if shorted (remove to test), otherwise normal screen. No sound or garbled sound. Mouse or graphics tablet pointer stuck or jitters. NOTE: for test purposes, computer will work without a SID plugged in. NOTE: 8580 and 6581 are pin compatible and somewhat interchangeable, but only with board component changes: pin 28 supply voltage either +9 or +12VDC, and two capacitors values must be changed for correct voicing. U10 and U11 (M41464) 2 RAM CHIPS (64K X 4 DRAM) Blank screen, no border. Shorted chips may get warmer (sometimes very hot) than the other RAM chips. Partial failure: will sometimes produce "garbage" screen, abnormal number of bytes free (lower than 38911) or "OUT OF MEMORY IN 0" error on startup screen. U18 4066 CMOS QUAD SWITCH (POTX/Y BUFFERS) Startup screen normal, but mouse/graphics tablet don't work. U19 LC3514A or MN2114-2 SRAM 1K X 4 (aka COLOR RAM) Shimmering colors on characters. U20 8701 or 7701 MASTER CLOCK OSC. Blank white screen, no border. U21 4066 CMOS QUAD SWITCH Wrong colors on characters. U22 7406 LOGIC Blank screen. No drive reset when computer switched on. U23 74LS14 LOGIC Blank screen. Drive spindle motor runs continuously with computer on. RUN/STOP-RESTORE doesn't work. M1 252405-02 RF MODULATOR Weak or no RF output (snowy screen) to TV. Internal defect in modulator can affect A/V outputs (no audio or video) since those signals go -through- modulator. Q1 2SD313 TRANSISTOR, CASSETTE MOTOR DRIVER Cassette motor will not turn when FF/REW or PLAY is pressed. Also check fuse inside computer. F1 1.5 AMP FB FUSE, 9VAC PROTECTION If fuse is open, computer appears to work normally, but no cassette operation, no 9VAC to user port, and no sound from SID. POWER PACK: +5VDC at 1.5Amps and 9VAC at 1 Amp Can produce many problems like blank screen (red power LED on, dim or off), program lock-up, "garbage" screen, hum bars moving on screen, hum in audio, damaged RAM chips, intermittant operation after warmup, etc. As common a failure as it is, the supply should be checked (by substitution) first. ******************************************************************* Troubleshooting any version of the C64 is often difficult because some symptoms can be caused by any one of a dozen chips. The most common, a blank screen with no border, is sometimes frustrating for even a tech to diagnose. We usually start by process of elimination, keeping in mind the chips most likely to fail in order. Each board is different in how many of the major chips are socketed, but that's a good place to start. Reseat all socketed chips by lifting up on each end slightly (pry between chip and socket with a small flat-blade screwdriver) and press the chip back down. There is no need to remove the chip just for reseating. You risk bending pins if you remove it. If you suspect a chip is bad based on symptoms, the best way to test it is by installing it in a working computer. If none are available, and/or you have no spare parts or boards, the job is nearly impossible. Substitution of chips or parts is the only practical way for most people to diagnose a fault. Don't overlook the power supply when you are troubleshooting. It deserves its bad reputation for damaging computer chips under some fault modes. Check by substitution only. The original supply is "potted" with epoxy inside and is not repairable. After-market power supplies are "beefier" and usually repairable. The 85xx series chips used in the C64C are, for the most part, not interchangeable with earlier C64 ICs. They represent a different logic "family" and in some cases have a different pinout. Substitutions should therefore not be made casually. As indicated above, the computer will work with a few of the major chips removed. This is helpful as a diagnostic. For example, the SID can produce a blank screen if shorted, but the computer will still work without it, except of course without sound, and the POTX/Y inputs will not work (mouse, graphics tablet, etc.) but a joystick will still work. The two CIA chips can be removed to test for shorted chips (they're bad for sure if they get hot) and the computer will come up if the chip is bad although the startup screen will be affected (see chip list for U1 and U2). A bad keyboard row or column is usually caused by a bad CIA. The two CIAs can be swapped to aid in diagnosis if no other parts are available, and the fault will follow the bad chip, but of course with different symptoms. Unlike the one in the C64, the PLA in the C64C runs cool to the touch. I've never seen a bad one. Of course new and old versions are not interchangeable. The older 6510 MPU seems to work in the C64C as a sub for the 8500. I have tried an 8500 in an old C64 and it worked (diagnostic test only), but I don't know how well these chips interchange in actual use. All of the 8500 series chips run cooler than the older 6500 series. The later C64C uses some of the same chips as a C128. Note that an open fuse in the older C64 computer will make it non-functional because the converted 9VAC runs the VIC and SID chips. An open fuse or failed 9VAC (bad power supply) with the later C64C may not be immediately apparent... the computer will still work but you will have no sound, the user port will not get the 9VAC, and the cassette motor will not work. The cabinet screws and the ones inside the computer may be the newer Torx (star-shaped) type rather than standard Philips. If so, you may need to get a T-10 bit to removed those screws. I've found that a standard small flat-blade screwdriver fits well enough if a Torx driver is not available. As mentioned before, the "blank screen" symptom is the hardest to troubleshoot because many different problems can produce that symptom. It would therefore be helpful to know what signals should appear on each pin of the microprocessor, the heart of the machine. If, for example, the reset line was being held low for some reason, that could be traced down to the specific problem and fixed. With that in mind, here is the pinout for the 6510/8500 microprocessor used in the C64 and C64C. Readings are taken with computer on and no external devices connected, keyboard unplugged. You'll need an oscilloscope or logic probe to make sense of some of these readings. 1 CLK 1 IN OSC SIGNAL 2 RDY DATA, ANDED BA FROM PLA + \DMA 3 \IRQ DATA (HIGH) 4 \NMI HIGH, FROM \RESTORE CIRCUIT 5 AEC DATA, ANDED AEC FROM VIC + \DMA 6 VCC ALWAYS +5VDC 7 A0 DATA \ 8 A1 DATA | 9 A2 DATA | 10 A3 DATA | 11 A4 DATA | 12 A5 DATA | 13 A6 DATA | 14 A7 DATA | 15 A8 DATA | ADDRESS LINES 16 A9 DATA | 17 A10 DATA | 18 A11 DATA | 19 A12 DATA | 20 A13 DATA | 21 GROUND 0 VDC 22 A14 DATA | 23 A15 DATA / 24 P5 +3V 25 P4 HIGH 26 P3 LOW 27 P2 HIGH 28 P1 HIGH 29 P0 HIGH 30 D7 DATA \ 31 D6 DATR | 32 D5 DATA | 33 D4 DATA | 34 D3 DATA | DATA LINES 35 D2 DATA | 36 D1 DATA | 37 D0 DATA | 38 R/W DATA / 39 CLK 2 OUT OSC SIGNAL 40 \RESET HIGH, FROM RESET CIRCUIT Ray Carlsen CET CARLSEN ELECTRONICS If you have any questions or comments, please feel free to write, especially if you spot an error here. Thanks!