CBM Repair Computer

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R e p a r a t u r t i p p s f r C 6 4 & C o. _COMPUTERS_C64 BLANK SCREEN C64C - CHIPS vs COMMON SYMPTOMS C64 CHIPS and COMMON SYMPTOMS A TESTER FOR THE C64 POWER SUPPLY A PROTECTION DEVICE FOR THE COMMODORE 64 Heat Sinking ICs in the C-64 ADDING A FAN TO COMMODORE EQUIPMENTIncluded are instructions for the C64, 1541, and SX64

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C64 BLANK SCREEN02-22-99 Blank screen is the most common symptom, and a failing PLA chip is the most common reason. However, quite a few other failures can cause it as well, such as a bad power supply (check with a known good substitute), bad RAM chip(s), and in general, just about any other chip in there because many chips share multiple data lines. If any one of those lines is loaded down or missing a signal for whatever reason, it can produce that symptom. To narrow it down a bit: Turn the computer off and back on rapidly about five times. If the screen ever comes up with flashing colors or all one color, the PLA is suspect. Replace it to check. Try a cartridge, such as a game. It essentially "replaces" some of the chips in the computer when it runs. If a cart works, check the ROMs. The screen may have a normal border even if the CHARacter or BASIC ROMs are bad. A bad Kernal ROM will produce no border. The internal RF unit outputs a signal that goes to the antenna input of your TV. If the picture is snowy, suspect the RF modulator, assuming the direct video output of the 64 is normal. If the computer is "dead" but is getting power (red LED on), the modulator will produce a black screen... darker than the blank screen of a failing chip in the computer. A missing 9VAC (power supply problem) is a possiblility. Note that the later C64C will still work without the 9VAC or if the internal fuse opens. However, you will have no sound, the cassette will not work, and the TOD clocks will not work. See if any of the RAM chips (there are eight of them) get warm or hot... feel each one with the back of your finger after the computer has run for about 5 minutes. Shorted chips will get hotter than the others. Note: bad RAM doesn't always get hot. See if the computer resets the other components in the system like the drive and/or printer. If so, try a "blind" disk command and see if the drive responds.. try formatting a disk. If that works, you may have a bad VIC chip (no screen display). Sometimes a bad SID chip will produce a blank screen... pull it out and try the computer. It will run without it, although you will have no sound, and a proportional mouse will not work. The few large chips that normally run hot have a high failure rate: in rough order... the PLA, SID, MPU and the VIC. Static zaps usually take out chips like the CIAs. A shorted CIA can produce a blank screen. Note that you will get the startup screen with the CIA's removed. You can use that as a diagnostic. The smaller so-called "glue logic" chips (TTL) run cool and are pretty rugged. Although they rarely fail, I've had a few that drove me crazy by making me unsolder a dozen IC's until I found the bad one. With the above noted exceptions, removing a chip will not allow the computer to produce the startup screen. Removing a chip will in most cases produce a blank screen. The only practical way to check chips is by substitution. The easiest way to do that is by inserting each suspected chip into a working computer that already has all chips socketed. (You can chase your tail doing it the other way around if you have more than one bad one.) I made a test board for just that purpose. Suspected chip(s) can be tested one at a time and only the bad ones need be replaced. At the very least, you need a source of known good chips for test purposes. Be careful... they are static sensitive. If you don't want to go to that much trouble to diagnose the problem, you will probably be better off hunting up another C64. Chips are hard to find and expensive. Keep a spare "breadbox" or two, even if only for parts. Ray Carlsen CARLSEN ELECTRONICS... a leader in trailing-edge technology.

C64C - CHIPS vs COMMON SYMPTOMSlast update and/or revision: 06-06-03There 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 U4 U5 74LS08 LOGIC Blank screen, no border. 251913-01 BASIC and KERNAL ROM (same as C128) Blank screen, no border. Cartridge works.

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 U7 318012-01 (8500) MPU aka CPU (6510 works as sub, see text) Blank screen, no border. Cartridge doesn't work.

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 U9 251715-01 (LH5062B OR YM3535) PLA, MEMORY CONTROL Blank screen, no border. Runs cool, rare failure.

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 c