C64 BLANK SCREEN Latest updates and corrections 11-20-07.
PACKING ELECTRONIC EQUIPMENT TO SURVIVE SHIPPING latest updates and corrections 2-1-2012 Blank screen is the most common symptom, and a failing PLA chip is the most common reason in the early version C64. 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. Fortunately, most of the time, the same few ICs are the cause of the blank screen symptom, so that narrows the search down a bit. You start with the statisticlly most common failures are work your way down. There are a few tests you can run to determine what the problem is. At a minimum, you will need a supply of known good replacement chips, some skill with a volt-ohm meter and a soldering iron. Repair is mostly educated guesswork and trial and error to see what works and what doesn't as you go along. 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 game cartridge. Some carts will essentially "replace" some of the chips in the computer when the game runs. If a cart works, check the ROMs. The screen can have a normal border even if the CHARacter or BASIC ROMs are bad. A bad Kernal ROM will show no border but a few game carts bypass the Kernal. Commodore Kickman and Jupiter Lander are the only two I have that work with a missing or bad Kernal ROM. I once repaired a C64 with one bad RAM chip, U10 on a 250425 board. The symptom was a blank screen. Remembering that some game cartridges would work with a bad Kernal, I tried a cartridge and the computer produced a garbled game screen. After swapping out the PLA and the Kernal without success, I tried the RAM "piggyback" trick (explained in detail later) with the cart plugged in and got the game screen with U10 piggybacked. I then got a normal boot screen after that RAM chip was replaced. 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, or bad cabling (switchbox, adaptor, etc.) to the TV. If the computer is "dead" but is getting its +5VDC source from the power supply (red LED on), the PS 9VAC source may be missing. If so, the modulator will produce a black screen... darker than the blank screen of a failing chip in the computer. Loss of 9VAC can be a bad PS or blown fuse in the computer. 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 boot 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 because those ICs are directly connected to the external ports. A shorted CIA can produce a blank screen. You will get a startup screen with the CIA's removed but you will have no cursor without U1 (keyboard interface) plugged in. 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 repair jobs that drove me crazy by making me unsolder a dozen IC's until I found the bad one. Sometimes there is just no easy way. With the above noted exceptions, removing a chip will usually 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, and you're really stuck if you end up having more than one bad IC. I made a test board for just that reason. 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. Salvage from used boards is all there is to keep our "obsolete" computers going. Ray Carlsen CARLSEN ELECTRONICS... a leader in trailing-edge technology.