C64 BLANK SCREEN       
                 Latest updates and corrections 11-27-21

     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 statistically 
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... but not a shorted one, of course.
     A diagnostic cart called the Dead Test can sometimes find a bad
RAM but other bad chips can give a false "one blink" indication. If 
RAM or other chips are shorted, even the DT cart will not function. 
Another diagnostic cart is more helpful after repairs are done but it
requires a harness called the "octopus" cable that cross-connects all
the ports. That cart will throw lots of errors without the cable. 
     After the above tests fail to restore normal operation, I drag
out the oscilloscope and check all eight data lines. There should be
activity on all lines which normally measures about 4VPP. If one or more
are missing or low (3VPP is not good enough), you need to find which IC
is dragging that line down. Note that many chips share data lines, so
each one must be checked. I desolder one pin of each suspected IC and
free it so it doesn't make contact. If the data signal returns, I swap
out that chip. You can't do that with anything but an oscilloscope. A
logic probe may not tell you enough. A standard meter is useless for
anything but voltage checks... the +5VDC and 9VAC from the power supply
and the various on-board regulators.
     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. If you ever find more than one bad RAM chip (often shorted) 
in a computer, it was more than likely caused by a failing power supply.
One failure mode of the Commodore black or white "brick" is due to 
over-voltage on the five volt output --after warmup--. The PS may work 
fine again after it cools down but then go on to kill another C64!
     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. 
     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.