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C64 BLANK SCREEN 02-22-99
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.
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.