Symptom: Screen remains black or doesn't show any video
signal at all.
Target : C-64
Possible sources of failure
- Power supply's fuse is blown
- continuous reset -> no startup
- Fuse -> no 12V for VIC-II (old C-64)
- Voltage regulator -> no 12V (old C-64)
- CPU -> blocks ø2 or bus
- SID -> blocks bus
- PLA -> well.... :)
- 8701 -> no clock cycles (not all models)
Power supply ok?
If the power LED is not lit, check the power supply's fuse and replace it,
Now measure the supply voltage at the 7406
in U8 (old boards) or U22
(new boards) between pin 7 (GND) and
pin 14 (Vcc). If it is higher than 5.2V, throw away your power supply
immediately and get a new one!. If it is below 4.8V, it is either due
to a bad power supply or a chip demanding a current too high.
To find out, unplug the power plug from the power supply jack of the C64
and measure the 5V DC directly at the
power supply plug.
While you are at it, you should also look for the 9V AC (between pins 6
and 7), which should range from 9V to 11V (as it is unregulated).
If either 5V DC or 9V AC are out of range, get a new power supply.
If the 5V DC at the power supply plug prove to be okay, but the voltage
measured at the 7406 isn't, check the voltages at a few other TTL chips,
they are labelled 74xx(x) or 74LSxx(x). If only one of the other TTL chips
(EXCEPT the ones near the VIC-II, they have a separate voltage supply) have
the 5V between their GND pin (which is the lower left pin when the notch
points to the top) and their Vcc pin (which is the upper right pin), your
board has one of more cold solder joints or haircrack.
In this case follow the trace(s) from the TTL chip which had the 5V at the
Vcc pin to the Vcc pins of the TTL chips without 5V using a continuity
tester and bridge the haircracks with a piece of wire.
pins to the other TTL chips (the ones without 5V) and resolder ith thein ththen check all chips on the board whether
one of them gets extremely hot ( = OUCH!-hot) and replace this chip.
If none of them gets extremely hot, replace the hottest chip. If the
supply voltage at the 7406 i
If you have an old C-64, check its
internal fuse now (near the expansion port).
Try accessing the floppy drive by typing blindly (you do not
need to close your eyes ;-)
LOAD"$",8 (assuming your device id is '8')
If the floppy motor starts spinning and the drive searches for the
directory, CPU, CIA's and the address manager (PLA) seem to work, so
that you can check section 'VIC-II okay?' now.
Reset line HIGH?
Since it is possible that the black screen is caused by a continuous reset,
you should check the voltage between pin 1 (GND) and pin 3 (/RESET) of the
user port. If it is below 2.4V, check the
reset switch, if you got one. If the switch proves to be okay and you have
a new board, check whether the output
levels of the 7406 (U22) and
74LS14 (U23) match their input
levels. Next, you should check the timer 556 in U20
(old board only) when I managed to
offer you a description for that chip :-) Until then, simply replace it, it
is not socketed, but cheap.
If that does not help, some other chips load the reset line so that the
level is below 2.4V which causes the CPU to reset. In this case try swapping
VIC-II, CPU, SID, and the two CIA's with the ones from another, working C64.
If you can't swap all the chips and don't want to desolder (some of) them,
try soldering a pull-up resistor (4.7 kohm) between pin 2 (+5V) and pin 3
(/RESET) of the user port. If that does
not make the blank screen vanish, *** check for other defective chips (which
probably become very hot, see later in this document).
Only VIC-II malfunctioning?
Then check the 5V at the power jack
(before the switch!) with the power supply plugged in and the C-64 switched
on. It should be between 4.9 and 5.1V. If not, a defective IC might demand a
current so high that the 5V level gets pulled below 4.8V. You should be able
to locate the very IC by simply touching all the chips and checking for
especially hot ones. Replace it.
Next, measure the 5V supply voltage between pin 7 (GND) and pin
14 (Vcc) of a 74xx chip. If the 5V show to be ok, then continue
with 'Voltage regulator ok?'.
In case you have an old C-64, check
the 12V voltage regulator VR1 (7812). At the output (right pin, 2) you
should measure around 12V (11.8 to 12.3V). If not, measure the input
voltage (pin 1, left), the voltmeter should show about 15 to 20V. If the
latter is the case, replace the VR. If the input voltage is out of range,
check the 9VAC on the power plug (with
the C-64 switched on). If they prove to be ok, then the two diodes CR5 and
CR6 before the voltage regulator could be damaged; replace them, their type
Otherwise, it is very likely that the cause for the low voltage level
is - believe it or not, it DID happen to a lot of people - the power
switch(!), so that it could be a good idea to either rock the switch
several dozen times, open and clean it or to replace it completely.
If you cannot find the 9VAC, make sure you have checked for
the C-64's internal fuse and otherwise get another power supply.
Other chips alive?
Check VIC, SID, PLA and, where applicable, the clock circuit 8701
near the VIC (not in very old PCB revisions), whether they get
extraordinaryly hot. If any of the chips get very hot, it is very
likely that they are defective and e.g. block the bus. They should
be replaced. In C-64s with the new board, you will find a 64-pin
multifunction chip, the MMU. If the other chips are ok, it is
probably the MMU which is damaged. Alas, this MMU is practically
impossible to replace, there are no spare chips that I am aware of.
You could only swap it with a working one from another C64 (with the
new board, of course).
Of course, every chip connected to the bus in any way could be
damaged and blocking the bus, therefore you should also check
if the CIAs, the ROMs, the RAMs, the RIMs or the RUMs (oops) are
getting extremly hot. Especially in the old C-64 board, the
various 74xx/74LSxx might be responsible for the failure.
If all else fails...
... you should consider the very low price of a used C-64 on
the free market or, if you live in a strange country with prices
over US$ 30 for a C-64 (including power supply), check for
hair cracks and dry joints.
Updated: February 9th, 1998
Created: January 1997
Status : NOT VERIFIED!
Site copyright © 1997 by Marc-Jano Knopp
This document is part of MJK's Commodore 64 & LCD Page
Brought back to life by Peter Schepers, Dec 10, 2005 because I really liked this site!