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Black Screen

Symptom: Screen remains black or doesn't show any video
         signal at all.
Target : C-64

Possible sources of failure

Analysis

Power supply ok?

If the power LED is not lit, check the power supply's fuse and replace it, if necessary. 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).

CPU running?

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?'.
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.

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 is 1N4001.
= 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!