When a C64 fails, the most common symptom is a black screen. While having a display of some sorts makes troubleshooting easier, a black screen gives away many useful hints. Here is how I usually start the diagnostics process, when I get a machine with a black screen.

Introduction

A black screen can be caused by just about any component on the mainboard. It is hardly possible to provide a complete guide on how to troubleshoot from that starting point, but rather to provide guidance on how to perform the initial steps in the diagnostics process and hopefully get closer to either solving the problem or isolating it.

In truth, I very often buy machines sold as defective only to discover that there either wasn’t a problem or that the problem was something trivial such as a blown fuse.

Test tools

Testing and repairing Commodore 64s isn’t something you can do with just a screw driver and a multimeter. You won’t make much progress without a minimum test setup.

First and foremost, there is no point in attempting troubleshooting anything without a known working power supply and a known working monitor and a known working screen cable.

Second, to get any sort of information out or a machine with a black screen, the following constitute a bare minimum:

• Dead Test Cartridge or even better a DesTestMAX cartridge.
• A multimeter
• Logic probe or oscilloscope

I also recommend to provide a way of testing chips. It could be either a test board with all ICs in sockets (best option) or a chiptester such as the BackBit Chip Tester or a Retro Chip Tester Pro (the fastest option).

A soldering iron and a desoldering station may become handy as well.

Check removable parts outside the board

The first thing I do is to remove all socketed chips and test them outside the defective board. If socketed, the SID, ROMs and CIAs should be left out of the board during the rest of the procedure. They are not required and there is no point it risking anything.

Voltages and fuse check

While in the basic department, the next step is to check that the voltages and the fuse are okay. A C64 power supply provides two power inputs:

• 9V AC (when measured directly on the PSU connector, expect 10V AC
• 5V DC (expect somewhere between 5V and 5.3V on a normal, healthy PSU without load

You should check that power make it past the power switch, as the power switch is one of the parts that fails often due to dirt building up inside.

On the user port, 9V AC should be present between pins 10 and 11 and 5V DC should be available between pins 1 (GND) and 2 (+5V). If you miss either power rail on the user port, test direcly on the power switch to ensure that it is working. If not, the power switch or the power connector are bad.

Internal power rails

Provided that you got the expected power measurements at the userport, the next step is to check is that the power rails generated internally from 9V AC, are present on the board.

On the longboards, 9V AC is transformed into:

• 12V DC. In the service manual, the 9V/12V DC rail is referred to as VDD.
• 5V DC rail referred to as CAN or VCC.

On a shortboard, 9V AC is transformed into 9V DC by through a bridge rectifier and a transistor (Q5). The 5V rail for the A/V and clock circuits is taken directly from the external power supply.

On longboards, VDD and VCC are used by the VIC, SID and clock circuits only. Unlike the regular 5V rail (VDC), VDD and VCC are protected by the fuse, meaning that if the fuse dies, only the 9/12 VDD and 5 VCC will disappear.

If you don’t see VDD and VCC, check the fuse for continuity.

On a shortboard, a dead fuse will allow the machine to work, albeit without sound and the tape motor won’t run.

The illustrations provice guidance on voltage measurements. The dots indicate where to put the multimeter probes. Ground is many places on the board, so you don’t necessarily have to put the black probes on the edge. It can be the shield for the cartridge port or the RF modulator as well.

The image below indicates where to measure on a shortboard:

Video output?

So, voltages are good and the fuse is good. Now what?

When troubleshooting, I always connect the computer through my upscaler. As seen on the picture below, the upscaler generates an overlay showing source, input and output. Notably we see that it has detected a PAL input. This is an important finding, as the C64 needs a working VIC II, a working clock circuit / 8701 Clock generator and a working RF modulator to generate a PAL signal. Hence, the below indicates that all these are good. And with that information, we can more or less focus on the left side of the board, provided we are working on a longboard.

If you don’t have an upscaler with similar features, a CRT monitor may be just as useful as it should generate a white line to the far left of the screen, provided you have the entire picture visible. A white line means that you have a PAL sync signal.

If there is no PAL sync signal, the immediate suspects are the VIC II and the MOS 8701 if present. If they turn out to work, the crystal and the RF modulator are next in line.

Dead test

Having determining that the video compartment is working, next step is to insert either a Deadtest cartridge or a DesTestMax cartridge. As these cartridges run in hypermax mode, they only rely on very few things to work, most notably:

• The CPU
• The PLA
• The VIC II and the video section (which we already established to be working)
• The reset circuit

If your hypermax cartridge won’t run, the primary suspects are the PLA, the CPU or the reset circuit.

While especially the PLA is known to fail often, the same could be said about other parts. As seen below, both the SID and the CPU run extremely hot and may take down the address bus or the data bus.

If Dead test / DesTestMax runs, the CPU and PLA are likely working to some extent.

Reset circuit

Having ruled out the PLA and the CPU, you should check for a reset signal. The easy method is to put a logic probe on pin 40 of the CPU. It should stay low (~0V) for a couple of 100ms after switching on the power and then go high.

If this doesn’t happen, I would recommend replacing the 74LS06 at U8. Please keep in mind that this is a hex inverter, meaning that it changes the input from high to low and vice versa. This means that if you run the logic probe over each pair of input and output pins, you should see the signal getting reversed for those pins that are connected. If that doesn’t happen, it might have failed.

If, on the other hand, the reset signal is present, you should change your focus to RAM, logic, ROMs (particularly the kernal) and CIAs if still present on the board.

Beyond the basics

Let’s assume that you made it to the point where you have established that the video section is working. What next?

If DeadTest makes it to the point where it displays text on the screen, you should suspect the kernal or another ROM chip.

If DeadTest boots with white flashes, this indicates a data line problem, which may be caused by a bad RAM chip, but could be caused by any part communicating on the databus.

Quite often, this is where things turn complicated, and you may find yourself doing some trial and error trouble shooting.

In my experience, it pays off to start the rest of your search by looking at the brand of chips used on the particular board. If the RAM is mT branded, that immediately raises the suspecion that you have a RAM problem. If any of the logic chips are MOS-branded, that’s an qually good indicator of where to start.