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Unfortunately, Clay's excellent site for System 80 repair has been taken down. However, there is a pinballwiki that has some good information. Be certain to perform all of the CPU tests. Only continue below if you are stuck (as I was).
The Alternative to the test EPROM.
A very handy thing to have for debugging 6502-based system is a NOP generator. As long as the processor has power and a clock, this should work.
The 6502 processor has an instruction set called NOP (No Operation). The NOP tells the processor to cycle through every one of the 65536 addresses. The end result is that the address bus will count in binary – each address line being the square wave at half the frequency of the previous address line (i.e. A0 will have the highest frequency, A1 will be half that, etc. all the way up to A15, the lowest frequency). These are best tested with a scope, but a digital probe can be used in a pinch. With a scope, all should be nice square waves.
Building the NOP generator is easy, as long as the board has an EPROM socket. First, get a 24-pin solder tail socket to plug into the EPROM socket. Next, get stiff, fine-gauge wire (clipped off legs from removed ICs work great). Push one wire into the new socket at pin 12 – this will be the ground wire. Push another wire into your new socket at pin 24, this will be the +5 wire.
Note: Do not use standard wire as that will 'spring' the IC socket and ruin it. Wire used must be the size of a standard IC chip leg - which is why I save those I clip off. If wires are not available, it is suggested inserting a IC socket of the same size into the existing socket. This second socket can be used as a permanent NOP replacement.
Push a wire into the new socket at each data line (pins 9, 10, 11, 13, 14, 15, 16 and 17 – eight wires and the +5 and ground wires).
The NOP instruction is EA in Hex. That means you are going to solder the wires from D7, D6, D5, D3 and D1 to the +5V wire. Then solder the wires from D4, D2 and D0 to the ground wire.
This means that pins 14, 11 and 9 will be grounded to pin12.
Pins 17, 16, 15, 13 and 10 will be hooked to +5 at pin 24.
Note that it is possible to assemble the NOP generator directly into the EPROM holder after the EPROM is removed. However, it is possible that the insertion of these wires into the EPROM holder will cause it to spring and ruin the holder. Plus, if this is assembled into a spare EPROM socket, it can be used over and over again. In the photo above, I used clipped off legs from removed ICs, so as to be careful not to ruin the IC holder.
After this is assembled, remove the EPROM from and insert your NOP generator into the EPROM socket. Insure that it is properly oriented and that none of the +5 wires are touching the ground wires.
Disconnect all plugs to the CPU board except for J1. Power up the machine.
If you are using a digital probe, the following pins on U1 (the 6502 processor) should be tested as pulsing:
The pulsing at pin 25 may be so slow that the individual pulses can be heard.
If all the addresses are pulsing, then check the other chips, U2, U3, U4, U5 and U6 to insure that the addresses at those chips are also pulsing. If the addresses are pulsing at the CPU, but not at one of these chips, then there is a broken trace.
Also check the address lines at Z10, Z12 and Z7 to insure they are not broken.
If one or more of these addresses are not pulsing, then either the CPU is defective, or one of the chips that are hooked up to the address are pulling it down. If you are lucky, your CPU is socketed. Then you could remove the CPU and bend the one lead out of the socket. But since these boards were manufactured with the CPU soldered in place, you are most likely out of luck.
The next best way to proceed is to take a sharp razor blade and temporarily cut the trace in such a way that it is easily repairable with solder. Check with a voltmeter to insure that the trace is cut and reinstall in the machine with all plugs removed except J1. Use the digital probe and see if the address at the CPU is now pulsing. If the CPU is now pulsing, then the CPU is good and one of the other chips is pulling down the line. If the CPU is not pulsing at this address, then it is likely defective and needs to be replaced.
If this address is not pulsing at the CPU, check the same address at one of the other chips. If you are using a digital probe, it may now appear to be pulsing, but that is likely a false reading. Take a 3.3k resistor and connect one side to the +5 volt supply, and the other to the address at one of the chips that now appears to be pulsing. The line as checked with a digital probe should now appear as high.
If after cutting the trace, the CPU is pulsing, then it will be necessary to determine which chip is at fault. This can best be done by repairing the trace previously cut at the CPU with solder (and maybe a thin wire). Then follow the trace from this address at the CPU and find where it splits. Cut one side of the trace and test again. Using this technique, it should be possible to narrow it down to the defective chip.