I recently had the chance to repair an NTSC VIC-20 that would not boot and just show a black screen.
The version I got to work on has a power brick containing a transformer. That power brick converts mains input to 9 volts AC, which then is fed directly into the computer. The rest of the (linear) power supply circuitry is inside the computer, including a very large capacitor. This capacitor wasn’t bulging but showed some signs of electrolyte leakage but that was not the cause of the problem. In fact I chose to skip replacing the capacitor for now.
In order to diagnose the problem I first checked the voltages, which all checked out perfectly.
Then I used an oscilloscope to check what the VIC and the CPU were doing.
The main clock signal is generated by the VIC chip from the output of a 14.31818 MHz crystal. Everything looked perfect in this area.
I decided to reseat all the socketed IC chips (CPU, ROMs, IO chips — I was not able to extract the VIC chip even though it was socketed), but that did not help.
The CPU has a reset pin which is held low for a few seconds and then goes high, this worked perfectly too. There is a tiny 555 IC placed on the board responsible for doing this.
However there was no activity on the address lines at all; it seemed like all address lines were held high. (There is a possibility that some lines (A0~A3) were low, maybe I did not check carefully enough.) However, occasionally, right after clicking the power switch there seemed to be some normal-looking activity on the CPU’s address lines, which very quickly faded away. This happened maybe once in 10 or 20 power cycles.
So then my first suspicion was that the 6502 CPU might have given up. Thankfully, I had access to another board using a 6502 CPU. (Actually this one was a 65CS02 CPU.) As the voltages looked normal it seemed very low risk to swap the CPUs to see what would happen. Much to my dismay at first, the 6502 CPU extracted from the VIC-20 worked on the other board.
However, I was dismayed only for a few seconds, as the 65CS02 CPU, when put in the VIC-20, didn’t quite make the computer work but I was able to see a lot of activity on the address lines of the CPU now!
The new theory was that the IC pins were much more oxidized than expected. We extracted all the ICs (including VIC) again and gave them a clean up with concentrated alcohol. And it still did not work! However on the oscilloscope most pins now showed normal activity.
Thinking there might be another problem, perhaps with the ROMs, I decided to insert a game cartridge into the cartridge port.
And it booted up!
Okay, is the BASIC ROM busted?
Well. After turning the computer off and taking out the cartridge and turning it on again, it would successfully boot into BASIC! What the?!
I thought that simply reseating ICs would immediately take care of most “bad contact” problems, at least temporarily. Well, turns out that oxidation can be pretty serious sometimes!
We had even checked continuity between IC pins and the socket’s pins on the other side of the board, and got beeps as normal.
So it seems this is not a very good test. Well, today (yesterday actually) I learned.
Another note: the computer and power supply was made with 120V/60 Hz in mind according to the labels on the back, but it worked fine at 100V/50 Hz.