I'm working on a friend's barn-find Varkon (yes, you read that correctly) and it's given me a couple of challenges that I've never encountered before. I figured I'd share with the group for future reference.

- I rebuilt the power supply, but the 5v would crash every time I plugged in any board. In other words, any load on the power supply at all resulted in the 5v plummeting instantly to 0vdc. Also, when watching the meter upon power up, I could see that the 5v was taking a solid 10 seconds to climb up to 5v, rather than coming on instantly. Eventually, I realized that the .12ohm, 5 watt resistor at R13 was broken clean off and I didn't even realize it. Added a new resistor and we're back in business.

- The MPU was giving me serious gas. I replaced the battery with a coin cell and holder. Pulled the roms and installed Leon's test rom. But I could not get ANYTHING on the LED's or the 7-segment display at all. It looked like the board was totally dead, both on the bench and in the game. I was getting solid 5v in the appropriate places, but there was definitely something wrong. Research indicated that the 7447 at IC34 or the 7400 at IC33 or the 6821 PIA at IC18 were to blame. I pulled each of them and tested them in a chip tester, but they tested good. In retrospect, I should have tested their inputs/outputs before pulling them, but I didn't. I replaced them even though they tested fine (with added sockets). I was getting 5v on all the output pins of IC34, and after doing some more research I realized that this would result in the the display being blank, since the segments light when the output lines are held low. At this point, I was stumped, but looking over the schematic I could see that the 7404 at IC7 was connected to the line that runs between PIA IC18 and IC33, which ultimately connects to IC34 and then onto the display. There was no activity on this line at all, so on a hunch I applied 5v to pin one of the 7404 to see if it would light the display (by holding the line high, which would turn the output lines of IC34 low) and it totally got the display to light. Surprisingly, after I removed the 5v from pin 1, the display continued to stay lit! WTF? Then I noticed the 7404 stated to get very HOT and I knew I had found my problem. I pulled the 7404 and it tested BAD. Replaced the 7404 and BAM! The display was now flashing in sequence, which is exactly what it should do with Leon's test rom installed. Elation ensued.

I am far from an electronics expert, and I don't really understand the internal workings of integrated circuits, but if I screw around enough I can usually figure things out. If I can do it, trust me, anyone can do it.