DrAM19
Experienced Member
Hello Folks,
I recently picked up a new VIC-20 that needs some work. When the VIC is powered on, my 1702 has a black screen. This is a 324003 rev board, so its an early model with the 2114 RAM.
I verified I have 5v and a good clock. I also reseated all socketed chips with some deoxit. I checked the 6502, the 6560, and the KERNAL ROM at UE12 in a working later model VIC, and it continued to work. I tried the good chips from the other VIC in the broken one, but that did not help.
Using an oscilloscope, I checked the reset pin on the 6502 and saw normal behavior. At the data lines at the 6502; there is a very consistent pulsing pattern on all of them. The address lines at the 6502 are all pulled high and there is no activity.
At seemingly random times, when I power it on, there will be activity on the address lines. Normal and consistent activity. However if I powercycle the system, it goes back to the high and inactive behavior. I cannot find a cause for this or repeat it at will. And of course, the random times when this behavior occurred, I was not hooked up to or near the 1702 to see if there was some kind of video out. It happened most often when I was removing a chip here or there, like a ROM or VIA. But again, it was not consistent or repeatable.
My next inclination is to desolder and socket the 65245s at UD8 and UE8, so I can isolate the RAM from the CPU. Maybe a stuck ROM is making the CPU lock up.
What's held me back from desoldering the 65245s is that even address lines CA 13, 14, 15 are stuck high and do not go to there; they only go to the 74LS138 deMUXes at UC5 and UC6.
Does anyone have another idea or theory about why those address lines are all stuck high? Or maybe the address line behavior is a red herring, and I should look elsewhere.
Thanks!
I recently picked up a new VIC-20 that needs some work. When the VIC is powered on, my 1702 has a black screen. This is a 324003 rev board, so its an early model with the 2114 RAM.
I verified I have 5v and a good clock. I also reseated all socketed chips with some deoxit. I checked the 6502, the 6560, and the KERNAL ROM at UE12 in a working later model VIC, and it continued to work. I tried the good chips from the other VIC in the broken one, but that did not help.
Using an oscilloscope, I checked the reset pin on the 6502 and saw normal behavior. At the data lines at the 6502; there is a very consistent pulsing pattern on all of them. The address lines at the 6502 are all pulled high and there is no activity.
At seemingly random times, when I power it on, there will be activity on the address lines. Normal and consistent activity. However if I powercycle the system, it goes back to the high and inactive behavior. I cannot find a cause for this or repeat it at will. And of course, the random times when this behavior occurred, I was not hooked up to or near the 1702 to see if there was some kind of video out. It happened most often when I was removing a chip here or there, like a ROM or VIA. But again, it was not consistent or repeatable.
My next inclination is to desolder and socket the 65245s at UD8 and UE8, so I can isolate the RAM from the CPU. Maybe a stuck ROM is making the CPU lock up.
What's held me back from desoldering the 65245s is that even address lines CA 13, 14, 15 are stuck high and do not go to there; they only go to the 74LS138 deMUXes at UC5 and UC6.
Does anyone have another idea or theory about why those address lines are all stuck high? Or maybe the address line behavior is a red herring, and I should look elsewhere.
Thanks!
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