Dead VIC
Posted: Sun Jul 08, 2012 10:56 am
Hello all,
I (like so many others, I see) have a dead VIC-20 which I would like to bring to life. I'd like some specific suggestions which I'll list below, but for starters, what I know is this:
This is an older (2 prong) NTSC system. The power brick tests ok. The rating on the brick states 10vac, and my multimeter shows a nominal and stable reading of 10.58vac.
All socketed chips test ok via the swap method with another VIC: the ROMs (chargen, BASIC and Kernal) are all good, as is the VIC, and the 6502. There was indeed a bad 6522 (UAB1), but I replaced it with a known good.
On power on, I see the monitor do "something", going all black, but never getting to the welcome screen. With a cart (Jupiter Lander), it is still dead.
I looked at Ray Carlsen's VIC-20 troubleshooting tips, and am narrowing it down to either the 555, as perhaps it is not resetting; or the 7402.
I do have a scope, but I can't see to find any timing diagrams handy, to know what a reset would look like on the 555, or what the clock would be on the 7402.
I've ordered a handful of 555's, simply because they're cheap and I don't have any spares, and will start with that. The 7402 is odd. Can one substitute a 7402 with an 74LS02?
I acquired this machine from someone who indicated it did not work, and it appears there was some previous attempt at repairing, altho nothing major. No cracked solder joints or scratched traces.
Thanks for reading.
I (like so many others, I see) have a dead VIC-20 which I would like to bring to life. I'd like some specific suggestions which I'll list below, but for starters, what I know is this:
This is an older (2 prong) NTSC system. The power brick tests ok. The rating on the brick states 10vac, and my multimeter shows a nominal and stable reading of 10.58vac.
All socketed chips test ok via the swap method with another VIC: the ROMs (chargen, BASIC and Kernal) are all good, as is the VIC, and the 6502. There was indeed a bad 6522 (UAB1), but I replaced it with a known good.
On power on, I see the monitor do "something", going all black, but never getting to the welcome screen. With a cart (Jupiter Lander), it is still dead.
I looked at Ray Carlsen's VIC-20 troubleshooting tips, and am narrowing it down to either the 555, as perhaps it is not resetting; or the 7402.
I do have a scope, but I can't see to find any timing diagrams handy, to know what a reset would look like on the 555, or what the clock would be on the 7402.
I've ordered a handful of 555's, simply because they're cheap and I don't have any spares, and will start with that. The 7402 is odd. Can one substitute a 7402 with an 74LS02?
I acquired this machine from someone who indicated it did not work, and it appears there was some previous attempt at repairing, altho nothing major. No cracked solder joints or scratched traces.
Thanks for reading.