Dead VIC

azog · Post by **azog** » 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.

6502dude · Post by **6502dude** » Mon Jul 09, 2012 5:48 am

If you have a spare 6560, try swapping it.

7402 can be replaced with 74LS02

azog · Post by **azog** » Tue Jul 10, 2012 6:36 am

Yes, I've tried all the socketed chips, including the 6560 aka the VIC. I have a handful of 555's in shipment, I'll plan to try that first, and it's also easier to replace... I can hopefully dig up an LS02 in my pile as a backup plan.

Nathan · Post by **Nathan** » Tue Sep 25, 2012 1:33 pm

Replace all the chips!

azog · Post by **azog** » Tue Sep 25, 2012 2:14 pm

Nathan wrote:Replace all the chips!

To which he replied, quoting the original message:

azog wrote:Yes, I've tried all the socketed chips, including the 6560 aka the VIC. I have a handful of 555's in shipment, I'll plan to try that first, and it's also easier to replace... I can hopefully dig up an LS02 in my pile as a backup plan.

crock · Post by **crock** » Wed Sep 26, 2012 4:30 pm

Hi azog. I just repaired a very old Japanese VIC-1001 showing the exact same symptoms. In my case the electrolytic capacitors were bad including the big 4700uf filter cap and the 1uf cap on the reset circuit. You can test the 555/reset circuit by putting your scope on pin 40 of the 6502 and powering on the machine. You should see the pin go low for about 1 second and then go high. If this isn't happening, suspect the capacitor or the 7406 that inverts the signal before you swap the 555, as they very rarely fail.

Rob