Denial

I have been working on restoring a heavily rusted VIC-20 ASSY 324003 computer. Initially the computer would not boot at all, there was a lot of rust on the motherboard.



Started by doing a deep cleaning with deoxid agent, baking soda and IPA to remove the rust as much as possible. There were some ICs that had pins very rusted, particularly those on the left bottom corner. So I went and decided to replace all the sockets and socketed many ICs to be able to test them. Unfortunately during this process, the two ICs on the bottom left got some of the pins loose because they were heavily rusted.

I ended up with a fairly clean and derusted motherboard:



All socketed ICs were tested on a working VIC 20. Fortunately none of them seem to have failed.

However, after all this work, I am now getting a black screen once the computer is powered on.

I have purchased a diagnostics cart on ebay... not sure if these carts work similarly to the C64 dead test card which will allow the computer to boot with fewer ICs (No ROMs) or other non-essential ICs. However, with the cart the behavior is the same.

There are scope captures from some of the PINs on the CPU and the VIC chips:

CPU:

• 6502 PIN 2:

• 6502 PIN 3:

• 6502 PIN 4:

• 6502 PIN 6:

• 6502 PIN 7:

• 6502 PIN 34:

• 6502 PIN 37:

• 6502 PIN 39:

VIC:

• 6560 PIN 2 (Chroma):

• 6560 PIN 3 (Luma/Sync):

• 6560 PIN 4 (R/W):

• 6560 PIN 35:

• 6560 PIN 36:

• 6560 PIN 37:

• 6560 PIN 38:

• 6560 PIN 39:

On the video output side, these are scope captures from the 5 PIN din connector:

• PIN 1: 5.16 VDC

• PIN 2: GND

• PIN 4:

• PIN 5:

Not sure where to take it from here, any advice or suggestions would be greatly appreciated.

"I ended up with a faily clean and derusted motherboard"
Probably some tracks gone etched away
BR

Using the schematics I traced each and every trace in the motherboard (or at least I think did) and found a few issues which I repaired with budge wires. At this point, I do not think broken traces may be the issue.

Any of the RAM chips hotter than the others?

beamrider wrote: ↑Fri Jul 16, 2021 4:42 pm Any of the RAM chips hotter than the others?

Follows an image of the MB after 1hr being turned on and the temps of the different ICs taken with an infrared thermometer.

The temperatures of UC2..UC5 look a bit high, but not exceedingly high.

Do you get a reset pulse from the 555 timer UB6? Pin 40 on the 6502 should be low for a fraction of a second after power on and then assume high level. If it remains low, the CPU is held in infinite reset.

Mike wrote: ↑Sat Jul 17, 2021 12:51 am The temperatures of UC2..UC5 look a bit high, but not exceedingly high.

Do you get a reset pulse from the 555 timer UB6? Pin 40 on the 6502 should be low for a fraction of a second after power on and then assume high level. If it remains low, the CPU is held in infinite reset.

Well... pin 40 remains low for about 5 seconds after which it goes to high and remains there until I power off the computer.

Mike wrote: ↑Sat Jul 17, 2021 12:51 am The temperatures of UC2..UC5 look a bit high, but not exceedingly high.

Ok, I went and socketed all RAM ICs. Now wondering how can I test them?

Do you get the same power on behaviour if you ground the reset pin?

Is the cart you have a diagnostics - I don't think that's the same as a dead-test?

You could try something like this..

https://github.com/StormTrooper/Vic20-dead-test

[edit - I don't think that's a proper dead-test cart as it relies on having a working screen], not sure if there's an equivalent of the C64 one that has its own RAM and flashes according to the detected faulty ram module?

Or I believe you can make/buy a cheap Arduino based SRAM tester.
https://myretrostore.co.uk/product/2114-tester/

Of course it's easiest if you can put the chips into a known working Vic-20.

Oh and I forgot to ask, have you tried any cartridge games to see if they work?

++ Another thing you can do to see if it's just a missing display or not is to power on the computer. Wait 10 seconds or so then press shift and run-stop to load a program from cassette. Then press play on the Datasette (the cassette motor should start). Then after a couple of seconds, press run-stop and then observe if the cassette motor stops. If it does stop then you know the computer is basically working and just not outputting a display.

Thanks for all these suggestions.. I guess I will go and get a SRAM tester, as switching them to the working VIC-20 would mean to desolder -at least- one SRAM IC from it.. as this is fully working, I would rather not risk damaging it.

I'm in the same boat but with a C64. It's a blank screen fault, but works with the Radar rat race cartridge. It can get quite expensive ordering all the diagnostic equipment and parts, not really worth it for one computer cheaper to just buy another working one but I quite enjoy the process.

beamrider wrote: ↑Mon Jul 19, 2021 2:25 am You could try something like this..

https://github.com/StormTrooper/Vic20-dead-test

I've got this... https://www.ebay.com/itm/264609006579?h ... Swq15eKhWe Which I believe is not a dead test.

beamrider wrote: ↑Mon Jul 19, 2021 2:25 am ++ Another thing you can do to see if it's just a missing display or not is to power on the computer. Wait 10 seconds or so then press shift and run-stop to load a program from cassette. Then press play on the Datasette (the cassette motor should start). Then after a couple of seconds, press run-stop and then observe if the cassette motor stops. If it does stop then you know the computer is basically working and just not outputting a display.

I tried this.. unfortunately, Datasette motor did not started.

So, as I mentioned before, I ended up removing all SRAM ICs, checked traces for continuity and fixed the broken ones found. Also replaced the ICs with "new" ones (as I have no tester I cannot tell whether the replacement ICs are indeed operational. However, all these replacements had no rusted pins.



Detail of the motherboard budge wires fixing some broken paths found:

I don't have any more ideas I'm afraid.

Have you covered most of the steps in this video and looked at Ray Carlson's page?

https://youtu.be/_rjjCkcXknE?t=280

http://personalpages.tds.net/~rcarlsen/cbm/vic20/

http://personalpages.tds.net/~rcarlsen/ ... /vic20.txt