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VIC-20 Blank Flickering Screen

robert_sissco

robert_sissco

Experienced Member
Joined
Oct 12, 2021
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342
Location
Midwest U.S.A.
I did a Vicky-Twenty build, and when I turn it on I got no picture. So, noting that this is a possibility, I installed the optional capacitor on the back of the board, now when I turn it on I get a blank, flickering screen.

I have a MOS produced VIC-20 CR board with the 6502, VIC, ROMs and PIA's all socketed, so when I switched them between boards, they all worked in the known working board, and I get the same issues on the Vicky-Twenty build board with the chips from the known working board, so it is something on the board itself. I can set up and shoot a bit of video later today, but since the issue seems to be on the board itself, any idea on where to start looking?

I do not yet have a scope to check signals, but I go have a logic probe. Since the issue appears to be with the video, I assume it is somewhere with that circuitry, but I am not sure where to begin.
 
One thing to note is that per my meter I am getting 4.89 volts on the VIC chips in the Vicky-Twenty built, and 5.09 in the known working board. I am not sure if that is a difference, but I believe that .11v is within tolerance for the 6560 VIC chip. (This is a NTSC build). It also has the 14 mHz crystal installed, so that should not be an issue.
 
The flickering is occurring ever couple of seconds, thought that it might be the 555 not properly switching the Reset line, but the probe shows that that is good. Voltages are all just shy of th5 5v mark on the CPU, VIC and at the User Ports, so the voltages across the board are looking good.
I am not sure what else it could be
 
Hi Robert.

What kind of flickering is it? Just black screen to white screen then back to black?

I don't know much about the vicky-twenty design, but given everything you've checked so far, I'd check some of the 74LS logic, like UC2.
 
The Vicky-Twenty is a clone of the CR Vic-20 board from Commodore (uses the same din connector to the power supply as the C64) with some minor changes, like the voltage lines going to the User Port without the need of the jumper wires across the boards.

And it is a black screen with an occasional scrolling (bottom to top) yellowish light, like something is trying to display.

I will take a look at the logic chips to see if I can find anything, thank you for a starting direction.
 
Gotcha. Being a good clone of the CR makes the info you'll find on the web more useful. Obviously the diagnostic cartridge would apply as well if you have one.

The bottom to top yellowish light could just be a junk signal due to something floating. Not sure.

You can look at this thread where thanks to good advice from daver2, rittwage, and eight088 I recently fixed a VIC that was giving me a black screen on boot, due to bad RAM. It just presented in an unusual way: https://forum.vcfed.org/index.php?threads/vic-20-with-no-video-out.1241720/

The two things that have proven super useful to this type of troubleshooting are the oscilloscope and a NOP generator. You don't have a scope, but can still test some things like the reset line on the 6502 with the probe. Do you have something to send NOP commands? Also, let it run for a while and check for hot chips. if a chip fails with a short, it'll get super hot.

You're likely looking at RAM or logic. And that's freezing the system on boot.

To get some ideas of where to look on the board: read this one from Ray Carlsen: https://portcommodore.com/rcarlsen/cbm/vic20/vic20.txt
Find your schematic here, and also take a look at the technical manual: http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/vic20/

Post what you find, I'm looking forward to it and how you fix it!
 
I do not have anything to send the NOP signals. I did let it run for 10 minutes and none of the chips felt hot to the touch.

I do have the TFW8B's Penultimate cartridge, and when I try to boot to dead test nothing comes up, no flashes or anything, that is what is leading me to believe that it may be an issue with the video signal over RAM. And since the VIC works in my known working CR VIC-20, I can at least eliminate the $100 as the fault of it.

I also do not have a scope yet, that is next on my to get list, and I found a good $80 from Adrian's Digital Basement I am likely going to get, but until there that is a no-go.

I already did test the reset line on the 6502 so I know at least the 555 is working.

I am 95% sure I got the right logic chips in the right sockets (part of why I love building my own systems like this, EVERYTHING IN SOCKETS), but my eye sight it crap, and on my list as well are a pair of light up magnifying glasses al a ADB again, but I can usually make them out with a bit of effort. I did redownload the BOM from Retro8BitShop and will verify I got everything in the right spot.

Otherwise I may just order the logic chips again since they are rather cheap and some spare RAM for it and just start doing individual chip swaps until I get that Cyan and White screen I am eager to see
 
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I went ahead and put together a Mouser order for most of the logic chips to try swapping them out on the board. I will try swapping them out one at a tie and see which is the new member of my dead chips bin
 
OK, I got a separate Dead Test cart for the VIC-20 and still nothing, and the LED on the cart does come on. So I am thinking that the RAM may not be the cause here since this is tested. At this point I am sure it is something with the video, and I know the VIC works, I tested in a known working VIC-20, so that is not the issue.

I have ordered a complete set of new logic chips and swapped them all out one by one, and I am still getting no video.

I may need to buy a scope at this point (Adrian's Digital Basement had a few good economy models for around $100 I am looking at), but any ideas on what I can check in the meantime?
 
So, just an update, did a lot of online digging and I found a YouTube video where the creator showed how he built his Vicky-Twenty to be switchable between PAL and NTSC, and in it he noted that the 6560 NTSC VIC chip doesn't seem to like the 74LS02 in the UB9 slot, and works better with the 74F02, and he used a 74F02PC for that IC, so I ordered a few as well to test.

I will update again once I get them and have a chance to swap the chips around.

Here is the video if anyone is interested

 
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