Dead VIC-20

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Thingumybob
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Dead VIC-20

Post by Thingumybob »

I have recently purchased a VIC-20 that appears to be dead. It's a 324002-01. I've worked on C64, but this is the first VIC-20 I've ever even messed with. (The springless keyboard is kinda sad.)

My question is, will this thing generate a background without the ROM chips or the CIA chips? In other words,if I pull UAB1, UAB3, UD7, UC11, and UC12, will just the 6502 and VIC try to generate a blank screen?

Thanks
Last edited by Thingumybob on Sun Jun 20, 2021 4:29 pm, edited 1 time in total.


banman
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Re: Dead VIC-20

Post by banman »

Hi Thingumybob,

Let me check for you....

In the meantime do you have access to an oscilloscope or logic probe ?
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Re: Dead VIC-20

Post by banman »

Hi Thingumybob,


My mainboard is a cost reduced NTSC version. I believe however all the Vic 20 computers share these same ROM chips (or very similar to the point of being interchangeable with other versions as I have done with mine).

I think they're easier to get going than the C64.

Please excuse the white dots on the monitor screen I have a bad RF modulator.
IMG_20210621_171114.jpg
BASIC ROM UE11 out.

IMG_20210621_171543.jpg
BASIC ROM UE11, Kernal ROM UE12 out and Char ROM UD7 out.
IMG_20210621_171343.jpg
BASIC ROM UE11 and Kernal ROM UE12 out.
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Re: Dead VIC-20

Post by banman »

Hi Thingumybob,


Here is a chip layout very similar to the board I have. I shamelessly lifted it from this site......


https://lyonsden.net/wordpress_s/wp-con ... scaled.jpg

VIC20-Motherboard-scaled.jpg
My mainboard is a cost reduced NTSC version. I believe however all the Vic 20 computers share these same ROM chips (or very similar to the point of being interchangeable with other versions as I have done with mine).

If you have access to an ocilloscope or a logic probe I found this following method for checking a possibility failed VIC display chip....

https://forums.overclockers.com.au/thre ... n.1220403/


It is important to take readings from the chip's legs AS WELL as soldered points on the mainboard. This will tell you if you have a dodgy socket or not. The factory sockets are renowned for becoming unreliable over time.

Basically I hooked up an oscilloscope to pin 38 and/or 39 which is a mainboard generated clock in. Check for activity. The clock signal generator is in fact a little rough looking. I believe it is within specifications though. I don't see why you could not use a logic probe for this. All one is checking for is activity.

Then check pin 35 for clock out from the 6560-101 VIC, in my instance. It will be the same for the PAL 6561-101.
If one gets a signal in but no signal out of the 6560/1-101 VIC chip it's fairly certain that the chip has died.



I hope this helps you out.
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eslapion
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Re: Dead VIC-20

Post by eslapion »

Thingumybob wrote: Sun Jun 20, 2021 4:25 pm My question is, will this thing generate a background without the ROM chips or the CIA chips? In other words,if I pull UAB1, UAB3, UD7, UC11, and UC12, will just the 6502 and VIC try to generate a blank screen?
As you can see on Banman's picture of a VIC-20 board, the CIAs on the VIC-20 are VIAs (6522 instead of 6526). Yes, the 6560/6561 should generate a blank screen if the clock signal it receives is good.

Perhaps this guide should help you put this board back in working operation: https://datassette.nyc3.cdn.digitalocea ... vic-20.pdf
Wealth, like happiness, is never attained directly. It comes as a by-product of providing a useful service. -Harland D. Sanders
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Re: Dead VIC-20

Post by Thingumybob »

Real quick here's what I know. It's not the CPU, ROMs, or CIA chips. The VIC was bad, but it's been replaced. Now I get a black screen with what could best be described as diagonal dashes. I'm using a 1701 CRT. I don't know how it would look on an LED monitor. Now I'm assuming there a problem with the RAM or bus drivers.
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Re: Dead VIC-20

Post by banman »

Hi Thingumybob,

Good to hear that your chips are ok.

Just something very random, maybe not.....


That monitor you are using. Am I correct in assuming that you are using a composite cable to make the connection between the Vic 20 and screen?

I think the Vic 20 has 2 composite video outs a high voltage and a low voltage.
If you're using a composite cable that worked (or made for) a C64 they are wired to the low voltage pin of the 8 pin DIN plug.

https://www.c64-wiki.com/wiki/A/V_Jack


Go down to the section marked "Pin Assignments".

I think the Vic 20's VHigh is the audio in of a C64.


That could be an issue...

I know that the composite cable that I made worked fine for the C64 with 8 pin DIN recepticle. However when I tried it with the Vic 20 on the very same TV you see in the above pictures there were problems.
It would only display a picture maybe 3 out of every 10 times I restarted the computer. It was fine when it did display though.

When I used the same TV with my very dodgy RF modulator the issue wasn't there. I suspect that the TV I have maybe needed a higher voltage level for the composite in signal.


I can only suggest at this stage trying to see if you have the cable arrangement I have to see if that could be a potential candidate.
Alternatively try power cycling the Vic 20 (leave it off for 10 seconds between retarts) many, many times and see what happens. Wait some time for the Vic 20 to initialise they take longer than the C64 computer.

I hope this helps you.
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Re: Dead VIC-20

Post by Thingumybob »

Thanks for the reply.

I know it's not an issue with the video cable. I have two similar VIC20s. One works fine. The other has no video. I am now waiting for a replacement VIC chip for the one that doesn't work. I'm not going to test it further with the VIC from the working unit. I also need to figure out how to source the RAM and bus driver chips. I'm not pulling anything that's not socketed out of the working unit.

I'll post what I find when the VICs come in.
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Re: Dead VIC-20

Post by banman »

Hi Thingumybob,


banman wrote: Fri Jun 25, 2021 9:06 pm
Alternatively try power cycling the Vic 20 (leave it off for 10 seconds between retarts) . :lol:
LOL!
That should have been restarts not retarts...... :lol:


The Vic 20's are much tougher than the C64's and one should be able to get the parts easier as well.



I would like to know how you go with your repairs. Good luck. :D
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Re: Dead VIC-20

Post by eslapion »

Thingumybob wrote: Fri Jun 25, 2021 7:05 pm Real quick here's what I know. It's not the CPU, ROMs, or CIA chips.
Perhaps I'm being redundant here... there is no such thing as a CIA chip in a VIC-20. There is a pair of MOS 6522 (the C64 uses the 6526) and they are called VIA chips.

Also unlike the C64 CIA the VIA is still in production and you can get new ones from Mouser with part # W65C22N6TPG-14 at a price of about 10$US each.

If you used C64 CIA chips on your VIC-20 then that's probably why it doesn't work.
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