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Vic20 text character problem

H

Hutch

Experienced Member
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Jul 1, 2018
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311
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California
So I pulled out my Vic 20 from the basement storage today for the first time in at least 15 years.

Purchased in 1982 or '83, It has an original 11.5vac power brick and the two pin power connector with the internal v-regulator heat sinks.
It starts up with a good picture. It responds to keyboard input and runs a simple basic program that I type in but all of the characters shown on screen are ampersand (@)
I have re-seated all of the socketed chips but it's still only showing @ (see my video of the problem below)
I was hoping someone here might have seen this before and have a solution or some troubleshooting tips.
Thanks


It looks like a problem with the character rom UD7 but in all other examples I've seen it showed random characters, not all the same one.
 
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Hutch said:
So I pulled out my Vic 20 from the basement storage today for the first time in at least 15 years.

Purchased in 1982 or '83, It has an original 11.5vac power brick and the two pin power connector with the internal v-regulator heat sinks.
It starts up with a good picture. It responds to keyboard input and runs a simple basic program that I type in but all of the characters shown on screen are ampersand (@)
I have re-seated all of the socketed chips but it's still only showing @ (see my video of the problem below)
I was hoping someone here might have seen this before and have a solution or some troubleshooting tips.
Thanks

It looks like a problem with the character rom UD7 but in all other examples I've seen it showed random characters, not all the same one.
Click to expand...

It looks either the chargen ROM gets always the same address or it's outputting always the same character. If the UC4 and UD3 address buffers were bad, I doubt the machine would boot at all, so I'd first test the chargen ROM out of the computer with an eprom programmer or similar.

Frank
 
poke 36879,8 reveals white @ symbols all over the screen, just changing colors when text is written.
for i=7680 to 8191:poke i,42:next doesn't change anything. Still just @ symbols

No eprom programmer available. I have an old O-Scope and can dust off to see what the address lines are doing.
I can order a replacement character rom but I want to be sure it's the chip first.
 
Hutch said:
poke 36879,8 reveals white @ symbols all over the screen, just changing colors when text is written.
for i=7680 to 8191:poke i,42:next doesn't change anything. Still just @ symbols

No eprom programmer available. I have an old O-Scope and can dust off to see what the address lines are doing.
I can order a replacement character rom but I want to be sure it's the chip first.
Click to expand...

You could check the address lines at the ROM but you need to trigger on both VA12 and VA13 low to be sure the ROM is really addressed.
The only way to know if that ROM is good is reading it outside that ViC-20 (maybe in another known good one) or replace it with a known good one (a 2532 EPROM with the right content is enough).

Frank
 
So tonight this happend...
It worked for a while and I was able to try loading some programs from old tapes, but the tapes didn't hold up well over the years.
Then the screen changed back to @ symbols just as quickly.
It seems to consistently start working when it warms up and stop working when it gets too hot.
Based on how the characters change back to normal, it seems to be the character rom, so I have a replacement on order.
 
Hutch said:
So tonight this happend...

It worked for a while and I was able to try loading some programs from old tapes, but the tapes didn't hold up well over the years.
Then the screen changed back to @ symbols just as quickly.
It seems to consistently start working when it warms up and stop working when it gets too hot.
Based on how the characters change back to normal, it seems to be the character rom, so I have a replacement on order.
Click to expand...

If it's temperature based, you better check the chips with a compressed air can (shot air to the chips to chill them and see if it starts working).
Also, if the ROM is socketed, try to clean its pins and reseat it into the socket

Frank
 
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Character rom didn't solve it.
Chip pins are clean and polished.
Chilling chips doesn't change anything. Maybe bad Vic or bad caps.?
 
Was able to test it today with a game cartridge.
Even in graphics mode, it's still a screen full of @ with only color changes.

Also, when the text stops working, I can get it to work by cooling the VIC chip. Even when text is working fine, graphics mode still shows only @
 
Got a replacement VIC chip and that solved the problem.

Does anyone know what the normal operating temp is for the Vic chip? I'm getting ~65c on the surface from an IR temp sensor after it's been on for ~20 minutes.
 
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Hutch said:
Why? Is 65c too high? 65 doesn't seem that high and is a perfectly safe temp for modern chips.
What's the typical surface temp of the Vic chip?

Anyone? ...
Click to expand...

I didn't mean the new one is unlucky, it was the previous one the unlucky one.
If the VIC circuit isn't enclosed in the metallic box and if the box cover doesn't have a tab that makes thermal contact with the VIC chip, by all means, put a heatsink on your VIC chip.
I heatsinked my VIC-20 chip, 65C on the surface means the internal chip is probably around 100C already.
Frank
 
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