Older Model Vic-20: Glitchy Graphics

Goro123 · Post by **Goro123** » Sun Apr 05, 2009 9:58 pm

Hi there!

I'm new to the forum and have a question regarding one of my VIC-20 computers. The one I like to use is an older model, and everytime I start it up I get crazy looking letters and characters all over the screen and in different colours as well. The background colour is still the cyan colour it's supposed to be.

When I shut down and power back on, the characters appear again in the EXACT same locations on screen. It never changes. Any ideas on what it could be and how it can be fixed?

Thanks for reading.

Bacon · Post by **Bacon** » Sun Apr 05, 2009 11:25 pm

According to Ray Carlsen's VIC-20 Diagnostics and Repair article there's a good chance it's the VIC chip (6560 if you're in NTSC-land, 6561 for PAL systems). If so, the only remedy is to get a new VIC chip.

6502dude · Post by **6502dude** » Mon Apr 06, 2009 6:33 am

This could also be a ram problem

Goro123 · Post by **Goro123** » Mon Apr 06, 2009 11:03 am

Done some further tests and it appears that game carts also do not work (still has garbage text around but it changes depending on the cart inserted). I'm thinking it's a RAM problem, too...

Since I'm here, I might as well post another Vic-20 problem I have (newer model). I use the same RF Modulator and same hook-up on the older Vic (from above post) for the newer one, and I can't get anything on the screen. Just lots of horizontal white bars and lots of buzzing noises coming from the TV. Oncve and a while I'll see a bit of colour flash across, it's like it's trying to work but it's being interfered with. It's weird because the same connection works fine with the older VIC that produces garbage text.

I was thinking this newer one may have a problem with the Vic Chip...

shem · Post by **shem** » Mon Apr 06, 2009 11:18 am

hey goro where in the world are you located???

Goro123 · Post by **Goro123** » Mon Apr 06, 2009 1:09 pm

Whoops! All this time I've been talking, forgot to mention location...

I have NTSC versions, not PAL. I live in Canada, near Toronto (driving distance).

Sorry about that.

channelmaniac · Post by **channelmaniac** » Mon Apr 06, 2009 3:37 pm

Sounds like you need to cobble together a test cart.

That test cart has some RAM of it's own in it that it uses to keep track of the test results and scratchpad info just in case the RAM on your system is hosed.

RJ

Goro123 · Post by **Goro123** » Mon Apr 06, 2009 8:00 pm

Bacon wrote:According to Ray Carlsen's VIC-20 Diagnostics and Repair article there's a good chance it's the VIC chip (6560 if you're in NTSC-land, 6561 for PAL systems). If so, the only remedy is to get a new VIC chip.

Just replaced half an hour ago. No difference. I'm wondering if it's this:

UB6 LM555 TIMER POWER-ON SYSTEM RESET
Will not reset on power up. May produce garbled or random screen
characters-graphics, or screen may start up "frozen". If drive resets when
computer is powered up, this chip is OK.

ral-clan · Post by **ral-clan** » Mon Apr 06, 2009 8:44 pm

The 555 chip is cheap and common, so it's worth swapping it to see if it's the cause of the problem.

Goro123 · Post by **Goro123** » Sat Apr 11, 2009 7:03 pm

OK, so I've got the soldering iron out and going to do some chip removal.

A question though: Can parts from the 2 different models (old and new) work in each other? Particularily RAM chips?

As you may know based on my other thread, I have 2 Vics down and want to get at least one up and running. Right now, my older model VIC can get a picture, so I'm going to perform surgery and take parts from my newer Vic.

6502dude · Post by **6502dude** » Sun Apr 12, 2009 8:33 am

ral-clan wrote:The 555 chip is cheap and common, so it's worth swapping it to see if it's the cause of the problem.

The 555 is a pretty durable chip. If a reset problem exists, I would suspect 10uf timing capacitor rather than the 555.