VIC 20 Repairs

[Phu]

Hi,

I'm hoping someone with some guruish experience can give me a few pointers on where to look to fix an ailing VIC 20.

It is a PAL version, later issue board (with the 7-pin DIN power socket). The machine powers up, 9VAC is present and correct as is 5VDC, which seems to be getting to all the right places (including the LED).

However, all I get is a blank screen, with patterning; not actual graphics just bands that make the blank slighter lighter (barely noticeable).

I've put the oscilliscope in on a few places, and the 6502 has been replaced with a working part. It is receiving a valid clock.

Likewise, the VIC (6561-101) chip is receiving a clock input from the 4.433Mhz clock circuit. It appears to be outputting a sync signal on pin 3, and barely anything on pin 2. (these pins are connected to the composite output). I can also see addresses being generated on the address pins, and data appearing on the data pins.

The machine is connected to a TV using composite video via an adaption lead to the 5-pin DIN socket. The TV is confirmed working with other sources, etc.

On suggestion from a fault-finding document, I've removed both VIAs and the character generator ROM to no effect.

Thats all the information I have so far. If somebody has a flash of inspiration of what else I can prod around, I'd be grateful :)

Thanks

-- Richard

[tezza]

Richard, do you have this document (http://personalpages.tds.net/~rcarlsen/cbm/vic20.txt)? It helped me to repair mine.

Tez

[Phu]

Richard, do you have this document (http://personalpages.tds.net/~rcarlsen/cbm/vic20.txt)? It helped me to repair mine.

Tez

Yup. Thats where I read to remove the VIAs and CG ROM.

Can't seem to put my finger on where the fault is. I'm hoping its not the VIC chip itself, as that would be a pain to replace. It seems to be doing its job, just no discernible video output.

-- Richard

[Druid6900]

It has been my (considerable) experience that, when dealing with computers, with Murphy's Law in full effect, that it's almost ALWAYS a chip NOT in a socket that is the problem.

GOS (Garbage On Screen) is usually (I'd venture 90%) a RAM related problem, again, in my experience.

[tezza]

Yes, even using that document I had to replace quite a few ICs (http://www.classic-computers.org.nz/blog/2009-02-20-bringing-a-vic-20-back-from-the-dead.htm)(non-socketed) before my Vic-20 was fixed.

Tez

[Phu]

Well, after a fun-filled weekend of hot air guns and soldering irons, I have replaced almost every chip. The only parts left are the VIC, the 74LS133 13-input NAND gate and the BASIC ROM. Everything else has either been replaced or removed for testing purposes.

Still a blank screen, although there appears to be *something* happening. About 1.5 secs after switch-on, there's a glitch in the black screen, like something being initialised. But then the blank screen persists.

Its going to be the VIC, isn't it?

-- Richard

[tezza]

Well, after a fun-filled weekend of hot air guns and soldering irons, I have replaced almost every chip. The only parts left are the VIC, the 74LS133 13-input NAND gate and the BASIC ROM. Everything else has either been replaced or removed for testing purposes.

Still a blank screen, although there appears to be *something* happening. About 1.5 secs after switch-on, there's a glitch in the black screen, like something being initialised. But then the blank screen persists.

Its going to be the VIC, isn't it?

-- Richard

Oh dear. Yes, although Murphy's law (extended) would mean it is going to be the very last IC you replace. In fact, Murphy's law will say it probably won't even be that...it will be a dry solder joint, a damaged track, faulty video cable/connector or power supply. :)

Look on the bright side. You're going to have the best socketed Vic around...very useful for repairing next time :)

Tez