Early VIC-20 motherboard repair

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rgrocha
Vic 20 Amateur
Posts: 55
Joined: Fri Dec 06, 2013 7:19 pm

Early VIC-20 motherboard repair

Post by rgrocha »

Hi,

I've been trying to repair an early VIC-20 model, with the same motherboard as the VIC-1001, with no luck.

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I've already checked the chips identified in the Carlsen guide as blank screen culprits. All of them are ok or replaced with working ones.

One special feature of this motherboard is a tiny piggyback near the VIC chip. Take a look at the pics:

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And with the piggyback desoldered:

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(you can see it here with another VIC chip, I don't want to risk the original one on the tests)

What does this little board does? the motherboard pictured in the Giacomo book does not have it and instead has some other passive components soldered in it's place.

Any clue or guide on what should I check now?
I can take new pictures of the motherboard or screen if needed.

Thank you!
Roberto
norm8332
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Joined: Sun Nov 13, 2016 11:04 am
Location: USA

Re: Early VIC-20 motherboard repair

Post by norm8332 »

Not sure about the board, maybe an output amp. I would break out a scope and start checking for clock, reset and data on the bus.
“In religion and politics people’s beliefs and convictions are in almost every case gotten at second-hand, and without examination... whose opinions about them were not worth a brass farthing.”

-Autobiography of Mark Twain
callan
Vic 20 Newbie
Posts: 16
Joined: Fri Oct 05, 2012 1:02 am

Re: Early VIC-20 motherboard repair

Post by callan »

I'm getting 404's (link broken) on all those images.
General path I follow when troubleshooting black screen failures:
Start with voltage rails, then RESET pulse and clock.
Then confirm ROM access.
Check I/O then RAM access.
Once they're working as expected, look back to VIC, and forwards into video output circuitry. (I've had a C64 blank screen caused by a failed RF modulator before!)
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