VIC-II Sync & Luma signal

From: Rob Clarke <>
Date: Sun, 03 Feb 2013 17:41:13 +0100
Message-ID: <>

As there is so many people on this list who are very familiar with the 
internals of the VIC-II, I hope someone can help me. I have 250425 C64 
board in which the VIC-II does not generate a sync and luma signal, 
hence no picture at all. It's not the chip itself as it works fine in 
other boards and I have tried several different VIC-II revisions with 
the same result.

The dot, colour, phi 0 and phi2 clocks all seem good and I have verified 
that the machine basically runs. I burnt a kernal that does nothing but 
initialise the VIC registers and then sequentially reads the address 
space so I can verify all the /CS signals, which all check out fine, 
including the VIC's /CS.

I wondered whether to replace all the caps on the board but the supplies 
seem free of excessive ripple, the 12v & 5v have about +/- 0.1v but at a 
very high frequency.

I then thought maybe something in the modulator was holding it low so I 
bent the pin out of the socket but it remains firmly low.

What is the minimum requirements a VIC-II needs to generate a sync 
signal? On other boards I can power it up with no cpu and I get still 
see a sync pulse, so I'm stumped as to what the problem could be. Any 
ideas guys?

cheers, Rob

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Received on 2013-02-03 18:00:03

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