Re: VIC-II Sync & Luma signal

From: Rob Clarke <>
Date: Sun, 03 Feb 2013 22:58:39 +0100
Message-ID: <>
On 03/02/2013 20:41, Gerrit Heitsch wrote:
> On 02/03/2013 07:58 PM, Rob Clarke wrote:
>> Hmm, I assume the pullup is in the modulator on this board. I just
>> powered it on without the vic present and there's nothing pulling it up
>> so that could be the problem.
> Maybe the diode died?
It was the modulator. At some point in the past someone tried to remove 
the modulator with brute force. I can see the screwdriver marks on the 
board where the modulator sat. Although they gave up before removing it, 
they broke the traces on the underside of the modulator where the pins 
go through to the MB.
>>> - Does the board contain any MOS-TTLs?
>> yes, it has a 7711 instead of a 74ls139, a 7712 instead of 74ls08 and
>> 7708's instead of the dram mux's. Why do you ask?
> Because the MOS-TTLs are known to fail. Quite often when I come about 
> a dead board, all I need to do is to replace the 77xx with the proper 
> 74LSxxx and I have a working C64 again.
> They also fail in interesting ways...
Well, after replacing the modulator, the machine runs again but after a 
few minutes the characters started breaking up. I suspected the 74ls258, 
which was in fact a MOS 7709, which I hadn't spotted before. Spookily, 
it corrects itself if I bring my finger within a cm or so of the chip. 
Take my finger away, characters break up again. Replaced it and voila - 

I would say that supports Gerrit's statement of failing in interesting 
ways. :-)


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Received on 2013-02-03 22:00:46

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