Re: VIC-II Sync & Luma signal

From: Gerrit Heitsch <>
Date: Sun, 03 Feb 2013 19:03:53 +0100
Message-ID: <>
On 02/03/2013 05:41 PM, Rob Clarke wrote:
> 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.

The luma pin needs a pullup to work. It will only sink current but not 
supply it. The datasheet specifies a 500 Ohm pullup. The usual circuit 
also contains a diode (1N4148 for example) between VIC and pullup.

> What is the minimum requirements a VIC-II needs to generate a sync
> signal?

It should do that once you supply power and clock. There will be no 
picture, but the sync signals should be there. Can be easily seen if you 
use a real CRT monitor. When you turn on an otherwise dead C64, you 
still get a twitch from the monitor once it locks onto the sync signal 
from VIC.

Also, on the 250425:

- +12V are present at Pin13 of the VIC?
- Clock signals are present (dot clock and color clock!)?
- Does the board contain any MOS-TTLs? (small ICs with MOS logo
   and a 77xx number)
- Modulator has power? Even if you use the video out, the modulator
   is needed to convert the video signals into Y/C and CVBS.


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Received on 2013-02-03 19:00:04

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