Apple II Plus color killer circuit

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Apple II Plus color killer circuit

I recently got my old Apple II Plus going with help from a member with a spare MM5740AAE/N.

Anyway, I noticed an odd thing with the color killer circuit.

It is "on" when booted (black/white only).
You can run something in color, like just "GR" from BASIC. Color looks fine.
Try to turn color off, like "TEXT" from BASIC. Color remains on.

Color mode won't turn off until I power the machine back off. The color killer is never re-engaged.

Easy fix?

EDIT: I think this may be a tuning issue between my monitor (Sony PVM) and this early board (Rev 4). My other Apple II+ will switch this monitor in and out, but not this one. I can power off the display and back on, and it will be black and white again. So the CK is working, just not "enough".

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Joined: Jun 5 2008 - 07:26
Re: Apple II Plus color killer circuit

The "color killer' doesn't completely eliminate the color burst part of the NTSC signal and some monitors can still lock on the attenuated color burst. A fix would be to stop the color burst completely with a logic gate before it's added to the rest of the NTSC output.

regards,
Mike W.

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Last seen: 2 years 7 months ago
Joined: Apr 10 2006 - 20:01
Re: Apple II Plus color killer circuit

rittwage wrote:

EDIT: I think this may be a tuning issue between my monitor (Sony PVM) and this early board (Rev 4). My other Apple II+ will switch this monitor in and out, but not this one. I can power off the display and back on, and it will be black and white again. So the CK is working, just not "enough".

I see this same behavior with some older rev mobos and my LCD monitors as well. It is definitely a tuning issue, as interrupting the signal and reconnecting (unplug/plug the RCA cable, power-cycle either the computer or the monitor) can fix it.

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Last seen: 35 min 55 sec ago
Joined: Dec 19 2008 - 21:01
Re: Apple II Plus color killer circuit

Thanks guys- good to know there is nothing "wrong" with this early board. I think my other is an RFI board, but this one is earlier. Still only a "plus".

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