Disabling the Color Killer circuit

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Joined: Oct 9 2011
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I haven't looked at the schematics yet, but is there an easy way to disable the color killer circuit on a II plus. It causes some issues with my projector when the color killer switches on/off where I have to I flip flop inputs for it to sync. It would be easier If I just disable it and live with multicolor text.

Thanks,
Corey

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speedyG's picture
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Joined: Nov 16 2011
Posts: 2376
Re: Disabling the Color Killer circuit

Hello Corey,
in the Apple II series there is no real "color killer"-signal... in fact the signal is just a mixture similar to the specs but not really identic... but the are 2 points to get influence to the signal:

at the end of the F and H row there is a trim capacitor called "color trim" it permits to take slight influence to the so calle "hue" ( i.e. Shifting-timing ) of the colorsignal and you might get a better cooperation with the projector by adjusting the signal there....

the second chance is the trimming resistor close to the back of the videojack... its responsible for the amplitude of the video signal.... if you lower the amplitude the colors get dumb and loose brilliance and it also reduced the difference between top-level and bottom-level of the signal and therefor reduces the peaks that are used for syncing the signal.... raising the signal enhances the peaks and the distance between top-level and bottom-level of the videosignal.

Any other solution would require you to buy a so called NTSC-decoder chip similar to the LM1889 which is a PAL-decoder chip for the european version, where you can input the apple videosignal and the chip splits/separates out the timing signals to extra pins. The according chip for the U.S. NTSC standard should be the TDA3562A chip from SGS Thompson. There is a datasheet availiable at:
http://html.alldatasheet.com/html-pdf/25060/STMICROELECTRONICS/TDA3562/1621/1/TDA3562.html

probably its a good idea to take a look here :
http://cds.linear.com/docs/Application%20Note/an57fa.pdf
this application note give a overview of such chips and the related standards.....
keep your eyemark to the conversion from composite NTSC to RGB ... that usualy needs the composite signal to be splitted up and the videosignals to be separated...

using that chips requires some developement work....

it might be better to try first if a solution is possible wuth the trimming-devices within the Apple....
good luck....
speedyG

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Joined: Jun 5 2008
Posts: 380
Re: Disabling the Color Killer circuit

Pull up the base of the transistor near location F-13 to +5 volts. That is the pin connected to the 4.7K resistor. You will want to do this with a pull up resistor. Easiest way would be to lift the other side of the 4.7K resistor and connect that to +5v. If you don't do this, when the input to the 4.7K gets driven low, the two resistors will act as a voltage divider - may be enough to prevent the color killer from kicking in, depending upon the value of the pull up you end up using.

Regards,
mIke Willegal

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Re: Disabling the Color Killer circuit

Thanks Mike, sounds simple enough to try.
I have some time on Monday I'll keep you posted.

Cheers,
Corey

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Joined: Nov 16 2011
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Re: Disabling the Color Killer circuit

Hello Mike,
thanks for the correction of my post above.... it turns out that my APPLE II must be a rev.0 due to the fact that Q6 is not present in my board.... after i read your reply I dragged out the circuitplans from SAMs from Winston D. Gayler and found out that in later revisions the transistor Q6 was added to the circuitary with the special function to shut off the color sync from the videosignal, if the display is generated from the textpages....

t least i have a question in this context related to that transistor Q6 used in the later revisions:
If the base of that transistor is tied up to "High" permanent, does this affect the videosignal too, when it is generated from the graphicpages... ? Up to the point i recognized from the circuitplans the transistor that is tied up to "High" just stops switching and behaves thereafter like beeing driven all the time from the softswitch for the display of textpages and also graphicpages behave like being monochrome ( i.e. no greyscale of graphics is possible - only black/white display enabled )..... Is that correct ?
sincerely
speedyG

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In early days I had a lot of money but no time - now I have no money but a lot of time....
the second part includes less friends but a lot more joy on life....

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Joined: Oct 9 2011
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Re: Disabling the Color Killer circuit

Speedy,

I nice discovery to find out you have a Rev'0', in my opinion the best of the Apple II boards!!! You should check out Mike's site about Rev '0' boards it's pretty interesting read.

Cheers,
Corey

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Joined: Jun 5 2008
Posts: 380
Re: Disabling the Color Killer circuit

Just checked schematics, which I should have done before. I had it backwards - grounding the base will prevent the color killer from kicking in. All this transistor does is, when switched on, is to prevent the color burst from getting added to the composite video signal. A bit leaks through, anyway, which some sensitive receivers can lock on. The rest of the signal isn't affected.

Regards, mike w