Apple II plus colour problem in text mode

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Apple II plus colour problem in text mode

I’m getting back into the apple II game after 30 years away from it. I purchased a clone and a real apple about a year ago. The clone works fine. The real apple has an issue in text mode on a colour monitor (Commodore). In HGR mode, the text is crisp with the normal colour fringing, but in text mode, the colour is so overdriven that you can’t read the text. I disconnected the 4.7K resistor to Q6 (colour killer transistor) and the text is the same in text mode or HGR (which it should be). I jumped the hue adjust control (which should ground the colour burst signal) and the colour is overdriven the same as in text mode which makes me think that the colour killer circuit is fine. Any ideas why grounding the colour burst would overdrive the colour on the screen?

 

Edit, I disconnected the 2.7K resistor that feeds the colour burst to the video out transistor and still the colours are completely overdriven.

 

I've attached 3 scope pictures. The 2 with the shorter fatter pulse are from the clone (one in text and one in hgr mode where you can see the colourburst). The one with the skinnier taller pulse is from the real apple with the resistor removed that feeds the colourburst to the video transistor.

 

Thanks

John

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Maybe pictures of the monitor

Maybe pictures of the monitor would be more helpful. Have you tried adjusting the video LEVEL pot?

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Is this on a PAL system with

Is this on a PAL system with the colour card, or an NTSC system?

 

The video signal width is the first thing to check, as are the resistor values. (Test to see if the resistors are operating in spec.)

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jeffmazur wrote:Maybe
jeffmazur wrote:

Maybe pictures of the monitor would be more helpful. Have you tried adjusting the video LEVEL pot?

Yes I have. If you turn it down just to the point, it loses sync, the colour will dissapear.

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Here are some pictures of the
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This is an NTSC system. How

This is an NTSC system. How do you check the video signal width? I have a basic background in electronics, but video is greek to me.

The 3 resistors that feed the video transistor (R6,7,8 on my schematic) as well as R27 that feeds the colour killer transistor check fine.

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I've come up with a

I've come up with a workaround. The clone I bought came with an 80 column card which has auto switchover. This gives me 2 composite outputs. The commodore monitor has a rear input with separate luma/chroma. I put one output to the luma input, and one output to the front composite input on the monitor. With the selector switch on the monitor, I can pick either text or graphics.

I was just going to use the clone, but it has a hires graphics problem which I will start a new topic on.

Thanks

John

 

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I found this on a google

I found this on a google group post:

 

Early ][+ main boards did not have sufficient isolation of the colorburst, so enough of it leaked through to trigger sensitive monitors togo into color mode.

Later revisions (7 and later?) had a mod which fixed this leakage.So if you have a pre-rev 7 (?) board, the over-eager switching to colormode is authentic.

 

My II+ has a sticker that says it came with 16K ram so it must be an earlier one?

 

What was modified on the later revision mainboards to fix the colour killer circuit sensitivity?

 

Thanks

John

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johndegroot wrote:I found
johndegroot wrote:

I found this on a google group post:

 

Early ][+ main boards did not have sufficient isolation of the colorburst, so enough of it leaked through to trigger sensitive monitors togo into color mode.

Later revisions (7 and later?) had a mod which fixed this leakage.So if you have a pre-rev 7 (?) board, the over-eager switching to colormode is authentic.

 Not much different.  The colour mode runs through one extra gate in Rev 7 and the colour killer circuit is identical to all models since Rev1.

If you're bleeding colour in text mode I'd look to see if Q6 is conducting properly to ground.  Grounding pin 12 at B12 (74LS11)  through a 1K resistor would accomplish the same thing if you want to test to see if there's something wrong with the colour killer circuit.

 

Video schematics here

(source: The Apple II Circuit Description by Winston Gayler)

 

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