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Re: 2600 Composite mod

Posted: Sat May 18, 2019 4:11 pm
by rubber_jonnie
PhilC wrote:
Sat May 18, 2019 2:13 pm
Too many projects, too little time.
I hear ya, same problem here :(

Lawns need mowing, diswashers repairing, taps fitting, have to do Karate as my next grading is potentially at the end of July, but I have to pass the assesment in June first, the list is endless with not enough time or energy for everything.

Re: 2600 Composite mod

Posted: Thu May 23, 2019 6:24 pm
by exxos
New output buffer chips finally arrived, but there is a slight problem :roll:

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Re: 2600 Composite mod

Posted: Thu May 23, 2019 6:51 pm
by stephen_usher
Wrong package type?

Re: 2600 Composite mod

Posted: Thu May 23, 2019 6:53 pm
by PhilC
Mount it upside down or on its side for testing at least? was that where the cap was supposed to go?

Re: 2600 Composite mod

Posted: Thu May 23, 2019 7:04 pm
by exxos
The capacitor is on the bottom of the board, it is the pads which are the problem, as the bottom of the IC as a metal pad, so it would actually short out VCC and GND on the cap vias.

So I did this..

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I get some strange black lines shooting across the screen at the moment. This chip is supposed to have automatic black level, it does have a adjustment, but it does not seem to do anything, and the black still does not look totally black either... Some more experimenting needed...

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Re: 2600 Composite mod

Posted: Thu May 23, 2019 7:41 pm
by exxos
Right, so its the AGC causing these issues kinda...

I put a couple of presets on the board so I can adjust the brightness and black level, but I can see the AGC slowly moving the brightness up and down basically counteracting what I am doing... And that some points it adjusts so fast it causes of raster lines in the black background...

So I am going to disable the AGC, and try again...

Overall I think the image does actually look better going via this new chip.

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Re: 2600 Composite mod

Posted: Thu May 23, 2019 7:49 pm
by PhilC
It does look pretty good on the colours but black doesn't seem as dark as you had it before but it could just be the camera?

Re: 2600 Composite mod

Posted: Thu May 23, 2019 8:13 pm
by exxos
PhilC wrote:
Thu May 23, 2019 7:49 pm
It does look pretty good on the colours but black doesn't seem as dark as you had it before but it could just be the camera?
I think its the camera. I can still see black isn't totally black on my monitor, but I have the black level turned right down, and it actually starts getting brighter if turned down to much.. I think the AGC freaks out and gives it a voltage boost when black level goes to dim.

I tried without the AGC, and the brightness is all over the place, hard to get it stable.

I think it is as good as it is ever going to get, I mean it is only composite video after all, but I think a lot of these issues are relating to my crappy monitor more than anything.

Current mash up..
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Overall I think I just need to add 2 presets onto the board, so people can experiment with the brightness and black level controls...

Re: 2600 Composite mod

Posted: Thu May 23, 2019 10:24 pm
by exxos
Pots added... SOIC package changed.. Data sheets says use 100nF and 1uF, I don't see the the point but I added in a second cap to each chip to try it out...

Now to wait for the next PCB order...

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I may remove the middle chip and see if it works without it.. its only doing the video mixing, but likely just linking the 2 signals together will work.. will try and do that tomorrow...

Re: 2600 Composite mod

Posted: Fri May 24, 2019 2:26 pm
by exxos
I tried removing the middle chip, and just wire linking the inputs and outputs, but I could not get video at all, even adjusting the presets etc, but these chips are not particularly expensive anyway, so I will just leave it in..

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I have tweaked the PCB again, basically adding in jumper pads to enable and disable the AGC, and to enable x2 brightness mode. However these adjustments have little effect when the AGC is enabled, as if you increase the brightness manually, AGC automatically decreases the brightness.

Overall I think just allowing the AGC enabled and just forgetting the manual settings is likely the way to go, but people will have to test this out on various monitors first.

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