Color Classic 640x480 questions / problems

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Color Classic 640x480 questions / problems

Hi,
I just recently modified my Color Classic to have 640x480 resolution, and was having some problems. I noticed after the modification, when the computer is powered on, the flyback transformer seems to be making a buzzing sound. At the same time, the picture sometimes has an almost interlaced look to it (except the screen actually is distorted (it looks like each even horizontal line of pixels are positioned the same, and the odd lines are positioned 1/4" to the left or right of the even lines)).

After trying to adjust the screen, the 'interlaced' (sorry, can't describe it any other way) distortion seems a little more minimized. (just lowering the horizontal width as much as possible)

I do have pretty bad barrel distortion as well. Everything with the modification seemed to go fine, however, it seems like I must've done something wrong. I have read about adding a capacitor to reduce the barrel distortion. Could that possibly fix the other problem as well though?

Thanks

eeun's picture
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I'd re-check where you solder

I'd re-check where you soldered on the underside of the analog board, and make sure there's no short in your bridging wire, and no small connection remaining where you cut the traces.

Putting a capacitor at CL26 might help your barrel distortion, but I'd get a clear picture first, then address the barrel distortion.

Did you do the VGA mod or the Hi-res mod? With one of my CCs, I had poor results with the VGA mod, and went with the Hi-res. YMMV, and my Takky was fine with the former.

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Hi, I did the VGA mod, (tryi

Hi,
I did the VGA mod, (trying to prep the computer for a takky conversion) after really spending some time doing some adjustments, the weird interlacing problem seems to have disappeared.. however, I had to reduce the horizontal width pretty much to the minimum. (also, I noticed the barrel distortion is gone. Apparently I just need to add that capacitor. The flyback transformer still buzzes.. Maybe it always had. I Never noticed it before, however, I never really ran the computer with the case apart either Smile

Since the adjustments, I have not been able to get the interlacing problem to occur any more, I just cannot get the screen wide whatsoever now.

Thanks

eeun's picture
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Glad it's working! I think I

Glad it's working! I think I read more into your first description than was there.

The capacitor fix will hopefully give you a bit more screen width. I tried that on mine, but I don't think the caps I had on-hand were within the voltage tolerances, and all I managed to do was make my monitor fuzzy.

martakz's picture
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[quote]The flyback hums becau

Quote:
The flyback hums because of an effect known as 'lamination rattle' caused by DC voltage on the line or poor construction or both. 'Lam' rattle occurs in all transformers to some degree, that degree being related to the quality of the transformer and the quality of the line voltage.

DC on the line causes a distorted magnetic field, which in turn increases the vibrations.

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fuzzy monitor

I also got mixed results from the capacitor fix on my VGA modded imaCC (imac motherboard upgraded CC). I had three ceramic disc capacitor types I tried, 1nF 2.2nF and 3.9nF (I couldn't find 2 and 4 nF values exactly) rated at 1000V. I tried the 2.2nF capacitor first and got a distorted picture when I tried to adjust the screens horizontal width. As you widened the picture, only half the screen would widen (the top half) and the bottom half would curve at the edges and get fuzzy. The 3.9nF capacitor just made the effects worse with more fuzzness and screen distortion. The last capactor, the 1nF didn't really look like it had any effect and was like having no capacitor at all. Being fustrated at this point after having ripped the machine apart three times, I just left the 1nF capacitor in there.

After seeing your comment on the voltage tolerances of the capacitors, I'm also wondering if I should be using higher voltage rated capacitors. Either way, the barrel distortion isn't too bad and I'm kinda getting used to it, so it's no big loss if I can't correct it.

I also noticed more noise coming from the flyback transformer after the VGA mod, but I just attributed that to the mod changing the voltage and sweep rate. Once you put the case back on, it's not that noticable. I get more noise from the second power supplies fan, which drones out any noise coming from my CC's flyback transformer.

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