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Old 29 June 2010, 18:34   #1
Zetr0
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A1200 Video Repair

Hello there my fellow forum chummies

I need some technical help on an couple of A1200 motherboards that I have here.

So what have we got? well both A1200 motherbaords are Rev 1B

Both work perfectly with an internal scan doubler / flicker fixer however via the Video Output / Composite or RF the signal is dire, no colour and very very Dark and only just distinguishable.

I have also used an SVideo Y/C adapter (amplifier) attached to the CXA1145 and still the same problem. I replaced the CXA1145 and the problem still exists.

Since both the boards exhibit the same problem I replaced one of the DAC's and still the problem exists.

The localized capcitors look okay and the multimeter seems to confirm this, has anyone got any ideas?
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Old 29 June 2010, 19:31   #2
8bitbubsy
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I know that I don't know much about this... but....
Even though caps look okay, they can be dried inside. How did you measure a capacitor with a multimeter? You loaded it up with energy and measured how long it managed to pump it out?
Well, I'd replace all the capacitors anyways, it's a common problem. And remember, 1B = old revisions. Old stuff!
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Old 29 June 2010, 19:45   #3
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Did you reflux the resistors, mate?
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Old 29 June 2010, 21:17   #4
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Hi Z,

Displaying in black/white could indicate that the cxa 1145 is in the wrong mode (Pal/NTSC) or there is a problem with the "Pal Goodies" section of tjhe schematics. The 23 pin video output will still display normally so i would start with the "Pal Goodies" section. Chances of both suffering the same symptons are slim unless they were blown in the same say.
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Old 29 June 2010, 23:16   #5
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Thanks for your helps Guys

Quote:
Originally Posted by 8bitbubsy View Post
I know that I don't know much about this... but....
Even though caps look okay, they can be dried inside. How did you measure a capacitor with a multimeter? You loaded it up with energy and measured how long it managed to pump it out?
Well, I'd replace all the capacitors anyways, it's a common problem. And remember, 1B = old revisions. Old stuff!
I have a feeling this might be the next move to be honest.

Quote:
Originally Posted by rkauer View Post
Did you reflux the resistors, mate?
No, they honely look fine, to be honest, If I am going to reflux them I might as well replace them my friend =)

Quote:
Originally Posted by kipper2k View Post
Hi Z,

Displaying in black/white could indicate that the cxa 1145 is in the wrong mode (Pal/NTSC) or there is a problem with the "Pal Goodies" section of tjhe schematics. The 23 pin video output will still display normally so i would start with the "Pal Goodies" section. Chances of both suffering the same symptons are slim unless they were blown in the same say.
I thought that the colour burst crystal might be dead, thus the dark-nearly black screen, however this was proven not to be the case as scan doubler worked fine ....

hmmmmm ponders...
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Old 30 June 2010, 01:42   #6
Magno Boots
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Zetr0,

Have you considered this mod..

http://eab.abime.net/showthread.php?t=43207

It worked on a 1d4 board for me (using a small LED alone) which had much the same symptoms as your boards.

Not sure if the 1B board has the same pads or not, seeing as the pic on BBOAH is poor.

Best of luck
Magno
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Old 03 July 2010, 07:00   #7
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My friend, today I swapped the capacitors on my A600 (common procedure each 3~5 years).

When I fire the Amiga again to test the image was pure cr4p! (imagine an At**i STfu running in 128 colours ). Guess what?

One of the capacitor's lead near of the CXA1145 encoder (10uF) was too close of one via and somehow was touching it.

Doing a small re-soldering in the area made the picture crystal clear again.
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Old 03 July 2010, 17:57   #8
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@Magnoboots

I have modified a 2B board to see if that works, thanks for the link, will hopefully get to post some pics of the jig when I get to test it today.


@rkauer

My friend, I have taken your advice and attempted to re-flow one of the 22uf caps next to the CXA1145 encoder as the leads were a bit dull...

slaped down some flux and attempted an iron on 320c... nothing ... just the flux sizzling... ramped it upto 380c and finally the solder began to melt... along with perhaps the worst fishy-capacitor smell I have hever had the missfortune to breath... I had to open the door... urgh....

unfortunately I would say it had rotted / dryed out internally and as a consequence destroyed the positive lead pad.

not worried to be honest as its an easy location to repair.

see included is a pic of the offending 22UF caps location
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Old 03 July 2010, 18:22   #9
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Okay, so the caps are dying, no doubt. Then you know how to act!
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