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Old 23 September 2018, 18:53   #1
Mick_AKA
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A1200 a mess, advice needed..

Hi everyone,

So I haven't opened up my A1200 for some time and this afternoon is simply presented me with a dark grey screen, obviously I think oh oh leaky caps.

Open her up and visually inspect caps with my magnifier, no sign of leakage around them but I know they can leak unseen.

So flip the board over and the plastic insulation layer thing is stuck to it with 2 tiny burn marks on it.
Seems the resistors at C47 and C16B have burned out big time!

On much closer inspection I found a load of light surface corrosion all around the 020 in U1!
And also some on one side of Gayle, so the board has now been scrubbed down with isopropanol, the legs on the chips with corrosion thankfully cleaned up really nicely and seem fine.

I've got a re-cap kit en route as that should be done while I have it apart, so my question is what was the likely cause of C47 and C16B blowing out and what resistors do I need to replace them?

This is a Rev 1B board.

Many thanks in advance.
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Old 23 September 2018, 19:32   #2
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Well, first off they're capacitors. They're both 220nF ceramic types, and they're just decoupling so in reality the board should work just fine without them. It's rare, but it does happen occasionally that capacitors like that short out, causing in this case a short between 5V and ground.
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Old 23 September 2018, 20:53   #3
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Ah I see, thanks for the info, I'm a bit out of my depth with this sort of stuff, used to working on very old amps and radios.
Looks like I need to look elsewhere for the reasons the board wouldn't fire up, I've desoldered about half the caps on the board so far and not found any leakage, although C821 was really bulging out.
Lets hope the re-cap and clean up does the trick, the damn thing was working fine a week ago.
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Old 24 September 2018, 00:50   #4
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Kinda wanna replace those 2 SMD caps on the underside of the board but I have to buy a bag of 50 haha
are they bi-polarity?
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Old 24 September 2018, 01:03   #5
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Yeah, they go either way round. As I said though, it should run without them so try and solve that issue first...
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Old 24 September 2018, 01:04   #6
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no not all SMD caps are bi-polar the metal CANs are not
just so you dont think all are bi-polar

just bought a bunch of powered risers and the cheap chinese installed 3 caps backwards on them
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Old 24 September 2018, 06:32   #7
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There are no metal cans on the underside of the board though, only ceramics.
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Old 24 September 2018, 17:21   #8
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Good news, recap went okay and A1200 is booting to workbench.

Bad news, here’s the RGB output:

[/url]


Here’s composite:


Last edited by Mick_AKA; 24 September 2018 at 18:59.
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Old 24 September 2018, 18:03   #9
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Somehow the synchronization seems to be way off... video DAC failure perhaps?

By the way, you've got some suspicious links underneath the photos. Is your image hosting service forcing spam in here?
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Old 25 September 2018, 17:49   #10
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Sorted the dodgy links.

If it’s a DAC failiure I may well have to scrap this board as replacing it is well past my soldering capabilities
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Old 25 September 2018, 17:57   #11
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Nah, don't scrap it. Replacing the DAC is well withing the capabilities of many people in the UK, though I suspect the problem lies elsewhere. You'll be able to get it repaired for a lot less than a new board would cost. I have a list of some of the repairers here.
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