A1200 Capacitor Trouble (again)....

So on the A1200 that I repaired the other week, I noticed a very slight flicker in the video output. It wasn't much but it was noticeable. It looked like the colour 'green' would disappear for a split second and then it would come back.

Strange.

I took the motherboard out and had a good look over the topside. Nothing out of the ordinary but I think that it definitely needs to be re-capped. In any case, there's nothing obviously wrong on the top.

Turn it over and look at the back. My attention is immediately drawn to this:

Ouch.

Holy crap. Another failed capacitor... This is a general power de-coupling cap that has clearly failed in a fairly spectacular way. I'm surprised I didn't see this when I did the previous repair but, to be honest, it could've happened after I did that.

Anyway, as it happens, I have a few of these value SMD caps in my box so I just replaced it:

Shiny.

Having put everything together, I plugged the A1200 back in and switched it on. This is what I got:

Now, I may not be an expert electronics engineer but this does not look like an improvement.

I dug the board out again and got the USB microscope on it for another survey, this time I was looking for ANYTHING that might look out of the ordinary. It didn't take long...

These two resistors have been replaced by someone:

361? 360 ohms? Are you sure?

According to the A1200 schematics for Revision 1 type boards, these resistors should be 2000 ohms, not 360 ohms.


R217 is very badly soldered and looks like it has been messed with:

 Bad Solder Joints

R345 has been replaced:

681? 680 ohms? Are you sure?

According to the A1200 schematics, R345 should be 22,000 ohms. Yes, that's right, someone has replaced a 22K ohm resistor with a 680 ohm resistor. What the hell? If anyone has any idea why somebody would do this, feel free to comment below.


 This one (and another nearby - close to the op-amps for the audio) have also been replaced:

Correct - but REALLY badly soldered..

This last one (and it's pair) are part of a known fix for the audio on these earlier models. The original schematics had these at 1.5k ohms but that value is too high for the op-amps and the sound comes out distorted. 681 - or 680 ohms is the correct value. Soldering standard sucks though..

Finally, this:

Caps

This little group of three 1206 size components are capacitors. They should all be exactly the same as they are part of the de-coupling circuits on the video encoder, with one capacitor each on the Red, Green and Blue inputs. So why are there three different colours and why are they all soldered differently? Bottom one looks like factory, second one up is a reasonable manual attempt and top one has way too much solder on the right side. Could this be the cause of the video problem?

Fortunately, these are all 22nF which just so happen to be the only value of SMD caps that I have (and was also the same as the one I replaced at the top of page!). So, out came the soldering iron again. And now we have this:

More new shiny...

And that completely cured the video problem. Rock solid. I left Frontier playing its demo for nearly six hours and no problems. Another great result.

Just waiting for the 'real' caps to arrive so I can do a complete job on this A1200 now. :)