So, a couple of weeks ago I went and got myself a Micronik A1200 tower thanks to ebay (and my superb wife). This tower had an A1200 motherboard in it - it was sold as a working unit after all - but I actually transferred my own A1200 into it.
The reason for this is that my own A1200 was re-capped by me. The soldering standards aren't high but at least the caps are only a year or two old. I don't know the history of this new A1200 board other than it's been in the tower for some time.
Anyway, during the changeover, I noticed something strange. On the opaque plastic sheet under the motherboard (the new one) there was a small, oval hole that shouldn't have been there...
|A cigarette burn? A micrometeorite?|
Closer inspection revealed that it had been melted by something. A hasty look at the back of the motherboard showed me this:
Something bad has happened here. After a look at the A1200 schematics, this capacitor is a de-coupling capacitor for U18 (a RAM chip). Most likely, it has failed, gone short circuit and then pulled a load of current directly from the +5v rail to ground. Release the magic smoke!
One quick order to Element14 for a strip of new caps and we're off on the next repair. :)
Here's a close up of the problem. From this it should be obvious that one end of the cap is likely to have damaged the solder pad on the board. It needs to come off.
|It doesn't look any better close up.|
As this next picture shows, the pad WAS destroyed. In fact, as soon as I touched the cap with the soldering iron, it basically fell off the board. The picture shows the scene after a cleanup.
|Wherefore art thou solder pad?|
Here's the offender in all his 1206 glory (1206 means 12 thousandths of an inch long by 06 thousandths of an inch wide in case you're wondering how flippin small these things are).
|Ah. There's my pad. Tell you what, you keep it...|
I don't have the tools, time or patience to carry out a proper repair. I'd need copper sheet, suitably rated epoxy for under-bonding, appropriate conformal coating etc etc which is all stuff I don't have. So here's my solution:
The replacement cap was soldered to the good pad and a small jumper wire (about 4mm long) was soldered between the end of the good cap at C18B and the end of the replacement cap at C18A. So far, everything seems OK. I have the A1200 playing the demo for Elite II over and over to give it a bit of a workout and it all seems OK.
Tomorrow I shall take it all apart again and check to make sure it has not repeated.