While soldering boards today one of our Metcal SP200 solder stations stopped working with a quiet pop and some magic smoke* came from the case.
We removed the case of the power unit and found that the internal fuse had blown. The fuse was soldered to the PCB so we removed the fuse and found a spare fuse holder for 20mm fuses which fitted into the old fuse holder holes. After fitting the new fuse holder we tried to power on the unit and after a flash of light from the new fuse, it blew again.
We have a couple of spare SP200 power units so we removed the covers from a working unit and proceeded to test the broken one to check the resistances of the components compared to the working unit.
The bridge rectifier was removed from the dead unit so we could test the resistance of the transformer coils to check for short circuits but this was found to be ok, on the positive and negative outputs of the rectifier, we found that there was a very low resistance short of approx. 0.2 ohms.
After tracking the route of the power output across the board and by checking each component to measure the resistances, we found that the power MOSFET (IRF840N N-Channel Mosfet Transistor) which appears to switch between the output to the iron and the power supply had the lowest resistance and after unsoldering it we found that all three pins on the MOSFET were short-circuited.
The PCB was rechecked again with the MOSFET removed and the short circuit problem had vanished so it appeared that we had tracked down the source of the problem and smoke.
After a hunt through our MOSFETs tray, we found a similar specification N-Channel Mosfet and this was fitted back onto the PCB and the unit was powered up.
The front led lit and after a few seconds, I checked the temperature of the soldering iron tip with a damp finger and burnt myself so it appeared to be working ok!
We let the unit run for 20 minutes before reassembling the case and now the Metcal SP200 soldering station power supply is ready to use again.
* magic smoke = http://en.wikipedia.org/wiki/Magic_smoke