I switched on an old computer yesterday. For those in the know, it’s an Acorn BBC Model B from around 1983. Whilst I’ve not had this one from new, I’ve used it plenty of times before.

So, there I am happily playing Galaxians when there is the most revolting smell followed by smoke from the computers underside; the computer did keep running.

Switching off, and removing the lid and it was obvious that the problem was in the switched mode PSU. I also recognised the smell; it’s quite distinctive.

The X2 capacitor that’s connected across the AC mains supply had failed. This is a really common problem in old PSUs.

The capacitor is usually a low value, 0.1uf in this case, and rated for 250v local mains voltage.

Fortunately I keep a small selection of these in my parts bin as it's a common failure and usually only a 5 minute repair once you’ve got the PSU out of the main chassis and dismantled.

Even though the PSU continues to work, this can be a sign that it may be time to overhaul the other capacitors. I removed each of the larger electrolytic capacitors (100uf @ 250v) and checked each one using an ESR & Capacitance meter. They were all well within specification which was good as I don’t have handy replacements for them.
Time to go parts shopping !!

I had to solder one of those retched 16-pin QFN packages recently. The IC in the picture measures 3x3mm

My normal soldering iron bit, which is pretty small, would still be too large for this job.

Manufacturers like to file off the part numbers on IC's to make reverse engineering their products harder, but it also makes reparing an item almost impossible.

A trick that sometimes works is to clean the area with surgical / white spirit, and then place a single drop of water over the component and if you're very lucky, the part number will be visible. You can see from the image on the right that this is a 4017 IC.

My email in-box contained a routine Elektor post email which contains a nice link to download a free Raspberry Pi Wall Poster and it containsa fair amount of useful information.
Funnily enough, it also contains some conflicting information.

The picture shows where the power input cable goes and it states
5VDC / 800mA… which is 100mA more than my Raspberry programming book states. 
However, at the bottom of the poster under “Troubleshooting” it states that it requires a “solid 5V / 1000mA” supply.

It’s a poster guys… it’s not rocket science to make sure the advice is the same across a single page. No wonder people get confused over these things.





You can download the poster here:      
http://www.elektor.com/Uploads/2013/POST/007/Elektor_Raspberry_Pi_Poster.pdf