About 15 minutes after power up there was a fizzing, popping sound from the computer and I ran to cut the power. Too late! A plume of blue/grey acrid smoke poured out of the left hand side of the machine right by the power supply.
I cut the power and moved the machine away from anything that might burn quickly and waited for the smoke to stop. It quickly dispersed and so I did the only thing a self respecting software engineer would do... I decided to turn it back on again.
But not without opening the power supply so I could take a look inside while it was running. Here's the PSU open after the fire:
wearing my safety glasses.
You'll notice that there doesn't seem to be anything burnt in the supply and when I powered on the machine worked perfectly. Close examination shows that one of the capacitors used for power supply filtering had cracked open and burnt up.
common failures on the BBC Micro power supply and easily repaired.
It's interesting to note that the capacitor used is rated by safety agencies because of the potential for failure leading to a short at mains potential and the metallized film is designed to self heal or fail safely. The device is also meant to fail safely at a peak potential of 2.5kV without burning with a flame.
EN132400 (IEC384-14): Active FlammabilityNot often you get to see an international safety standard take effect.
The capacitor under test is connected to rated voltage through a transformer and filter. 20 transients are then introduced across the capacitor at random intervals while rated voltage remains applied. The amplitude of the transient is dependent on the class of capacitor. The capacitor may not flame during this test.
Here's the power supply circuit diagram.