Monday, 15 May 2017

BBC Micro Power Supply Repair

If the wonderful BBC micro computer has an Achilles heel, its the power supply.
And the Achilles heel of the power supply is the Rifa brand mains filter capacitors. This is a rare photo of one which hasn't failed. Yet.
These have a tendency to fail and go bang and create large clouds of noxious fumes. It's not pleasant when it happens. And it will happen. These are all failing. If it hasn't gone bang yet, it will do. 
There are similar problems with mains filters on Commodore PETs and the same capacitors on the monitor in an ICL One Per Desk. Here you can see the cracks in the side of the case where the magic smoke escaped from.
This one has been removed from the case, you can see where it has bulged and shattered.
My standard practice whenever a BBC comes in is to replace these capacitors before I even switch it on. The one time I deviated with 'just a quick check before I do the power supply caps' resulted in one going bang as soon as power was applied, so now I stick to the rule.
There are two X class capacitors filtering the mains input, a 0.01uF and a 0.1uF. X class capacitors are rated to be fitted across the mains supply (as opposed to Y class, which is rated to be fitted in series with the mains supply for capacitive droppers etc.). This means they have mains voltage across them, so when they fail, they really fail.
The set on the left are from one that must have been quite loud, the centre set just a minor bang, and the ones on the left, the modern X2 class replacements. Also shown is an electrolytic capacitor which sometimes gives issues with slow or intermittent power supply startup. I normally replace those at the same time.
You can see the gold band on the new electrolytic capacitor in the centre of the picture, and the grey boxes of the new X2 caps. The modern 100nF cap is physically smaller, but there are sets of holes in the board for either size.
These power supplies are a little tricky to work on as you need to remove several earth bolts, push out the floppy drive power connector and mains switch in order to remove the board.
A tip here is to push out the switch (by pushing flat the retaining tabs) before removing the spade connectors.
All refitted, ready to begin testing the rest of the system.

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