Date   : Tue, 17 Mar 2009 04:55:34 +0000
From   : mike@... (Mike Tomlinson)
Subject: Dead BBC B

In article <015301c9a622$eb16ab00$1b04a8c0@...>,
Anders Carlsson <anders.carlsson@...> writes

>However, do you think it would be wise to replace the 220uF electrolytic 
>capacitor at C9 as well as the C2 and/or C1 capacitors while I'm onto it?

Yes.  Don't think about it, just do it.  Use a 105 degree C replacement.

>The power supply didn't yet emit any smoke (that I can remember) but since 
>all three faults are somewhat common perhaps it would be best to refurbish 
>it completely?

Yes.  It's 25 years old, give it a little TLC :)

>It has been unplugged from the mains for a half year, so I expect most high 
>voltage capacitors have discharged by now or would one still need to take 
>extra measures before digging into it?

It should be OK, but the mains reservoir capacitor can hold a charge for
a surprisingly long time if there is a fault causing the PSU to not
startup (e.g. an open cct startup resistor, very common in the Master
PSU.)  

You could carefully disassemble the PSU, trying not to touch anything
that could bite, but it's fiddly to get apart.  The board is held in the
metal frame by three (four?) screws, but there is a triangular piece of
metal forming part of the frame that makes it awkward to lift the PCB
out cleanly without touching anything.

Once you get at the underside of the board, as Mark says, you can ensure
the mains reservoir caps are discharged using a bulb or similar.

> I realize high voltage circuits 
>shouldn't be fixed at all by unqualified people but perhaps if one is very 
>careful?

Well, even qualified people are careful :) The bite from the mains
reservoir cap is *nasty*.

If you're not confident about doing the work, it's best to find someone
who is.  If they will let you watch while they work, you can see what is
involved and pick up some tips for the next one.

-- 
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(='.'=)  Bunny says Windows 7 is Vi$ta reloaded.
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