Date   : Tue, 14 Feb 2006 18:28:20 -0000
From   : "Mike" <profpep@...>
Subject: Re: Pop goes the PSU

Those caps are fine.

The caps to really worry about are the ones after the bridge rectifier.
According to a drawing I have here, that would be C5, C6, C7, and C8.

The best discharger is a little 'jig' which is composed of a bleed
resistor of (say) 22K 1watt wited between a pair of decently insulated
test probes. Make sure the unit is actually unplugged from the mains, (I
lmow it's opbvious but ...), and not connected to the BBC.  Connect your
test meter (digital for pref), on it's maximum votage range, across the
resistor. apply the probes across each cap in turn, and watch the
voltage tail off. 

If you're not clear about this I can email you a drawing.



If it's a very old supply, it might be worth replacing the electroytics
with some modern 105 degree temp rated models. Electrolytics are
essentilly an electrochemical device and they 'dry up'  reducing their
capacitance. I had some like that in the Viglen power supply I used with
my hard disc. It used to keep trying to 'rev up' - a bit like an old car
not quite starting. Four new caps and it fires up cleanly every time. 

I've got a few spare PSU's and was wondering about re-manufacturing
them. Not sure about the public liability issue though.


  ----- Original Message ----- 
  From: Paul J 
  To: BBC-Micro Mailing List 
  Sent: Tuesday, February 14, 2006 5:49 PM
  Subject: Re: [BBC-Micro] Pop goes the PSU


  I need to replace the filter caps on a model b PSU which went pop a
few days ago. There are two filter caps on the board, the one remaining
reads 0.1uF. Not sure what the other one is as the casing has gone on
the capacitor.  

  From an earlier post on this board, someone mentioned the maplin codes
for the filter caps, see SC93B and SC89W respectively at ...

  http://www.maplin.co.uk/Module.aspx?ModuleNo=26584 

  These caps are 0.1uF and 0.01uF, are these the correct replacements
for the two filter caps ?

  Just thought I would double check before soldering them out.

  Which components do I need to dis-charge before working on the board,
and what is the best way of discharging them ?

  Regards,
  Paul.