Date   : Mon, 16 Mar 2009 14:47:26 -0000
From   : jumbos.bazzar@... (Mark Haysman)
Subject: Dead BBC B

Hi.

As I've said in another post, whenever I refurb a machine, I ALWAYS replace 
the 2 X2s and the startup cap, even if the PSU works perfect, because if I 
don't then the original X2s are going to explode, and I'll only have to take 
it back and do them. I'd rather get it out of the way as a preventative 
measure, then it's done and the PSU can go on for many years.

Half a year it's been unplugged? The voltages usually become safe after 5 
seconds being unplugged if the PSU is okay. If the switching transistor is 
open circuit or not switching, then sometimes the DC rectified manis can be 
stored in the 2 HV caps, and it's best to discharge them - easiest way is 
put a 60w light bulb across them for 5 seconds. Don't use a screwdriver to 
short them unless you like sparks!

Mark.



> Mark Haysman posted the link to Sprow's webpage:
>
> http://www.sprow.co.uk/bbc/howto.htm#powersupply
>
> I also have a BBC B with a failing power supply. Last year I measured it 
> and
> found it gives about 1.2V on the 5V lines when conneted, but close to 5V 
> if
> unloaded. Based on the trouble shooting above, it sounds like fault #2, 
> the
> BUA208 transistor which I'll try to replace.
>
> 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?
> 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?
>
> 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? I realize high voltage circuits
> shouldn't be fixed at all by unqualified people but perhaps if one is very
> careful?
>
> Best regards
>
> -- 
> Anders Carlsson