Date   : Sat, 23 Feb 2002 16:33:48 +0000
From   : John Woodgate <jmw@...>
Subject: Re: Repairing PSU

I read in !bbc-micro that Mike Tomlinson <mike@...>
wrote (in <1GYeVhG8WBd8Ew23@...>) about '[BBC-Micro] Repairing
PSU', on Wed, 20 Feb 2002:

>WRT BBC PSUs - the most common fault on those is capacitor C9.  If you
>can find someone able and prepared to replace it for you, this fixes 70%
>of dead BBC PSUs.  Another common problem is that the mains cable breaks
>at the strain relief grommet at the back of the PSU, usually due to
>people winding the cable around the machine to store it.
>
>Don't work on PSUs if you have no prior experience - they can have a
>very nasty bite even if left unplugged for a long time.

Agreed: there are several electrical safety issues. The persistent
electric shock issue involves C6 and C8. R2 provides a discharge path
for C6 so, **provided R2 and L1 aren't open-circuit**, it should be well
enough discharged after 10 time-constants. We have 100 uF and 100 kohms,
which gives a time-constant of 10 s, so wait 2 minutes after switch-off
**and then measure the voltage across C6 itself**. Similarly, R3 and R5
discharge C8 with the same time-constant. If you find either capacitor
retains its charge, discharge it through a 10 kohm resistor, NOT a
screwdriver or your finger!

BE SURE to replace C9 with a high-temperature (105 C) part (such as
Farnell 921-026), and maybe extend the leads on R4, assuming it gets
hot, to move it away from C9.

I have the circuit diagram of the power supply (although there may be
variations) if anyone wants it.
-- 
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk 
Interested in professional sound reinforcement and distribution? Then go to 
http://www.isce.org.uk
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