Date   : Wed, 15 Nov 2006 20:40:27 -0000
From   : robincommander@... (Robin Commander)
Subject: Master 128 Power Supply

Hi,

That will almost certainly be dried out electrolytic capacitors in the psu.

I've seen this many times, not in Beeb Masters, but in video recorders, tv's
etc. If you're handy with a soldering iron it's entirely possible to replace
them but:

** Health warning !! _ I'm an electronics engineer so I know which bits not
to touch..... Never do anything on the PSU with it plugged into the mains
!!! ***

Disconnect the psu from the mains and leave it for a couple of days so that
everything has a chance to discharge (the main smoothing capacitor(s) can
hold a 400v DC charge for a long time particularly under certain fault
conditions - and it's just sat there waiting for you to discharge it with
your fingers !) ***

>From experience I've found that it's best to blanket replace all of the
electrolytics as they are quite cheap. The fault is probably being caused by
a relatively small one (physically) with a value between 1 and 10uf as these
tend to dry out first. You can identify them easily, they are normally
aluminium cylindrical devices with a plastic sleeve and two connections both
of which may come from one end of the cylinder or alternatively one wire
from each end. You may see that the metal can is bulging slightly on a
suspect capacitor, or sometimes you'll see brown gunge around the leadout
wires.

For replacements I normally use "Low ESR" devices which are rated for 105
degrees C temperature. Companies such as RS components (rswww.com) and
Maplin Electronics (www.maplin.co.uk) can help out with new ones.

Given the age of these machines it's inevitable that these sort of problems
will crop up now - with a set of new caps they'll go on for another 25 years
:-)

Your spare power supply might be an easier alternative - no guarantee it
won't do the same eventually though !

HTH

Regards,
Robin


 

-----Original Message-----
From: bbc-micro-bounces+robincommander=blueyonder.co.uk@...
[mailto:bbc-micro-bounces+robincommander=blueyonder.co.uk@...
] On Behalf Of Richard Gellman
Sent: 15 November 2006 20:10
To: bbc-micro@...
Subject: [BBC-Micro] Master 128 Power Supply

Hi,

My Master 128 has developed a rather peculiar fault.

Onces its on, it runs fine. Getting it on however is proving problematic.

Flick the switch... nothing. Dead.  Take machine apart, all the 5v lines 
read 0.1 volts on the voltmeter. Try powering up supply separately, 5v 
lines now read 5v. Try reconnecting PSU to M128, switches on fine. Looks 
of confusion abound. Drink consumed, though sadly no alcohol present. 
Reassemble M128, and slide it back into position. Power on. Dead. 
RAMPANT looks of confusion now. Go to unplug unit, M128 suddenly powers on.

Its almost like the PSU has developed a random time delay before it is 
willing to provide power to the system.

Any ideas what might be causing this, or should I just give up now and 
swap it for the spare in the battered M128 that is (rightfully) buried 
in a cupboard?

-- Richard

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