Date   : Wed, 06 Dec 2006 08:31:36 -0000
From   : robincommander@... (Robin Commander)
Subject: [OT-ish] A3020 won't turn on

Hi Richard,

*** Usual caveats apply - watch your fingers with mains voltages ! ***

A PSU who's supply rails are coming up really slowly is indicative of a
dried out electrolytic capacitor.

I've never been inside an A3020 psu I'm afraid so can't be certain if it's
linear or switched mode.

If it is a linear as you suggest (which is likely if there's a big
transformer) I'd look very hard at the main smoothing capacitor. On a linear
psu you'll have the mains transformer secondary winding feeding into a
bridge rectifier arrangement (could be 4 discrete diodes or one bridge
block, if the latter there'll be four leads, two are the AC in and the other
two positive and negative.)

The main smoother (aka reservoir) will be connected across the +ve and -ve
outputs of the bridge.

If this cap has dried up the psu will be unable to maintain the correct
output voltage with a load present. If you have access to an oscilloscope to
look at how "clean" the power rails are you'll see that there will be a
significant amount of AC ripple present.

There will probably also be smaller value capacitors on the input and output
sides of the + and -12v regulator IC's. These can cause similar effects
though as you say both supplies come up slowly it points to the problem
being further upstream.

If it is a switch mode psu in disguise I'd still be looking for a dried out
capacitor. Often (as has been mentioned a few times on this list) in these
circumstances, if you leave the unit switched on long enough they may well
start up eventually.

Good luck !

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: 05 December 2006 21:40
To: bbc-micro@...
Subject: [BBC-Micro] [OT-ish] A3020 won't turn on

Hi,

I'm looking for the expertise of a power supply expert...

I have here an A3020 that was working fine until I switched it off, then 
back on again (not quickly, leaving adequate pause between) to check the 
cold start boot sequence.

But it didn't come back on.

The supply fuse is fine, and no burning smells are too be found. A quick 
test with a voltmeter shows that the +5V line stays constantly at 0V, 
and the +12V and -12V lines seem to *very* slowly build up (about 0.01V 
every 10 seconds for -12V, 0.01V every 1.5 minutes for +12V).

The power supply itself looks to be a linear type (there is only one 
huge transformer and a single toroid coil present), unless its a very 
deceptive looking switch-mode supply. There doesn't seem to be a -5V 
test point, so I assume it doesn't require one.

Anyone have any ideas?

-- Richard


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