Date : Wed, 06 Dec 2006 09:32:32 -0000
From : robincommander@... (Robin Commander)
Subject: [OT-ish] A3020 won't turn on
*** EDIT ***
I was curious so went looking for some info on the web and found this
picture : http://acorn.chriswhy.co.uk/Pics/A3020C-9.html
The PSU is built into the main pcb on the left hand side. The circuitry on
the left hand bottom corner does look like it is a switched mode power
supply. I can't see the number on the top of the integrated circuit there as
the picture isn't close up enough but if you google the part number you can
confirm what it's doing from the manufacturer info.
It's quite unusual to have a transformer along with a switcher, normally
switchers just rectify AC mains in and use a reservoir capacitor to provide
a 400v DC feed which is chopped by the switcher. Possibly Acorn wanted to
they drop the mains voltage down first then use the switcher to provide
multiple outputs at high currents.
Maybe the transformer is providing plus and minus 12v supplies and the
switcher is providing (or maybe not in your case) a +5 volt supply.
My money's still on a dried up capacitor though if there's no blown fuses or
smoke.
Regards,
Robin
-----Original Message-----
From: bbc-micro-bounces+robincommander=blueyonder.co.uk@...
[mailto:bbc-micro-bounces+robincommander=blueyonder.co.uk@...
] On Behalf Of Robin Commander
Sent: 06 December 2006 08:32
To: bbc-micro@...
Subject: Re: [BBC-Micro] [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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