Mike wrote:
Hi there, I am based in the high voltage UK ! The PCB has 620-0120
printed on it which I guess is the part number.
It looks to me like it might have had a couple of new capacitors in
the past. The photographs don't do it justice, it is very clean inside.
BTW: If you're going to work these make sure you apply the proper
precautions - you don't want a nasty, possibly fatal shock. Some of
those caps can have deadly voltage/amperage stored in them, etc. Best
to unplug the power supply for a couple of days before doing work on it,
and even then, you should drain the large caps by using a resistor
(don't short out the leads, that works too, but can damage the caps),
see previous messages in the LisaList archives, etc.
That looks very much like a 1.2A power supply. Check the capacitors
carefully, look for any cracking or warping. Mine actually had a small
tiny silver square capacitor that was visibly cracked down the middle
(was C32 or C34)!
Replacing the large capacitors that are marked with 220V in marker is
likely a good idea if you suspect them. Do they look ok? Any warping or
inflating means it's time to replace them. One of mine also had a
hidden defect that I couldn't see until I desoldered it as it was on the
side where the two caps are near each other. So look closely.
There is a schematic online for the 1.2A supply. Get your hands on it
if you don't have it, it'll be useful in finding out the uF values for
the caps. When replacing them be careful, some caps have to be AC - the
square amber/yellow/clear cased ones on the right side of the middle of
the picture (i.e. 0.22uF C1 and neighbors). You'll want to replace them
with ones that can handle higher temperatures as they'll last longer.
Since you're in the UK and that's a 220/240V power supply be careful to
replace capacitors with ones that match - i.e. not sure if the 1.2A
schematic will say the right values, so read what's written on the caps
you're about to replace.)
Also wiring up a small fan to help keep it cool is advised. I'm using
an old mini 5V CPU fan without the heat sync. to blow air out the back
of the power supply. See the schematic and wire it up to +5V and GND,
but not for the trickle/standby power.
Testing capacitors is a bit painful if you don't have ESR equipment - I
don't, so under James's advice, I just replaced the obviously damaged
ones and ones similar to them. See: http://www.awiz.com/cwinfo.htm
Also check any diodes, the ones that form bridges could be broken too.
Reading them can be a bit tricky unless you desolder them, but you can
usually get away with just using an OHM meter and reading in both
directions. One should show up as a very low resistance, the other as a
very high.
BTW: What happens when you push the power button? Does the button light
up at all? Any sound from the speaker when you push power?
Take a bit of cardboard and stuff it into the power supply switch slot
that acts as a case-open in the power supply, then plug in power. Next
take a VOM and measure the voltage on the first and last pins on the
COP421 chip - if you don't see 5V, the standby/trickle linear power
supply portion is bad, so you'll want to look at the schematic and check
those parts first. If you do get 5V, it's likely something in the
switching supply portion.
Good luck. :-)
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