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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