15 Sep
2008
15 Sep
'08
2:08 p.m.
The Attache's power supply is non-functional. I
turn it on and get
absolutely nothing out of it, not even the power supply fan is
spinning. I've verified the power switch is working and that the little
220/110V "card" is flipped the correct way. Anyone have power supply
schematics (I have logic board schematics)? Anyone seen similar
problems with this machine?
No, but assuming this is an SMPSU I've seen similar problems in other
machines.
There are 2 main causes of an SMPSU being totally dead. Firstly a shorted
semiconductor (rectifier diode or chopper transistor) on the mains side
of the supply. This will, of course, blow the fuse -- I assume you've
checked that!. Occasionally a low-value series resistor fails too and
either the fuse holds, are a replacement fuse will hold. Typically the
owner replaces the fuse, sees it's still dead, and gives up.
So start bu finding the chopper -- the large power transistor on the
primary side of the uspply -- and testing it.
I asusme you've foudn the mains smoothing capacitors (normally a few
hundred uF at 200V each). _Carefully_ (you are dealing with live mains!)
measure the voltage across those with the machine on. Expect about 350V
across the 2 in series.
The sceond cause is an open-circuit startup resistor. Start from the +ve
side of that mains smouthing capacitor pair and look for a high value
resistor (a few hundred k) from there to a point in the chopper circuit.
These resistors have a habit of failing open. Desolder it and test.
-tony