17 Sep
2008
17 Sep
'08
12:18 p.m.
On Monday 15 September 2008 17:08, Tony Duell wrote:
I have had one supply fail, a small switcher in a VCR. Checking found a
blown fuse and the one TO-220 cased transistor in there shorted. Replacing
those with exact duplicates gave me another blown fuse and another shorted
transistor. There were at that time kits available at rather reasonable
prices which included a bunch of other stuff, diodes and caps mostly, and
replacing all of the parts in the kit solved the problem. I'm still not sure
quite what it was.
Other supplies that I've dug into showed me blown power transistors or
rectifiers, usually.
--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, a critter that can
be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin
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.