Hello eagle1109,
I need to comment in regards to the second PSU you have with those shorted MOSFET's.
You need to check the main fuse to see if its open circuit.
You need to also check for a short across the main filter capacitors.
Going by E-design's attached schematic in Post#7, you need to check D27 for a short,
and reverse leakage.
Also check Q1 for any shorts as well.
When ever you get ANY appliance that shows no signs of life, here are the steps you should follow:
(1) When the unit is open, immediately check the area for any burn smells. If there is something
burned out, you can usually smell it as soon as you open the unit up.
(2) Even if there is no smell of burned components, do a close visual check to see if there is any
evidence of split open semiconductors, burned or overheated resistors, as well as overheating on
the PCB itself. Use a magnifying glass with good area lighting.
(3) Check visually for any electrolytic capacitors that may have bulges at the top, as well as the
bottoms of them to see if the rubber bung has been partially forced out.
(4) Flip the PCB over and check the solder side for any faulty solder joints, especially with any
through-hole components and high current areas. You can usually tell of high current sections
by the track widths. The wider the track and/or the track/s themselves have solder on top of them
will determine high current spots.
(5) Grab your multi-meter and set it to its highest ohms setting, then measure the main fuse to
see if it has gone open circuit.
(6) Measure across the main filter capacitor/s for any shorts. This will determine if your B+ rail
is shorted. If it is, you need to check the bridge rectifier, or individual diodes within the rectifier
circuit for the short.
(7) Measure any diodes for shorts, as well as doing a reverse bias punch-through tests for any
reverse leakage. One leg of the diode MUST be lifted from the circuit to perform an accurate
punch-through test.
(8) Check any transistors or MOSFET's for shorts.
If any one or more of the above is confirmed, then more intensive tests need to be performed.
Any shorted components that has support circuitry attached to it has to be checked.
i.e. Resistors, electrolytic and non-electrolytic capacitors MUST be tested.
In regards to resistors, one leg MUST be lifted from circuit, and for SMD types, they have
to be removed entirely and measured.
You would only need to lift a leg or remove the SMD component entirely only if the in circuit
readings you're getting are a little ambiguous. eg. A 10k resistor is reading 173k.
If they read close to the value indicated on the component, then more than likely they
won't be faulty.
Non-polarized capacitors can be measured for their capacitance values, but electrolytic and
tantalum caps need to be tested for their ESR values as well as capacitance.
Your main goal in the above is to determine if just one component is the culprit for the failure
or there are other components that are responsible, as well as causing a part to go short
circuit.
If you find nothing in the list above, then the only course is to look on the secondary side
of the PSU to see if any components are causing the shut-down.
If there's nothing obvious in the secondary side, then the only way to determine what may
be wrong, is to fire the unit up and perform voltage checks, preferably with a load.
Doing the above is dangerous, and unless you know exactly what you're doing, you risk
a nasty electric shock, perhaps even a fatal one.
If you are unsure, then you need to give the unit to a qualified technician.
Sorry for the story, but I felt that doing the above via a structured procedure should help
you or anyone else to diagnose the problem in a short time. Without having to poke around
measuring components and the like that have nothing to do with the actual fault.
Regards,
Relayer