"petrus bitbyter" wrote in
news

"BigTom" schreef in bericht
...
Jerry G. wrote:
This is not something that can be done over an email. It is the
skill and experience of using the proper tools and knowledge to be
able to trace through the operation of the supply, and to be able to
check and or try out
the various suspect components.

Many times in these supplies the electrolytic caps give problems.
You can use an ESR meter to test them.


ive checked esr on all the caps and theyre ok.
ive got a another working monitor to test voltages/resistances etc.
ive read the sci.electronics.repair.of.smps.
are there any other good web sites where i can learn more about smps.



From sci you should know the SMPS is starting but switches off before
it is really up and running. From the datasheet of the 3842 you should
at least have a general idea of the workings of that chip, especially
from the example circuit. Now you have two main causes that makes the
regulator switch off:

- Overcurrent. Somehow the voltage across the current sense resistor
becomes too high. Maybe a short in the transformer but an overload

Bad filter caps on secondary,very common.ESR meters are good for these.

or
short on the secondary side is also possible. I ever had a current
sense resistor that has got to high a value.

- Power failure. These regulators start using a start circuit that
provides just enough current just to start. When the circuit comes up
it requires more current which usually is provided by the transformer.
(Just look at the example on the datasheet .) This piece of power
supply may fail itself but the failures mentioned above may also cause
this problem.

Usually the housekeeping supply's electrolytic cap,by my experience.


Failing transformers can be found by using a ringer.

A rare failure,IMO.

Secundairy
overloads or shorts can often be found using an Ohmmeter. Pay special
attentiion to the secondary rectifiers and their smoothing capacitors.

First place to look,IMO.(other than obviously burned parts.)

Keep in mind that parts that look all right when measured this way
nevertheless may fail under load. Also keep in mind that the cold
filament has a low resistance. If this all does not give a clue you
can try to find the failing secondary part by temporary removing the
rectifier. (Which is not riskless but your last chanche.)

petrus bitbyter





--
Jim Yanik
jyanik
at
kua.net