ticking smps
 

how can i fix a ticking smps on a mitac L1450PD
it uses a UC3842 controller CNX82A optocoupler and TL431A shunt.
there are four lines on this monitor, 150v 87v 24v 6.3v all reading low.
how do i find the line causing the problem?

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.

--

Jerry G.
=====

"BigTom" wrote in message ...
how can i fix a ticking smps on a mitac L1450PD
it uses a UC3842 controller CNX82A optocoupler and TL431A shunt.
there are four lines on this monitor, 150v 87v 24v 6.3v all reading low.
how do i find the line causing the problem?

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.


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.

BigTom wrote:
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.


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.



Did you check for an over current condition causing the PS to shut down?

i dont know the circuit well enough to test for it.
the output voltages on the smps are less than half of normal.
and even the voltage on the diode side of the optocoupler is low.


Ken wrote:
BigTom wrote:

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.


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.



Did you check for an over current condition causing the PS to shut
down?

BigTom wrote in :

how can i fix a ticking smps on a mitac L1450PD
it uses a UC3842 controller CNX82A optocoupler and TL431A shunt.
there are four lines on this monitor, 150v 87v 24v 6.3v all reading low.
how do i find the line causing the problem?


ticking indicates the PS is trying to start,but tripping a protective
current limit.

I'd check for shorted diodes on the secondaries and use an ESR meter to
check the electrolytics.

--
Jim Yanik
jyanik
at
kua.net

BigTom wrote in :

i dont know the circuit well enough to test for it.
the output voltages on the smps are less than half of normal.
and even the voltage on the diode side of the optocoupler is low.


Download the application notes for the UC3842,it's a common switcher
control IC.

That will give you an idea of how the PS works.

--
Jim Yanik
jyanik
at
kua.net

"Jim Yanik" bravely wrote to "All" (03 Jan 05 17:07:22)
--- on the heady topic of " ticking smps"

JY From: Jim Yanik .
JY Xref: aeinews sci.electronics.repair:5701

JY BigTom wrote in :
how can i fix a ticking smps on a mitac L1450PD
it uses a UC3842 controller CNX82A optocoupler and TL431A shunt.
there are four lines on this monitor, 150v 87v 24v 6.3v all reading low.
how do i find the line causing the problem?


JY ticking indicates the PS is trying to start,but tripping a protective
JY current limit.

JY I'd check for shorted diodes on the secondaries and use an ESR meter
JY to check the electrolytics.


Weak switching pass transistors can also result in ticking though the
psu might even start intermittantly but the regulation will be poor.

A*s*i*m*o*v

.... If all else fails, hurl it across the room a few times!

"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 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.

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

"petrus bitbyter" wrote in
:


"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

****After many hours of studying the circuit and with the help of
everyone here, the ps problem has been SOLVED.
It was R907 o/c, part of the current sense circuit of UC3842.
Many thanks to everyone involved in this project.



Jim Yanik wrote:
BigTom wrote in :


how can i fix a ticking smps on a mitac L1450PD
it uses a UC3842 controller CNX82A optocoupler and TL431A shunt.
there are four lines on this monitor, 150v 87v 24v 6.3v all reading low.
how do i find the line causing the problem?



ticking indicates the PS is trying to start,but tripping a protective
current limit.

I'd check for shorted diodes on the secondaries and use an ESR meter to
check the electrolytics.