Need help with switching power supply repair

S

senator richards

Guest
I am trying to troubleshoot a small SMPS that came from an A/V
switcher. Input is 120vac and it is supposed to output + and - 15vdc
at .8A. Currently it is outputting +17 on one output and somewhere
between +22 and +30 on the other output. My experience with SMPS has
usually been shorted rectifiers or bad output filter caps so the first
thing I did was check all the diodes and replace the output caps.
Obviously that didn't fix the problem. The high voltage is about
170vdc. The supply to the pwm chip is fluctuating between 7-15v, so
i'm thinking this might be the problem, but maybe its something else.
In case its not obvious, i'm fairly new to tinkering with these
things. Thanks in advance for any help.

Randy
 
"senator richards" <rnewman36@gmail.com> wrote in message
news:dd3e3ab9-1f98-41b8-a7ec-08431af9a33d@u20g2000yqj.googlegroups.com...
I am trying to troubleshoot a small SMPS that came from an A/V
switcher. Input is 120vac and it is supposed to output + and - 15vdc
at .8A. Currently it is outputting +17 on one output and somewhere
between +22 and +30 on the other output. My experience with SMPS has
usually been shorted rectifiers or bad output filter caps so the first
thing I did was check all the diodes and replace the output caps.
Obviously that didn't fix the problem. The high voltage is about
170vdc. The supply to the pwm chip is fluctuating between 7-15v, so
i'm thinking this might be the problem, but maybe its something else.
In case its not obvious, i'm fairly new to tinkering with these
things. Thanks in advance for any help.

Randy
Click to expand...
The cap that filters the supply to the pwm chip on the primary side, maybe ?
It's pretty common on most designs of switcher. Work on the thing on an
isolation transformer if at all possible. They are potentially very very
dangerous if you are not fully competent with them

Arfa
 
senator richards wrote:

I am trying to troubleshoot a small SMPS that came from an A/V
switcher. Input is 120vac and it is supposed to output + and - 15vdc
at .8A. Currently it is outputting +17 on one output and somewhere
between +22 and +30 on the other output. My experience with SMPS has
usually been shorted rectifiers or bad output filter caps so the first
thing I did was check all the diodes and replace the output caps.
Obviously that didn't fix the problem. The high voltage is about
170vdc. The supply to the pwm chip is fluctuating between 7-15v, so
i'm thinking this might be the problem, but maybe its something else.
In case its not obvious, i'm fairly new to tinkering with these
things. Thanks in advance for any help.

Randy
See if you have an optical coupler in there. I've seen these used as
Click to expand...
part of the regulation and it's possible it isn't working any more.

Also check for bad caps, ripple could be causing a reg issue.

Jamie
 
"senator richards"

I am trying to troubleshoot a small SMPS that came from an A/V
switcher. Input is 120vac and it is supposed to output + and - 15vdc
at .8A. Currently it is outputting +17 on one output and somewhere
between +22 and +30 on the other output.
Click to expand...
** That is very odd.

You real sure of that measurement?


The supply to the pwm chip is fluctuating between 7-15v,
Click to expand...

** Means the supply for the IC is not holding up.

Check for open resistors or diodes in the supply chain to that IC.

Any yellow glue in sight ?


..... Phil
 
senator richards wrote:
I am trying to troubleshoot a small SMPS that came from an A/V
switcher. Input is 120vac and it is supposed to output + and - 15vdc
at .8A. Currently it is outputting +17 on one output and somewhere
between +22 and +30 on the other output. My experience with SMPS has
usually been shorted rectifiers or bad output filter caps so the first
thing I did was check all the diodes and replace the output caps.
Obviously that didn't fix the problem. The high voltage is about
170vdc. The supply to the pwm chip is fluctuating between 7-15v, so
i'm thinking this might be the problem, but maybe its something else.
In case its not obvious, i'm fairly new to tinkering with these
things. Thanks in advance for any help.

Randy
Try to load both sides with about .1amp, low load on a switcher
Click to expand...
can confuse the hardware sometimes.
 
On Wed, 21 Sep 2011 13:31:46 -0700 (PDT), senator richards
<rnewman36@gmail.com> put finger to keyboard and composed:

I am trying to troubleshoot a small SMPS that came from an A/V
switcher. Input is 120vac and it is supposed to output + and - 15vdc
at .8A. Currently it is outputting +17 on one output and somewhere
between +22 and +30 on the other output. My experience with SMPS has
usually been shorted rectifiers or bad output filter caps so the first
thing I did was check all the diodes and replace the output caps.
Obviously that didn't fix the problem. The high voltage is about
170vdc. The supply to the pwm chip is fluctuating between 7-15v, so
i'm thinking this might be the problem ...
Click to expand...
Just to add to what Arfa said, the supply to the PWM chip often starts
with a simple resistor feed from the +170V rectfied mains voltage, and
is then augmented by a regenerated supply from a tap on the switchmode
transformer. If this is the case, then there will be a small
electrolytic filter capacitor (~10uF) that often develops high ESR.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.
 
On 2011-09-21 22:31:46 +0200, senator richards <rnewman36@gmail.com> said:

I am trying to troubleshoot a small SMPS that came from an A/V
switcher. Input is 120vac and it is supposed to output + and - 15vdc
at .8A. Currently it is outputting +17 on one output and somewhere
between +22 and +30 on the other output. My experience with SMPS has
usually been shorted rectifiers or bad output filter caps so the first
thing I did was check all the diodes and replace the output caps.
Obviously that didn't fix the problem. The high voltage is about
170vdc. The supply to the pwm chip is fluctuating between 7-15v, so
i'm thinking this might be the problem, but maybe its something else.
In case its not obvious, i'm fairly new to tinkering with these
things. Thanks in advance for any help.

Randy
Click to expand...
try to check the main capacitor with an esr meter, or change it !
build yourself easy an esr meter grab any you want on my web site below
regards,
--
----------
Kripton

the ESR Repository @ http://kripton2035.free.fr/esr-repository.html
the Geiger Repository @ http://kripton2035.free.fr/geiger-repositor.html
 
On Wed, 21 Sep 2011 13:31:46 -0700 (PDT), senator richards
<rnewman36@gmail.com> wrote:

I am trying to troubleshoot a small SMPS that came from an A/V
switcher.
Click to expand...
A photo of the PS would be helpful. Mostly, I'm looking for what
manner of regulator chip is being used and whether there's a
transformer or optoisolator involved.

Input is 120vac and it is supposed to output + and - 15vdc
at .8A. Currently it is outputting +17 on one output and somewhere
between +22 and +30 on the other output.
Click to expand...
If you have +30VDC on an output circuit that is suppose to only
tolerate +15VDC, it's possible that the output caps are now dead,
especially if this PS has been run for quite some time. The output
caps are probably rated at 25WV, which doesn't apprecitate the 30V (or
more if there are voltage spikes on the output).

My experience with SMPS has
usually been shorted rectifiers or bad output filter caps
Click to expand...
Yeah, that's the usual problem.

so the first
thing I did was check all the diodes and replace the output caps.
Obviously that didn't fix the problem. The high voltage is about
170vdc. The supply to the pwm chip is fluctuating between 7-15v,
Click to expand...
Fluctuations like that are sometimes arcing in the xformer or spurious
oscillations in the regulator chip. High output voltage usually means
a failure in whatever is regulating or sensing the output voltage. As
others have suggested, get an isolation xformer before you kill
yourself. Then, attach a scope probe to the output lines, regulator
pins, and optoisolator, and see what's causing the fluctuations.
That's not normal.

--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
 
Jeff Liebermann <jeffl@cruzio.com> wrote in
news:p8mm77dur3hb6af5n9i275438jkbrqo2sj@4ax.com:

On Wed, 21 Sep 2011 13:31:46 -0700 (PDT), senator richards
rnewman36@gmail.com> wrote:

I am trying to troubleshoot a small SMPS that came from an A/V
switcher.

A photo of the PS would be helpful. Mostly, I'm looking for what
manner of regulator chip is being used and whether there's a
transformer or optoisolator involved.

Input is 120vac and it is supposed to output + and - 15vdc
at .8A. Currently it is outputting +17 on one output and somewhere
between +22 and +30 on the other output.

If you have +30VDC on an output circuit that is suppose to only
tolerate +15VDC, it's possible that the output caps are now dead,
especially if this PS has been run for quite some time. The output
caps are probably rated at 25WV, which doesn't apprecitate the 30V (or
more if there are voltage spikes on the output).

My experience with SMPS has
usually been shorted rectifiers or bad output filter caps

Yeah, that's the usual problem.

so the first
thing I did was check all the diodes and replace the output caps.
Obviously that didn't fix the problem. The high voltage is about
170vdc. The supply to the pwm chip is fluctuating between 7-15v,

Fluctuations like that are sometimes arcing in the xformer or spurious
oscillations in the regulator chip. High output voltage usually means
a failure in whatever is regulating or sensing the output voltage. As
others have suggested, get an isolation xformer before you kill
yourself. Then, attach a scope probe to the output lines, regulator
pins, and optoisolator, and see what's causing the fluctuations.
That's not normal.
Click to expand...
determine what output rail the supply regulates from.
then check it's filter caps for ESR.

I've seen where the filter cap(s) on that rail had a high ESR,and the PWM
control tried to drive it back to the nominal voltage,and the OTHER output
rails climbed high because of that. And that caused high dissipation in
other circuits that eventually burned a hole in the PCB.

you could also have a bad electrolytic on the reference output on the PWM
chip. or a reference divider resistor changed value.

Find the datasheet for the PWM Ic and learn the inputs and what levels they
should be at,that will tell you how your supply is supposed to work.


--
Jim Yanik
jyanik
at
localnet
dot com
 
On Sep 21, 8:08 pm, "Arfa Daily" <arfa.da...@ntlworld.com> wrote:
"senator richards" <rnewma...@gmail.com> wrote in message

news:dd3e3ab9-1f98-41b8-a7ec-08431af9a33d@u20g2000yqj.googlegroups.com...

I am trying to troubleshoot a small SMPS that came from an A/V
switcher. Input is 120vac and it is supposed to output + and - 15vdc
at .8A. Currently it is outputting +17 on one output and somewhere
between +22 and +30 on the other output. My experience with SMPS has
usually been shorted rectifiers or bad output filter caps so the first
thing I did was check all the diodes and replace the output caps.
Obviously that didn't fix the problem. The high voltage is about
170vdc. The supply to the pwm chip is fluctuating between 7-15v, so
i'm thinking this might be the problem, but maybe its something else.
In case its not obvious, i'm fairly new to tinkering with these
things. Thanks in advance for any help.

Randy

The cap that filters the supply to the pwm chip on the primary side, maybe ?
It's pretty common on most designs of switcher. Work on the thing on an
isolation transformer if at all possible. They are potentially very very
dangerous if you are not fully competent with them

Arfa
Click to expand...
Thanks for the help. I borrowed an ESR meter and checked the cap that
supplies the pwm chip. The cap is a 47uf 50v and esr measures .5 which
appears to be about right.

I don't have an isolation transformer but will look into getting one.

Thanks for your help.

-R
 
On Sep 21, 8:34 pm, Jamie
<jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:
senator richards wrote:
I am trying to troubleshoot a small SMPS that came from an A/V
switcher. Input is 120vac and it is supposed to output + and - 15vdc
at .8A. Currently it is outputting +17 on one output and somewhere
between +22 and +30 on the other output. My experience with SMPS has
usually been shorted rectifiers or bad output filter caps so the first
thing I did was check all the diodes and replace the output caps.
Obviously that didn't fix the problem. The high voltage is about
170vdc. The supply to the pwm chip is fluctuating between 7-15v, so
i'm thinking this might be the problem, but maybe its something else.
In case its not obvious, i'm fairly new to tinkering with these
things. Thanks in advance for any help.

Randy

See if you have an optical coupler in there. I've seen these used as
part of the regulation and it's possible it isn't working any more.

  Also check for bad caps, ripple could be causing a reg issue.

Jamie
Click to expand...
There is an optical coupler. Its a CNX82A chip. Is there an easy way
to test it? I looked at Mouser and the part is discontinued and NLA.

I've checked all the caps for low ESR and they all check out OK.

Thanks.
-R
 
Just to add to what Arfa said, the supply to the PWM chip often starts
with a simple resistor feed from the +170V rectfied mains voltage, and
is then augmented by a regenerated supply from a tap on the switchmode
transformer. If this is the case, then there will be a small
electrolytic filter capacitor (~10uF) that often develops high ESR.

- Franc Zabkar
Click to expand...

Hi Franc,

I found the 10uf cap you mention and measured the ESR. It was very
high (around 50), so i replaced it with another cap i had around. it
is not a low esr cap and only an 85 deg one, but figured it would be
ok for testing. it measures about 4.5 on the esr meter which is still
a little high (should be closer to 2). I still get the same result
when its hooked up.

-R
 
On Sep 21, 11:41 pm, "Phil Allison" <phi...@tpg.com.au> wrote:
"senator richards"

Thanks in advance for any help.

** FFS  -  get yourself an ESR meter.

....  Phil
Click to expand...
I actually have one, but didn't have it with me the other day. I have
it now and have gone through all the caps. The only one that measured
bad was the 10uf 16v cap that Franc mentioned above. I changed it with
another cap and it didn't appear to fix the problem.
 
try to check the main capacitor with an esr meter, or change it !
build yourself easy an esr meter grab any you want on my web site below
regards,
Click to expand...

The main filter cap (68uf 400v) tests ok for ESR.

-R
 
determine what output rail the supply regulates from.
then check it's filter caps for ESR.

I've seen where the filter cap(s) on that rail had a high ESR,and the PWM
control tried to drive it back to the nominal voltage,and the OTHER output
rails climbed high because of that. And that caused high dissipation in
other circuits that eventually burned a hole in the PCB.

you could also have a bad electrolytic on the reference output on the PWM
chip. or a reference divider resistor changed value.

Find the datasheet for the PWM Ic and learn the inputs and what levels they
should be at,that will tell you how your supply is supposed to work.
Click to expand...
I've checked all the caps. i replaced all the output caps initially
before I had the ESR meter. Now with the meter i've gone through all
the caps (there are 8 total on this smps) and only one tested bad. It
was the 10uf 16v that Franc mentioned previously. Replacing it didn't
seem to fix the problem. I have the datasheet on the pwm. It is a
UC3842BN I checked the input voltage and it was erratic as mentioned
previously. I checked the two resistors, diode, and filter in the
supply and they all seem ok. Here is a link to some pictures I took
with my phone:

https://picasaweb.google.com/115370641579278903570/SMPS?authuser=0&authkey=Gv1sRgCJ2014unnrOWvgE&feat=directlink
 
"sci.electronics.repair" <rnewman36@gmail.com> wrote in message
news:26b39f80-37e4-4dd2-bdae-46c118a3c01c@1g2000vbu.googlegroups.com...
On Sep 21, 8:08 pm, "Arfa Daily" <arfa.da...@ntlworld.com> wrote:
"senator richards" <rnewma...@gmail.com> wrote in message

news:dd3e3ab9-1f98-41b8-a7ec-08431af9a33d@u20g2000yqj.googlegroups.com...

I am trying to troubleshoot a small SMPS that came from an A/V
switcher. Input is 120vac and it is supposed to output + and - 15vdc
at .8A. Currently it is outputting +17 on one output and somewhere
between +22 and +30 on the other output. My experience with SMPS has
usually been shorted rectifiers or bad output filter caps so the first
thing I did was check all the diodes and replace the output caps.
Obviously that didn't fix the problem. The high voltage is about
170vdc. The supply to the pwm chip is fluctuating between 7-15v, so
i'm thinking this might be the problem, but maybe its something else.
In case its not obvious, i'm fairly new to tinkering with these
things. Thanks in advance for any help.

Randy

The cap that filters the supply to the pwm chip on the primary side,
maybe ?
It's pretty common on most designs of switcher. Work on the thing on an
isolation transformer if at all possible. They are potentially very very
dangerous if you are not fully competent with them

Arfa

Thanks for the help. I borrowed an ESR meter and checked the cap that
supplies the pwm chip. The cap is a 47uf 50v and esr measures .5 which
appears to be about right.

I don't have an isolation transformer but will look into getting one.

Thanks for your help.

-R
Click to expand...
A brand new cap might be a little better than that, but certainly right ball
park at 0.5 ohms, and would not be an issue at that figure. About the only
other thing that you could try, assuming that it uses a startup resistor
from the 170v, is to disconnect the self-feed diode from the switching
transformer. That way, you will force it to run from the startup supply
only, just in case the self-run voltage is fluctuating, and interfering with
your reading on the pwm chip's supply. Bear in mind though, that you can't
run it for too long on the startup resistor, as it will get quite hot, being
normally intended to supply current to the chip, only for as long as it
takes the supply to fully start up and settle. If the voltage supply to the
chip still jiggles around when it is only being fed by the resistor, then
after the 47uF cap, which may yet be faulty, but not in an ESR way, the next
prime suspect would have to be the chip itself.

Arfa
 
On 2011-09-23 02:56:37 +0200, "Arfa Daily" <arfa.daily@ntlworld.com> said:

"sci.electronics.repair" <rnewman36@gmail.com> wrote in message
news:26b39f80-37e4-4dd2-bdae-46c118a3c01c@1g2000vbu.googlegroups.com...
On Sep 21, 8:08 pm, "Arfa Daily" <arfa.da...@ntlworld.com> wrote:
"senator richards" <rnewma...@gmail.com> wrote in message

news:dd3e3ab9-1f98-41b8-a7ec-08431af9a33d@u20g2000yqj.googlegroups.com...

I am trying to troubleshoot a small SMPS that came from an A/V
switcher. Input is 120vac and it is supposed to output + and - 15vdc
at .8A. Currently it is outputting +17 on one output and somewhere
between +22 and +30 on the other output. My experience with SMPS has
usually been shorted rectifiers or bad output filter caps so the first
thing I did was check all the diodes and replace the output caps.
Obviously that didn't fix the problem. The high voltage is about
170vdc. The supply to the pwm chip is fluctuating between 7-15v, so
i'm thinking this might be the problem, but maybe its something else.
In case its not obvious, i'm fairly new to tinkering with these
things. Thanks in advance for any help.

Randy

The cap that filters the supply to the pwm chip on the primary side, maybe ?
It's pretty common on most designs of switcher. Work on the thing on an
isolation transformer if at all possible. They are potentially very very
dangerous if you are not fully competent with them

Arfa

Thanks for the help. I borrowed an ESR meter and checked the cap that
supplies the pwm chip. The cap is a 47uf 50v and esr measures .5 which
appears to be about right.

I don't have an isolation transformer but will look into getting one.

Thanks for your help.

-R

A brand new cap might be a little better than that, but certainly right
ball park at 0.5 ohms, and would not be an issue at that figure. About
the only other thing that you could try, assuming that it uses a
startup resistor from the 170v, is to disconnect the self-feed diode
from the switching transformer. That way, you will force it to run from
the startup supply only, just in case the self-run voltage is
fluctuating, and interfering with your reading on the pwm chip's
supply. Bear in mind though, that you can't run it for too long on the
startup resistor, as it will get quite hot, being normally intended to
supply current to the chip, only for as long as it takes the supply to
fully start up and settle. If the voltage supply to the chip still
jiggles around when it is only being fed by the resistor, then after
the 47uF cap, which may yet be faulty, but not in an ESR way, the next
prime suspect would have to be the chip itself.

Arfa
Click to expand...
I've never seen a chip 3842 dead...
but the small cap 10uF beside it has really often been solving the case !!!!!

--
----------
Kripton

the ESR Repository @ http://kripton2035.free.fr/esr-repository.html
the Geiger Repository @ http://kripton2035.free.fr/geiger-repositor.html
 
<Kripton>


I've never seen a chip 3842 dead...
but the small cap 10uF beside it has really often been solving the case
!!!!!
Click to expand...

** I have seen about 5 dead ones.

And I do few SMPS repairs.



..... Phil
 
"Arfa Daily" <arfa.daily@ntlworld.com> wrote in
news:bPQeq.11661$fR1.8387@newsfe24.ams2:

"sci.electronics.repair" <rnewman36@gmail.com> wrote in message
news:26b39f80-37e4-4dd2-bdae-46c118a3c01c@1g2000vbu.googlegroups.com...
On Sep 21, 8:08 pm, "Arfa Daily" <arfa.da...@ntlworld.com> wrote:
"senator richards" <rnewma...@gmail.com> wrote in message

news:dd3e3ab9-1f98-41b8-a7ec-08431af9a33d@u20g2000yqj.googlegroups.co
m...

I am trying to troubleshoot a small SMPS that came from an A/V
switcher. Input is 120vac and it is supposed to output + and -
15vdc at .8A. Currently it is outputting +17 on one output and
somewhere between +22 and +30 on the other output. My experience
with SMPS has usually been shorted rectifiers or bad output filter
caps so the first thing I did was check all the diodes and replace
the output caps. Obviously that didn't fix the problem. The high
voltage is about 170vdc. The supply to the pwm chip is fluctuating
between 7-15v, so i'm thinking this might be the problem, but
maybe its something else. In case its not obvious, i'm fairly new
to tinkering with these things. Thanks in advance for any help.

Randy

The cap that filters the supply to the pwm chip on the primary side,
maybe ?
It's pretty common on most designs of switcher. Work on the thing on
an isolation transformer if at all possible. They are potentially
very very dangerous if you are not fully competent with them

Arfa

Thanks for the help. I borrowed an ESR meter and checked the cap that
supplies the pwm chip. The cap is a 47uf 50v and esr measures .5
which appears to be about right.

I don't have an isolation transformer but will look into getting one.

Thanks for your help.

-R

A brand new cap might be a little better than that, but certainly
right ball park at 0.5 ohms, and would not be an issue at that figure.
Click to expand...
IIRC,the values on the DSE meter are MAX values,so IMO,if a cap is reading
what's listed,it's marginal.

About the only other thing that you could try, assuming that it uses a
startup resistor from the 170v, is to disconnect the self-feed diode
from the switching transformer. That way, you will force it to run
from the startup supply only, just in case the self-run voltage is
fluctuating, and interfering with your reading on the pwm chip's
supply. Bear in mind though, that you can't run it for too long on the
startup resistor, as it will get quite hot, being normally intended to
supply current to the chip, only for as long as it takes the supply to
fully start up and settle. If the voltage supply to the chip still
jiggles around when it is only being fed by the resistor, then after
the 47uF cap, which may yet be faulty, but not in an ESR way, the next
prime suspect would have to be the chip itself.

Arfa
Click to expand...
usually,the startup resistor doesn't supply enough current to keep the IC
running,just enough to start,that's why they have the housekeeping supply
from the XFMR. Otherwise,you get a "burst" or "chirp" mode of operation;the
PS starts,then dies because the IC isn't getting enough power.
Often,it's the housekeeping supply electrolytic cap that's gone bad,as you
said.

I've also seen where the start resistor goes high in value,even
open,preventing starting.(but not this guy's problem)

--
Jim Yanik
jyanik
at
localnet
dot com
 
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