Switchmode powersupply problem

[Creamed_emu]

My Laptop's external powersupply recently stopped working. upon measuring
the output voltages they are correct until a load is placed on them, eg the
laptop. they all drop down to zero volts. i measured the output of the
transformer and the voltage droped from 28v to around 2 or 3 volts. the
secondary uses a dule op-amp as a pwm to drive the opto isolator. With the
scope on the primary side of the opto it showed the narrow PWM. under load
it does not become wide but instead lowers in voltage eventually down to
zero volts. there is no frequency change either. On the primary side it uses
a 8pin PWM chip which appears to we working fine. could it be that there is
a short in the laptop. could i inject my own PWM signal into the opto and
widen it or would that cause damage to the laptop because of the excessive
voltage/current?

There are voltages on all the input and output pins of the op-amp. What do i
try next?

Thanks, Ben

 

[Rheilly Phoull]

"Creamed_emu" <shazzy@iprimus.com.au> wrote in message
news:4010f5c3_1@news.iprimus.com.au...

My Laptop's external powersupply recently stopped working. upon measuring
the output voltages they are correct until a load is placed on them, eg
the
laptop. they all drop down to zero volts. i measured the output of the
transformer and the voltage droped from 28v to around 2 or 3 volts. the
secondary uses a dule op-amp as a pwm to drive the opto isolator. With the
scope on the primary side of the opto it showed the narrow PWM. under load
it does not become wide but instead lowers in voltage eventually down to
zero volts. there is no frequency change either. On the primary side it
uses
a 8pin PWM chip which appears to we working fine. could it be that there
is
a short in the laptop. could i inject my own PWM signal into the opto and
widen it or would that cause damage to the laptop because of the excessive
voltage/current?

There are voltages on all the input and output pins of the op-amp. What do
i
try next?

Thanks, Ben


Why not try the supply into a load resistor and check it that way ??

--
Regards ........ Rheilly Phoull

 

[Jerry G.]

Test the supply with a properly calculated dummy load resistor(s) on its
outputs. Most of the time, many of the electrolytic caps in these go high in
their ESR causing failures of this nature.

Note that the manufacture will not support servicing these types of
supplies. If you need proprietary parts, or service information these will
not be available. These are normally replaced as a complete module.

--

Greetings,

Jerry Greenberg GLG Technologies GLG
=========================================
WebPage http://www.zoom-one.com
Electronics http://www.zoom-one.com/electron.htm
=========================================


"Creamed_emu" <shazzy@iprimus.com.au> wrote in message
news:4010f5c3_1@news.iprimus.com.au...
My Laptop's external powersupply recently stopped working. upon measuring
the output voltages they are correct until a load is placed on them, eg the
laptop. they all drop down to zero volts. i measured the output of the
transformer and the voltage droped from 28v to around 2 or 3 volts. the
secondary uses a dule op-amp as a pwm to drive the opto isolator. With the
scope on the primary side of the opto it showed the narrow PWM. under load
it does not become wide but instead lowers in voltage eventually down to
zero volts. there is no frequency change either. On the primary side it uses
a 8pin PWM chip which appears to we working fine. could it be that there is
a short in the laptop. could i inject my own PWM signal into the opto and
widen it or would that cause damage to the laptop because of the excessive
voltage/current?

There are voltages on all the input and output pins of the op-amp. What do i
try next?

Thanks, Ben