Fluctuating Voltages

[onyxx]

Hello, I am working on a SMPS from a PC monitor. Voltages across the
circuit are all wrong and not only that but they are fluctuating as
well. I started by checking the AC voltage from the outlet through the
monitor power cable, it is a steady 221Vac (I am in South America,
things are a little more dangerous here..lol) but when i plug in the
monitor, ac voltage starts fluctuating from 215Vac to 221Vac and all
voltages in between. I can trace this problem from the ac plug, all the
way to the SMPS controller, bridge rectifier and main filter cap as
well. So the problem occurs both on the ac and dc sides of the PS.
Another thing that is happening is that if i measure voltage on the
chopper resistor´s Drain i get over a thousand volts!!!!! DMM goes
blank....I've compare the MOSFET to one that is working and this one
seems fine.
I thought that the chopper transformer was shorted but I´ve replaced
it with a good one and the same thing happens. Can any one help me?

Thank you...

 

[James Sweet]

Hello, I am working on a SMPS from a PC monitor. Voltages across the
circuit are all wrong and not only that but they are fluctuating as
well. I started by checking the AC voltage from the outlet through the
monitor power cable, it is a steady 221Vac (I am in South America,
things are a little more dangerous here..lol) but when i plug in the
monitor, ac voltage starts fluctuating from 215Vac to 221Vac and all
voltages in between. I can trace this problem from the ac plug, all the
way to the SMPS controller, bridge rectifier and main filter cap as
well. So the problem occurs both on the ac and dc sides of the PS.
Another thing that is happening is that if i measure voltage on the
chopper resistor´s Drain i get over a thousand volts!!!!! DMM goes
blank....I've compare the MOSFET to one that is working and this one
seems fine.
I thought that the chopper transformer was shorted but I´ve replaced
it with a good one and the same thing happens. Can any one help me?

Thank you...

The high voltage at the transistor is normal, it's caused by the inductive
kick of the transformer, I've never actually seen a switching transformer
fail so that's never really been a suspect.

How are the capacitors on the output side? The power supply should easily
compensate for the fluctuating input voltage.

 

[onyxx]

Hello and thanks. You say that the high voltage is normal...but is it
normal if it stays like that always? If it is caused by the inductive
kick of the trans...should it not just be a temporary thing?
And yes, all caps SEEM fine, all i have to check them is my little
piece of $&·% DMM.