Sanken SMPS PSC10089G M faulty

[spdf]

This is my first time I am trying to repair switching mode power supply that I pulled off from old LG plasma TV. After a week or so I am still at it and I cacn't make thibgs right.

Model Sanken PSC10089G M I couldn't find any schematics of it.

At first it was completely dead no output voltage and no standby voltage what so ever. I tried to measure some voltages and I found out that I have ~300V on primary filter caps and that's all. After changing power IC KA7553 standby voltage came a live BUT after trying to powering it on in series with mains 100W bulb relay clicked and bulb began to shine at full brightness - dead short. (I disconnected all smps loads and soldered some pull down resistors to start power supply) I literally checked every component with DMM and couldn't isolate any shorted components in circuit so now I am beginning to suspect switch mode power transformer, but I already know from reading that it is highly unlike that this transformer went bad.

it would be great if someone could give me advice or point out where to look for a clue. ;]

[IanMacdonald]

SMPS transformer failure is extremely rare, at least in quality kit. It's more likely you have a short of some kind on the primary side.

[spdf]

Kinda running out of options where to check. In primary side Bridge rectifier is good (220V on AC ~300DC) primary filter caps ~300V
3x power mosfet's all good, diode (in photo) good voltage regulation IC's has no obvious short on them. Measured voltages on first transformer got ~300V on both sides. Fuse never was blown.

After relay klicks it stays on with bulb lit at full brightness for minute or so than it turns off (protection?)

[Armadillo]

Kinda running out of options where to check. In primary side Bridge rectifier is good (220V on AC ~300DC) primary filter caps ~300V
3x power mosfet's all good, diode (in photo) good voltage regulation IC's has no obvious short on them. Measured voltages on first transformer got ~300V on both sides. Fuse never was blown.

After relay klicks it stays on with bulb lit at full brightness for minute or so than it turns off (protection?)

How come one device is not installed.?

There are labels on those connectors with voltages shown. After powering up, measure the connector pin voltages and note all the connectors voltages, from there you can tell which one of the circuit/power is out of spec or shorted.

[spdf]

How come one device is not installed.?

I measured it out of circuit, just to be sure.

There are labels on those connectors with voltages shown. After powering up, measure the connector pin voltages and note all the connectors voltages, from there you can tell which one of the circuit/power is out of spec or shorted.

Did that. stand by voltage pin - 5V as expected and
5V pin shows 1.2V everything else 0.

[spdf]

Ok I've found something interesting

On L4981A power factor corrector IC if I place my meter probe on pin 15 suddenly bulb in series with mains dim away but relay stays activated.

PIN 15 P-UVLO Programmable under voltage lock out threshold input. A voltage divider between supply
voltage and GND can be connected in order to program the turn on threshold.

So I gues something is not right with input voltage for this IC, or does that mean faulty IC on it's own? Voltagte at Vcc pin 19 10V

I atached datasheet for IC and bord's photos.

[IanMacdonald]

PFC circuits can be difficult to test this way. If the input voltage drops due to series resistance, the step-up converter starts to drive like crazy to bring it back up, which results in the bulb dropping all of the voltage and the PSU none. You may need a couple of bulbs in parallel, or to disconnect the PFC transistor collector/drain  from the series coil whilst doing first tests. Or just take it out, possibly easier.

[spdf]

And that gentelmen there a short was! Power correction IC. Took me awhile..

Thanks for the input and help.