Benq T905 power supply repair

Repairing LCD monitor Benq T905 Q9T4 power supply, it had the mains filter capacitor open circuit and leaky and the fuse and the switching transistor blown. Replaced these components, for the transistor I used a P6NC60FP in p lace of the original P7NK80ZFP, some people used 2SK2645 that is more simil ar to mine than the original. Checked all capacitors for ESR and all diodes and resistors in the supply section.

Here is its schematic:

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As usual I wired a light bulb in series with L605 to protect the transistor and it almost works. In standby the supply runs and regulates fine and wit h the backlighting disabled it runs the video processor fine. But with back lighting enabled runs for a short time while it displays the BenQ logo, the n all goes off with a flash from the transistor protection light bulb. I ca n repeatedly press the power button and it keeps doing that, it seems the t ransistor+fuse would blow without the light bulb.

I've observed that increasing the light bulb wattage increases the time the supply runs. A 100W bulb runs for 1 second, 160W gets 2 seconds and 300W h alogen runs for 4 seconds. Could the inserted bulb or its wiring somehow di sturb the power supply as to fail discharging the mains filter capacitor th rough the transistor? I'm reluctant to try without any bulb as I don't want to fry the transistor.

Reply to
Jeroni Paul
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r capacitor open circuit and leaky and the fuse and the switching transisto r blown. Replaced these components, for the transistor I used a P6NC60FP in place of the original P7NK80ZFP, some people used 2SK2645 that is more sim ilar to mine than the original. Checked all capacitors for ESR and all diod es and resistors in the supply section.

** If the mosfet blew, R615 must have too.

Did you maybe replace it with a wirewound type ?

That would explain most of you symptoms, running OK at low loads but failin g if the current in the switch is higher - also made worse if the AC supply voltage is lower as it will be with a lamp in series.

R615 ( 0.22 ohms 2 watts ) must be a metal film or composition type.

IOW *low* inductance is crucial.

..... Phil

Reply to
Phil Allison

Thank you Phil. The resistor you mention was good and is the original. Finally I got it working by wiring 4 100W bulbs in parallel, so I removed the bulbs and all is fine.

Reply to
Jeroni Paul

** That SMPS is unusually sensitive to low AC supply voltage.

Normally they work from 85V and up.

.... Phil

Reply to
Phil Allison

r capacitor open circuit and leaky and the fuse and the switching transisto r blown. Replaced these components, for the transistor I used a P6NC60FP in place of the original P7NK80ZFP,

Before shipping it monitor the temperature of that mosfet. The current, vo ltage, and wattage ratings are similar but sometimes a sub just isn't happy .

Reply to
ohger1s

I did, after two hours the mosfet heat sink felt warm to the touch, comparatively the secondary rectifiers were quite hotter.

Reply to
Jeroni Paul

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