Benq T905 power supply repair

[Jeroni Paul]

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 place of the original P7NK80ZFP, some people used 2SK2645 that is more similar to mine than the original. Checked all capacitors for ESR and all diodes and resistors in the supply section.

Here is its schematic:
http://www.rom.by/files/q9t4-fp91g-power_bd_1.pdf

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 with the backlighting disabled it runs the video processor fine. But with backlighting enabled runs for a short time while it displays the BenQ logo, then all goes off with a flash from the transistor protection light bulb. I can repeatedly press the power button and it keeps doing that, it seems the transistor+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 halogen runs for 4 seconds. Could the inserted bulb or its wiring somehow disturb the power supply as to fail discharging the mains filter capacitor through the transistor? I'm reluctant to try without any bulb as I don't want to fry the transistor.

 

[Phil Allison]

Jeroni Paul wrote:

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 place of the original P7NK80ZFP, some people used 2SK2645 that is more similar to mine than the original. Checked all capacitors for ESR and all diodes and resistors in the supply section.

Here is its schematic:
http://www.rom.by/files/q9t4-fp91g-power_bd_1.pdf

** 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 failing 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

 

[Jeroni Paul]

Phil Allison wrote:

Jeroni Paul wrote:
Here is its schematic:
http://www.rom.by/files/q9t4-fp91g-power_bd_1.pdf


** 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 failing 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

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.

 

[Phil Allison]

Jeroni Paul wrote:

** 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 failing 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.



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.

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

Normally they work from 85V and up.



..... Phil

 

On Tuesday, April 18, 2017 at 6:18:30 PM UTC-4, Jeroni Paul wrote:
> 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 place of the original P7NK80ZFP,

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

 

[Jeroni Paul]

ohg...@gmail.com wrote:
> Before shipping it monitor the temperature of that mosfet. The current, voltage, and wattage ratings are similar but sometimes a sub just isn't happy.

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