Backlight constantly On after repair

[kingzzz]

Hi guys, My Benq Fp91g had the backlight problem. Ordered a repair kit from Ebay and replaced all 4 c5707 transistors and the 3A picofuse. One of the transistors was shorted on the pins so I guess that was the problem. Now it works but the backlight is always on even when I press the button to turn off the monitor and the LED indicator shows it is off. If leave the monitor alone and the PC goes to sleep, the same thing happens with the backlight being fully on when the LED indicator is orange. There appears to be no problems during normal usage ( only tested for about 30mins though) but the monitor seems to get quite warm, I cant remember if this was normal or not.

Any ideas on what could be causing the backlight to be constantly on?

[kingzzz]

Re: Backlight constantly On after repair

ha, looks like i didnt take enough care when testing, It appears one of the fqu11p06 fets was shorted, replaced both and now monitor is all good. When taking out of the the fets one of the tracks got lifted. Had to ghetto wire it directly to the pad the trace it was connecting to. Hopefully nothing blows up in my face . This was the middle leg of the fu024n mosfet

[Krankshaft]

Re: Backlight constantly On after repair

No worries I've had a few Benq boards with a surface mount FET that did the same thing. When there was over current and a C5707 transistor shorted the heat incinerated one of the traces on the On / Off FET.

Extending the leads and soldering directly to a trace or jumper wiring is fine.

If it's a through hole component and you solder directly to a thin trace instead of using a stranded jumper wire just make sure that you don't bend the component after soldering since the tiny trace is more likely to rip off due to less surface area.

[kingzzz]

Re: Backlight constantly On after repair

Yea it was a through hole component. I pretty much just bent the leg and it happened to reach the next component directly. So the track was bypassed altogether. Monitor is not running so warm now either so looks to be a good success.

[retiredcaps]

Re: Backlight constantly On after repair

Benq boards that require a c5707 replacement also *must* have their inverter transformer pins all resoldered. Otherwise, you may be ordering another repair kit in the near future,

[kingzzz]

Re: Backlight constantly On after repair

Haha speak of the devil, the monitor just died again today, and have ordered another kit, good thing they arent expensive. Live and learn I guess.

regarding re soldering. Just heat the solder up/apply a bit more to all the pins and that's it?

[jetadm123]

Re: Backlight constantly On after repair

Since the manufacturer probably used lead free solder, ideally, you should remove the old solder using desoldering braid or a solder sucker bulb and reapply fresh lead/tin solder.

[Wizard]

Re: Backlight constantly On after repair

The flux that is used with the no-lead solder is so ineffective, I prefer in some situations to remove these transformer and lightly scrape the pins with razor knife.

Cheers, Wizard

[kingzzz]

Re: Backlight constantly On after repair

have replaced all 4 c5707s and resoldered the transformers, monitor is back to life. We'll see how long it lasts this time.

this time however none of the replaced c5707s had any shorts between the pins so i wasnt sure which one was actually broken. if there a good way to test?

[Toasty]

Re: Backlight constantly On after repair

There is a thread on the forums about MOSFET testing, but I can never find it when I need it.

Google: mosfet test procedure

The first few results are good enough. You're going to use your DMM on Diode test to switch the MOSFET on and off. Hopefully yours produces enough volts to cause the device to work. If you have a 2nd meter, test them against each other to see what volts are produced when in Diode test position.

The 4QD-TEC page explains this. The ezine page does not, but has you check the protection diode if it exists in the C5707.

Toast