Benq FP51g turning off completely after few seconds

[Joe Black]

Just as in title. When power ON is pressed, it turns on, but just to turn off after 4-5 seconds... (no LED, no faint image, no nothing... dead as dodo.)

It will work OK, after 10 minutes of ON-OFF cycles, though.

However, when I heat up upper right corner of the board with blow-dryer, it works perfectly every time. (those 5 elcos, inductor and diode)

Tested caps on ESR,they all look good. Tried replacing (with used ones, but known good), no result. But heating up works as charm.

Apart from caps, I don't know what else to try.
(tried resoldering all in that area-no result)

Thanks in advance for ideas!

[budm]

Re: Benq FP51g turning off completely after few seconds

'But heating up works as charm' heated the top side or the bottom side of the board?
You have some SMD parts on the bottom side that may be affected by heat.
BTW, when it does not work, do you lose the Voltage feeding the main board too, or lose the control signals from the main board?

BTW, did you replace the main filter cap? Isn't standing it up will still fit into the chassis?

[ReeceyBurger123]

Re: Benq FP51g turning off completely after few seconds

Some solder joints to me dont look so good either on the power devices. Try resoldering them:

[Joe Black]

Re: Benq FP51g turning off completely after few seconds

'But heating up works as charm' heated the top side or the bottom side of the board?

Heating only corner of the board, directly into caps.

BTW, when it does not work, do you lose the Voltage feeding the main board too, or lose the control signals from the main board?

Will check and report tonight or tommorow (GMT +2)

BTW, did you replace the main filter cap? Isn't standing it up will still fit into the chassis?

Replaced it a year ago or so, worked good till now. It fits easily

...and will do some more resoldering as sugested.

Thanks, guys! You rock!

[Joe Black]

Re: Benq FP51g turning off completely after few seconds

Here goes nothing

Monitor turns on, and shuts down immediately as backlight is lit.

On the connector to control board I measure 3.4 V, which drops for a second when backlight is lit, and stabilizes back as it shuts down. (I think it drops to 0V, but DMM can't catch it that fast).

Also, there's a fuse or something (pf 751) above the heatsink (cant be seen on pic) which reads near 0 Ohm, but as backlight turns on jumps to 170 Ohms or so, and back to zero as backlight shuts down.

Confirmed that heating with hair dryer works, but now picture gets lost, only the LED stays lit. (Was working good before if I heat corner with output caps, now it wont shut down completely but still loses image)

Resoldered mosfet receey pointed to. Immediately after that, monitor worked flawlessly, for a 10 or 15 mins. Then it all broke down again Tested mosfet -OK.

Tried to pull output caps out one at a time, no difference in behaviour whatsoever.

Got frustrated at myself 'cause I just seem to make no progress in repair skills, although I did some monitors before, and read a lot, every time it seems like I never did anything like this before.
Get confused so easily for whatever reason

[Joe Black]

Re: Benq FP51g turning off completely after few seconds

If I unplug either CCFL bulb, monitor will stay on, but without any light, just dark image that can be seen under flashlight...

I can unplug any bulb, and it works, logically bulbs are OK? (monitor won't shut down with upper plugged in nor with lower plugged in, just shuts off with both)

Meanwhile, changed caps to new, checked unfamous c5707s and their caps, and it seems power supply is OK...

[lion_sta]

Re: Benq FP51g turning off completely after few seconds

I saw in many videos that we should check that there are 12V and 5V on the appropriate exits. But there are no indication which one is ground and which ones are 5 and the 12.

How can we deduct that?

[Joseph cauchi]

Re: Benq FP51g turning off completely after few seconds

Check capacitors C723 and C739 sometimes the Benq have this problem I think these are 33j 6KV these are for backlight.

[Joseph cauchi]

Re: Benq FP51g turning off completely after few seconds

Sorry I think I mixed the caps these are 15j 3kv

[Joe Black]

Re: Benq FP51g turning off completely after few seconds

They are both 27J 3KV and measure 30 and 31 pF on ESR meter, repsectively.
Here is the shematics if someone experienced wants to take a look

[viziks]

Re: Benq FP51g turning off completely after few seconds

What you are saying is that if either of the lamp lights up,the monitor switches off.
Please check voltage on big mains filter 68uf 450V which should be stable around 320V. If this voltage drops when backlight lights up ,the Capacitor needs to be replaced.

[Joe Black]

Re: Benq FP51g turning off completely after few seconds

What you are saying is that if either of the lamp lights up,the monitor switches off.

Well, technically, lamps only light up in pair, I can't light up single one because when i unplug one the whole inverter wont start. But yes, when they light up (they both are lit for that half a second) monitor instantly switches off (no led).

Checked large cap, V is rock solid, doesn't drop at all.