BenQ Q7C3 (FP767Ver2) can not be turned off by the front push button

[ArizaazirA]

BenQ 17" LCD, Model: Q7C3, made on October 2004 Rev:BBD, Part #FP767 Ver2

This monitor started to develop problem with the front push button power switch about a month ago. Initially, it can not be turned on by pushing the power switch on the front panel, however, after swapping the power cord and other stuff, it can be turned on by the front power switch and worked for about a week.

After it refuse to turn itself on again, it was taken apart, all the components, both on the power supply board and the signal board, looks OK. There are no any burned parts or bulged capacitors. It was put back together, and this time, instead of not being able to switch it on, it become not possible to turn it off by the front push button, the only way to turn it off is to pull the power cord.

Any suggestions are highly appreciated.

[retiredcaps]

Re: BenQ Q7C3 (FP767Ver2) can not be turned off by the front push button

Any suggestions are highly appreciated.

It would help to have clear focused pictures of your boards so we can indicate out test points. Please use the manage attachments function to upload your pictures (2000x2000 is the max resolution). Please do not post pics inline as they slow down the loading of pages.

[ArizaazirA]

Re: BenQ Q7C3 (FP767Ver2) can not be turned off by the front push button

Here is goes, let me know if it would be helpful to focus on any particular part on the board.

Thanks.

[retiredcaps]

Re: BenQ Q7C3 (FP767Ver2) can not be turned off by the front push button

Can we also get a picture of the logic board and the on/off panel board?

Do you have a multimeter to take voltage measurements?

[Th3_uN1Qu3]

Re: BenQ Q7C3 (FP767Ver2) can not be turned off by the front push button

My best guess would be that the button is simply stuck. Do you hear/feel it clicking when you push it? If not, there's a good chance it's stuck.

[ArizaazirA]

Re: BenQ Q7C3 (FP767Ver2) can not be turned off by the front push button

Here is the other two boards, they look OK.

Just checked the power push button sw306 and it functions normal.

All bus voltage are there, +5, +3.5, +/-5, +/-16 V.

[mockingbird]

Re: BenQ Q7C3 (FP767Ver2) can not be turned off by the front push button

Looks like you have a plague of Elite and Lelon capacitors.

On your PSU, everything should be changed. If you are feeling adventurous, you should also recap the logic board.

The second last pic, can you turn it around and take a photo? I have a feeling that there might be a cap or two on there as well that warrants replacing.

[ArizaazirA]

Re: BenQ Q7C3 (FP767Ver2) can not be turned off by the front push button

Yes, there are tow capacitors on the board with all the push button switches.

I will replace all the Elite and Lelon capacitors.

[mockingbird]

Re: BenQ Q7C3 (FP767Ver2) can not be turned off by the front push button

Make sure you check out the specs of the Lelons and Elite so you know what to replace them with.

[kaboom]

Re: BenQ Q7C3 (FP767Ver2) can not be turned off by the front push button

Don't forget to resolder the area around the inverter! Specifically the transformers, tuning caps, jumpers, 2SC5707s, and the PWM brightness buckverter. As well as anything else that looks questionable.

-Paul

[ArizaazirA]

Re: BenQ Q7C3 (FP767Ver2) can not be turned off by the front push button

Thanks everyone for the helpful suggestions and insightfulness.

All the Elite and Lelons' capacitors have been replaced ( total 39) on both the power board and the logic board. The monitor is working again and , hopefully, it will keep working for a long time.

[ArizaazirA]

Re: BenQ Q7C3 (FP767Ver2) can not be turned off by the front push button

Here is the update, after working for about 2 hours, the monitor can not be turned off by the front push button again.

It seems that there are some components that is fine when everything is not hot, but start melfunctionion after warmed up. All the button on the front panel is not functioning.

Will try again after everything cooled down to test the above hypothesis.

Any ideas and suggestions are highly appreciated.

[mockingbird]

Re: BenQ Q7C3 (FP767Ver2) can not be turned off by the front push button

It sounds like you may have used the incorrect replacements.

[kaboom]

Re: BenQ Q7C3 (FP767Ver2) can not be turned off by the front push button

After you run it til it no longer responds to the front panel controls, cycle the power by unplugging it, then plug it back in.

Does that make the controls work? If not, try leaving it unplugged for anywhere from 1 to 10 minutes; now plug it back in and check the controls again.

If the controls only become responsive after letting it 'cool,' try to run the unit naked. Get some wooden blocks or polypropylene/polyethylene items (tupperware) to hold things in place. This will change things since you've got ambient air keeping things cool, but look for hot spots on that signal/logic board.

Speaking of which, on the logic board, U5 is a voltage reg. Check if it's shorted between 'I' and 'O' pins. If ckt resistance prohibits you from getting a meaningful reading, check that the voltage at the 'O' pin is less than that of 'I.' If those ICs get too hot from too high a supply voltage, they may still work but slowly overheat. You've got 5 & 3.3v from the PS board, so it's probably either 5 or 3.3v down to 2.5 or 1.8v.

If after your controls freeze up, try changing your resolution on the computer. Will change the scan format being sent to the monitor, forcing the logic to readjust to the different signal. Go from, say, 1280x1024 to 800x600 and see if it still expands/resyncs.

I'm thinking a heat issue with those IC(s)...
-Paul

ps- What did you use for replacement caps, brand and series?

[ArizaazirA]

Re: BenQ Q7C3 (FP767Ver2) can not be turned off by the front push button

After overnight cooling, the monitor start to response to the front power push button again, this indicates that the melfunctioning is reversible event(s)?

U5 has 3.3 V and 2.51V and is reasonably stable.

The replacements are Rubycon SL360.

[Tracy]

Re: BenQ Q7C3 (FP767Ver2) can not be turned off by the front push button

zener diode did the same thing to me this week. I was working on a musical keyboard and one of the c sharp key was stuck on high volume. I had resoldered the diodes last week thinking maybe just a cold joint it worked so i sent it on with the customer. the unit came back with the same problem. I realized that when I heated the diode up, it worked fine, but leave it sitting for overnight and the key was stuck high again. I touched the legs with the iron to heat it up and it worked fine. So, I changed the zener which was a 4148 and everythings fine even cold start. p.s. the zener checked fine cold and hot with the meter.

[ArizaazirA]

Re: BenQ Q7C3 (FP767Ver2) can not be turned off by the front push button

More up-date for this monitor:

1) the front push button is completely out of services now, the blue LED lite up as soon as the power is supplied to the monitor; if PC signal connected to the interface board, the monitor works like normal, if no PC signal, the blue LED turns into yellow. In either way, pushing the PW button does absolutely nothing.

2) hooking up a multimeter to these connector pins between the power board and the interface board shows that the bus voltage +5 and +3.3V are there; P12 and P3.3V could not been read by multimeter; BLT_ON has 3.3V while the LED is blue and 0V when the LED turns into yellow; PW_DOWN has ~4.4 mV reading on the meter while the LED is blue and 0V when the LED turns into yellow.

3) Check all the transistors on the interface board and they seem OK, at least there are no shorted one;

Here is the rub, it is likely that the power board is OK, and the interface board is where the problems resident, how can one check the interface board without a gears?