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Benq FP737S LCD monitor backlight - won't go off (longish).

Received a Benq Q7T3-FP737S for repair, symptom no backlight. These are notorious for inverter failures. Schematics at

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Found PF751 open, FETs Q743/751 OK, D751 OK, Q739/740 OK, and one of Q759/760 (2SC5707) shot. All four inverter transformers showed expected (and matched) readings on an ESR meter, which of course doesn't completely exclude a shorted turn or two. Replaced fuse, Q759 and Q760. Unit came to life on test signal, and I commenced smoke test aka burn-in.

Note that unlike some LCD's (such as many Acers where there is the opportunity to run the thing with the back shield removed), it is not practical to access the pcb - either side - while all connections are in place.

Noticed a charring smell after about ten minutes, but left it running as I wanted a smoking gun. I'd rather have a blackened/dead component than a mystery. Left it running for several hours, no smoke, no failure, and the temperature above the vents was fairly much normal. Eventually I did a quick "shut-down, tear-down and feel" I found nothing showing residual overheating signs or anything that suggested a charring smell.

On next powerup, with the soft-on switch ON, the screen illuminated with the (normal) BENQ logo on a purplish background, then the expected "no signal detected" on a black background, and shortly - when I would have expected a normal black screen to appear - the screen illuminated blank white. The backlight was not inhibited! On turning the soft-on switch OFF, the LED extinguished but the backlight remained on still. This behaviour is repeatable - with the soft-on OFF and AC applied, the backlight is on.

From the schematic, the backlight_on signal simply kills Vcc to the TL1451 PWM controller, which *should* turn the FETs Q743/Q751 off and hence kill the +15V rail to the inverters. But being unable to check these lines while powered up, it's a bit of a puzzle. I don't want to resort to trial-and-error replacement if I can avoid it - a 17" LCD isn't going to be worth the extra time.

Anyone with experience or constructive suggestions?

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who where
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AArgh! Didn't realise what Flickr was going to do to the schematic.

Apologies for the ads, try

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Reply to
who where

On Mon, 14 Dec 2009 12:48:51 +0800, who where put finger to keyboard and composed:

Here is another service manual.

Diamond View LCD Monitor DV172 / DV172B Service Guide:

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- Franc Zabkar

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Please remove one 'i' from my address when replying by email.
Reply to
Franc Zabkar

Franc, I'm right for the full Benq SM too. The schematic I posted ("Diamondview") was a better image than the Benq, and identical - presume they use the same Benq board.

What I am really looking for is some inspired suggestions on what else is prone to die in these models other than inverter bipolars and SMPS caps. Maybe I'll need to tack some extender leads onto the points of interest so I can check them when the unit is reconnected .

Reply to
who where

zlava had written this in response to

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: I met same problem. PF751 open, FETs Q743/751 OK, D751 OK, Q739/740 OK, and Q759 shot. I replaced Q759 and fuse but after a couple of minutes problem appeared once again. So I replaced once again Q759 and Q760. I checked Q760 and it was little suspicion because of different resistivity measurements. After these replacements everything works good.

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zlava

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