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

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

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?

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

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

Apologies for the ads, try

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

finger to keyboard and composed:

Quoted text here. Click to load it

Here is another service manual.

Diamond View LCD Monitor DV172 / DV172B Service Guide:

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

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

On Mon, 14 Dec 2009 16:06:51 +1100, Franc Zabkar

Quoted text here. Click to load it

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 <sigh>.

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

zlava had written this in response to
I met same problem. PF751 open, FETs Q743/751 OK, D751 OK, Q739/740 OK,
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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