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Dodgy Benq/Dell monitor fixes?

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LAN_403

Conundrum

Mon Mar 17 2008, 08:18PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

Hi all.

I have six 17" TFT panels here which are dead.
Of those, one turns on but "flashes" and the rest seem to be stuck in standby.

Has anyone else here attempted to fix these beasties?

I include this snippage, taken from

also

is handy as it details a particular failure mode of a BA9741F driver IC which can result in the "3 seconds of light" fault.

"following this thread and finally decided to contribute. I hopefully this may shed some
light on thing or twoâ€¦ or possibly confuse some. To my knowledge the following is
pretty accurateâ€¦ I tried to simplify the operational aspect to the best of my ability w/o
going too overboard. So
This is for those who have had no success after replacing/testing Q759 Q760 Q739 Q740
(2SC5707), and the inverter transformers T752 T754 T753 T751 (type: 9.5uH EE19).
{In order to test the transformers youâ€™ll need to use a ringer and or a Henry meter, most
electronic repair shops have the ability to test these components using a component
analyzer}. Ideally shorts in these components result in IC751 detecting the â€œshortâ€ or
â€œopenâ€ (or just a bad lamp) and shutting down the PWM in order prevent further damage.
Keep in mind there are two separate but identical inverter circuits driving two CCLF
outputs.
Also, when testing components such as transistors, capacitors, resistors, etcâ€¦ Desolder
them and test them separate from the circuit. Tip; if you are testing a two terminal device
(i.e. resistor) you only need to remove one leg of the device before testing.
The key component of this inverter circuit is IC751 (TL1451AC) a PWM dual power
supply controlâ€¦ For those unfamiliar with PWM; PWM is when a voltage or current
source is supplied to an analog load by means of a repeating series of on and off pulses.
The on-time is the time during which the DC supply is applied to the load, and the off time
is the period during which that supply is switched off. A feedback is provided from
the output in order maintain a specified voltage and frequency by varying the on-time
and off-times.---Anyway, IC751 pulses Q751 or Q743 which act as electronic switches
(small amount of power to control the flow of a large amount of current). The Pulsed
signal is then fed into an inverter circuit i.e. Q759 Q760 or Q739 Q740â€¦ (Basically each
set compose a push-pull circuit) which is tied to two paralleled transformers boosting the
Voltage to around 600Vrms in order to fire the CCFL lamps. Feedback from CCFL
lamps is provided via D753 D764 R762 and D767 D758 R742 respectively (**very
important** - a faulty component in this section will overdrive the output i.e. Q759â€¦).
The figure/diagram below shows the type of configuration in this board (Royer circuit). â€“
{Feedback Diodes are 1N4148â€™s}
Basically, IC751 contains an oscillator circuit in whoseâ€™ frequency is controlled by C765
(470p 50v) and R765 (10K 1%). ***(these values are critical for proper operation of the
oscillator circuitry)*** If the values of C765 and/or R756 drift out of tolerance. The
frequency of the circuit will exponentially change. This is of particular importance when
selecting a replacement transistor Q759 Q760â€¦ check the frequency band-width, also a
fast switching transistor type is a must. When dealing with frequency in the 50 â€“ 100+
KHz range internal resistance and capacitances can cause excessive Vce leakage
therefore causing increased heat dissipation (eventually leading to thermal
runawayâ€¦voltage punch-thru). Check these things; C765 and R765, which control the
oscillator frequency oscillator and may be the reason for many of the â€œHOTâ€ Q759
Q760//Q739 Q740 transistors??? {The tables below were obtained from the component
data-sheet showing the effects of changing Ct~C765 and Rt~R765}
I have read that some of you are checking voltage across D751 or D761â€¦ This is a bad
point to accurately check the circuit unless you are checking with an Oscilloscope. A
typical voltmeter has a relatively low impedanceâ€¦ meaning anything above the 1 KHz
range is will not be a valid measurement. At this point is a high frequency pulse width
modulated (PWM) signal, not a typical AC sinusoidal voltage. â€œOkâ€, well if you see
voltage across bothâ€¦ this means that a PWM signal driving the notoriously failing
2SC5707 trannyâ€™s; Q759 Q760//Q739 Q740.
However, if you want to check voltage; measure across IC751 pinâ€™s 8~ (ground) and 9~
(+15Vdc) there should be 15Vdc. If there is NOT voltage check Q761 (p-type bipolar
transistor A733)â€¦ {Q761 regulates the DC voltage to IC751 via biasing resistors R752
R754 and â€œturn onâ€ transistor Q757}.
If there IS +15Vdc, check/replace Q751 Q743 and IC751â€¦ Q751//Q643 are MOS type,
they are the â€œswitchâ€ which induce the PWM voltage into the transformers by injecting
Q759 Q760//Q739 Q740 respectively.
Also, test/check/ (replace if unsure): C764 (1uF 50v #105C), C754 C755 C734
C735 (all .15uF 250V MKP type), R768 R769 R748 R749 (all 1Kohm 1/2w),
D761 D761 (SB360 schottky diode), C771 C772 (220pF 50V)
â€¦THE FOLLOWING IS ONLY IF YOUR MONITOR IS COMPLETELY â€DEADâ€ no
LED light ON indication flash etcâ€¦
(Assuming youâ€™ve checked the fuse) Check for voltage on IC601 (3842B) pins 5-
gnd 7-Vcc; If there is voltage; test/replace Q603 (n-type bjt H945) Q602 (n-type
MPSA44) R602 (68Kohm 2w) and IC601.
If there is NO voltage on IC601â€¦ replace optical isolator IC602 (PC123F1), and
check/replace IC702 (TL431) and D701 (UGF10DCT).
The following is a schematic from a very similar model Benq Inverter Circuitâ€¦
Something that I find interesting is that on my board is a 100uF 25v cap for C772
however a 22uF 50v (#105C), is on this boards schematic for an LG monitorâ€¦
Interestingly this capacitor determines the protection enable time characteristics
for the inverter limiting an over current condition. (Graph was taken from IC
751â€™s datasheet)."

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