LCD monitor won't turn on

Don't know if it helps but Google the number stamped near the top of the board (3150-0122). Got several hits and at least one source for a replacement board ($80).

IC18 to the left of the power socket? I can't read the type number other than it's made by International Rectifier..

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Adrian C

On Mon, 6 Oct 2008 09:51:14 -0700, "james" put finger to keyboard and composed:

It took my browser forever to render your X3 image (in fact I gave up waiting for it), but I finally worked out how to link directly to the photo:

The fact that the something is happening when you press the on/off button suggests that the uP is alive, and that the +5V(?) supply is OK. I suspect that one of the AmTRAN chips (AM30 or AM100B) controls the power to the backlights. It appears that Q15, the PNP (?) smt transistor near the lamp connector, switches the +12V power to an off-board inverter. I'd monitor the +12V supply on its Emitter (input) and Collector (output) pins. The Base should be getting an on/off signal from the uP via a resistor. Perhaps the inverter has a short circuit which has taken out Q15 and/or a fusible resistor.

BTW, the Mitsubishi M52743BSP chip is a "I2C BUS CONTROLLED 3-CHANNEL VIDEO PREAMPLIFIER", so I don't think it handles power control:

FWIW, a Nokia LCD 510L monitor appears to use the same chips, so maybe your Viewsonic is a rebadged unit.

See

- Franc Zabkar

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The first thing I would check are all those electrolytic caps. About half the LCD monitors I see just need caps. Andy Cuffe

snipped-for-privacy@gmail.com

Forget what I said about 2V. I measured the wrong pin.

Surprisingly viewsonic gave me the repair manual, but I have to sign a NDA. So I can't reveal too much here.

There is an IC that implement the power switch toggle effect. That is where the problem lies. The front panel power switch is connected to the clock input of this flip flop via some RC network but for some reason it doesn't want to flip. If I manually connect the +12V to the set or reset pin of this flip flop, I can turn the monitor on / off at will. So I can add a toggle switch to do this; it would just look a little ugly.

I hope other LCD manufacturers are as nice as viewsonic in providing circuit diagram. Has anyone ever obtained repair manual for other brand of LCD monitors?

On Wed, 8 Oct 2008 08:32:48 -0700, "james" put finger to keyboard and composed:

I thought you may have been measuring the +12V at the lamp/inverter connector. It may have helped if you were more specific.

BTW, does Q15 switch power to the inverter?

So you are talking about IC30, the MC14013B dual D-type flip-flop?

AFAICT, the circuit looks like this:

|----------------------| | | | |-- R -- Vcc | | | | | ____o___ | Vcc | | S | | | | | | | R |---| D Q*|-- R94 --+ | | | +_|_ C11 +----+---|>Clk Q |--> ___ 10uF 16V | _|_ | | on/off | / SW ___ |___R____| | | | C o _|_ _|_ _|_ | = = = |--- R --- Vcc

R94 and C11 effectively debounce the switch. If the chip is OK, then I'd suspect that C11 is open. Try connecting a 10uF cap in parallel with C11.

Mitsubishi Electric Australia will sell me a manual for mine, without an NDA. LG supply manuals for their LCD TVs, without an NDA. The FCC database has many circuits for LG monitors and other manufacturers, all of which you can download ... without an NDA.

- Franc Zabkar

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You're a walking IC encyclopedia? :)

Close enough. I tried assuming each of the component is bad and did a patch test (e.g. short the cap to Vcc or to gnd) but still couldn't make it work. I hooked up a scope on the Clk and it shows a pulse when the power switch is pressed. If I use a frayed wire to short the switch (this creates many pulses) then it sometimes work after many attempts. Increasing or decreasing the voltage by 1.5V doesn't help (use an ohmeter Rx1 range to short the switch).

I don't know if it's a good idea to try to replace this chip. It's surface mount and the chip is small (about 1cm) and the leads are even smaller.

I'll play with this until I get tired of it. If I can not figure out the definite cause then I'll just glue on an additional switch.

Thanks for the info. I didn't know FCC has circuits in their database.

On Wed, 8 Oct 2008 20:13:26 -0700, "james" put finger to keyboard and composed:

4000 series CMOS logic has been around for decades. Anyone from the old school would recognise that chip. BTW, I think that the R and S inputs should be active high, not active low as I've drawn them.

If you short the cap to ground, then the D input is always 0, which means that Q will always be a 0. If you tie the cap to Vcc, then Q will always be a 1 ... if the chip's clock input is working.

Practice on a scrap board.

Being CMOS, and the fact that the chip is connected to the real world (ie the switch), it may be that the clock input has been damaged by ESD. (???)

Neither did I until recently. Most circuits appear to have been added after 2000. Not only that, but there are user manuals, block diagrams, operational descriptions, and internal and external photos. They're not always good quality, but they might be better than nothing.

One major downside is that the "searchability" of the database is very poor. For example, I'd like to be able to locate all LG LCD monitors for which circuits have been submitted. Or I'd like to find all ADSL modems which use a Globespan Virata chipset. Maybe the FCC should let Google manage their database.

- Franc Zabkar

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On Thu, 09 Oct 2008 15:02:47 GMT, "spd" put finger to keyboard and composed:

This is it:

Grab the FCC ID off the label on your device and type the characters into the Grantee Code and Product Code boxes.

Note that most products will not have circuit diagrams because their manufacturers often choose to hide behind confidentiality provisions. I still managed to find about 100 circuits for LG LCD monitors and TVs, though.

- Franc Zabkar

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On 10$B7n(B8$BF|(B, $B2 >I have an old broken viewsonic VG150 LCD monitor. Basically, it cannot be

Could you please provide me Viewsonic VG150 repair diagram or manual? I also have a broken VG150.