Sharp Aquos LC-37LE320E no back light?

[Dang]

Just bought this today for £20 so well worth a punt, everything seems to work as I can hear sound and shinning a light on the screen shows that a picture is being produced, there's just no back light, I've checked visually as best as I can all the components, mainly PSU, but nothing is obvious so I ask if anyone know's where to start with this issue, please, I would really appreciate your help, thanks.

[philliesfan30m]

Re: Sharp Aquos LC-37LE320E no back light?

good clear pictures of the back of the tv with the cover off and individual pics of each of the boards in the tv

[Dang]

Re: Sharp Aquos LC-37LE320E no back light?

Here's some photos of the Tv, I've measured the voltages from the red and yellow wires coming out from the top of the power supply, it says on the board that they should be +60v, I'm measuring 24v for all four wires, is this correct or has it dropped from the required 60v?

[tom66]

Re: Sharp Aquos LC-37LE320E no back light?

Grr... Nothing about this made by Sharp. Just another rebadge! And I thought their LED sets were OK. If measuring only 24V, it sounds like the 24V -> 60V boost converter has failed.

Schematic in SM. 1st result.
http://www.tgohome.com/ServiceManual...&Search=Submit

use 42" schematic as that is same PSU, 37" is different even though your TV is 37".

Check points, power on and out of standby:
- one of C907/C916/C918 on primary/HOT side -- around 370~410V when TV powered up
- voltage across C943 on secondary (1000uF/35V) -- around 24V
- voltage across C945/ on secondary (100uF/160V) -- around 60V
- pin 1 of CN902, red on pin, black on chassis, should measure >2V

Power off, unplugged:
- D985: check not shorted
- R904/R906: check should be shorted (<0.05 ohm) in parallel though meter would show around 1 ohm, this is OK

[Dang]

Re: Sharp Aquos LC-37LE320E no back light?

Thank you once again Tom66, I'll get on it in the morning, Andy.

[Dang]

Re: Sharp Aquos LC-37LE320E no back light?

Grr... Nothing about this made by Sharp. Just another rebadge! And I thought their LED sets were OK. If measuring only 24V, it sounds like the 24V -> 60V boost converter has failed.

Schematic in SM. 1st result.
http://www.tgohome.com/ServiceManual...&Search=Submit

use 42" schematic as that is same PSU, 37" is different even though your TV is 37".

Check points, power on and out of standby:
- one of C907/C916/C918 on primary/HOT side -- around 370~410V when TV powered up
- voltage across C943 on secondary (1000uF/35V) -- around 24V
- voltage across C945/ on secondary (100uF/160V) -- around 60V
- pin 1 of CN902, red on pin, black on chassis, should measure >2V

Power off, unplugged:
- D985: check not shorted
- R904/R906: check should be shorted (<0.05 ohm) in parallel though meter would show around 1 ohm, this is OK

Hi again, right, I've checked R904/R906 and both show as expected, with D985, on each pin it seems to be charging up, so I'm guessing a capacitor is taking a charge and this will happen on all three pins.

When it comes to powering up the Tv and checking the voltages on the capacitors hot side, do I use a ground screw and then the red probe to positive pin?

[tom66]

Re: Sharp Aquos LC-37LE320E no back light?

For the hot side capacitors please check with the meter probes on each end, i.e. red on positive black on negative of capacitor, you only need to check one of those.
If all those suspects have been ruled out, then it's probably the little 8 pin IC for the boost converter. Hopefully you can buy that on ebay or elsewhere otherwise you'll have to source a PSU.

[Dang]

Re: Sharp Aquos LC-37LE320E no back light?

For the hot side capacitors please check with the meter probes on each end, i.e. red on positive black on negative of capacitor, you only need to check one of those.
If all those suspects have been ruled out, then it's probably the little 8 pin IC for the boost converter. Hopefully you can buy that on ebay or elsewhere otherwise you'll have to source a PSU.

OK, so the C916 measures at 400v, C945 is at 24v, should this one be at 60v?, C943 is also at 24v but CN902 measures 4.18v using the very bottom pin, 5v is present at the very top but for 2v it's the yellow wire second from bottom, any thoughts? also this 8 pin IC, is it just to the right of the 3 450v caps or the underside of the PSU? once again thank you, always helpful and of course appreciated, Andy.

[tom66]

Re: Sharp Aquos LC-37LE320E no back light?

Yes, so if the diode is testing OK, I suspect the chip on bottom for it is bad. Page 207 of that file linked, IC902. Available here: http://www.ebay.co.uk/sch/i.html?_tr...at=0&_from=R40

If you have a scope you can do some checks on the various waveforms produced.

You can also try checking pin 8, Vcc. There looks to be a linear reg circuit which could go bad. Should be around 12~16V.

[Dang]

Re: Sharp Aquos LC-37LE320E no back light?

Yes, so if the diode is testing OK, I suspect the chip on bottom for it is bad. Page 207 of that file linked, IC902. Available here: http://www.ebay.co.uk/sch/i.html?_tr...at=0&_from=R40

If you have a scope you can do some checks on the various waveforms produced.

You can also try checking pin 8, Vcc. There looks to be a linear reg circuit which could go bad. Should be around 12~16V.

Thanks Tom66, yet to test the voltage at PIN 8 VCC but can I ask, is there anything else it might be? I've studied the schematic as closely as I know how and I can't really tell where this 60v is coming from, forgive my lack of knowledge but how does this IC work and from what PIN might I find this illusive 60v or is there some other mechanism at work to produce this?

[tom66]

Re: Sharp Aquos LC-37LE320E no back light?

It is a boost converter.

An inductor is the core of such a converter. An inductor is a device which resists change in current by creating a voltage drop; internally, it works by building up a magnetic field. When current through an inductor is interrupted, a voltage change is produced counter to the change in current.

It's converting the 24V into 60V.

So, the IC902 turns on MOSFET Q991 or Q980 (gate pin goes HIGH to +Vcc), pulling current through the inductor L807. At a certain point, it will hit a time limit (on the order of microseconds) which will turn the MOSFET fully OFF. The inductor wants to maintain this level of current, so a spike of voltage is created. This spike is rectified (made positive, and accumulation only) by D985 and then dumped in Q945/C950. IC902 adjusts the ON-time, as above, to regulate the output to a fixed 60V.

It's reasonably easy to see that if the MOSFET never switches on, because of one fault or another, the output will just pass through that diode and you'll get approx 24V out instead of 60V.

[Dang]

Re: Sharp Aquos LC-37LE320E no back light?

Managed to measure Vcc, it's just over 12v measured from the top of R9120, I believe this to be correct, I can't find Q980 on the board. I managed to find the PDF on the IC in question: , is PIN 7 the offending party here?

Thanks for the explanation by the way, very informative.

[tom66]

Re: Sharp Aquos LC-37LE320E no back light?

That's quite close to the threshold of not working...

[tom66]

Re: Sharp Aquos LC-37LE320E no back light?

Can you locate Q906 and check voltage on each pin?

[Dang]

Re: Sharp Aquos LC-37LE320E no back light?

Can you locate Q906 and check voltage on each pin?

Yes, checked from the top again via the resistors attached to it nearest side, they measure: Base @ 14.5v to 14.6v, Emitter @ 13.9v to 14.0v, Collector @ 24.1v to 24.3v, I give you the voltage variances due to their timing, I'd say they fluctuate between those small differences every one second, up/down/up/down and so on every one second, could this be an issue? If it's of any relevance the resistor R9120 measures out at 22.6 ohms.

[tom66]

Re: Sharp Aquos LC-37LE320E no back light?

Looks like IC902 is bad. There's a 22 ohm resistor between emitter and Vcc, and approx 2V drop, so Iq=90mA... quite a lot for that kind of chip. However, one other thing to check is gate-source resistance of Q980/Q991 in case that is low/shorted.

[Dang]

Re: Sharp Aquos LC-37LE320E no back light?

Looks like IC902 is bad. There's a 22 ohm resistor between emitter and Vcc, and approx 2V drop, so Iq=90mA... quite a lot for that kind of chip. However, one other thing to check is gate-source resistance of Q980/Q991 in case that is low/shorted.

OK, managed to test the MOSFET Q991, Q980 is not in place, just blanked off with solder so I guess it's not needed. So looking at Q991 pins down they measure while on: left and right pins show 0v yet the middle is at 24.2v, also while on I tested the resistance of the right pin and it showed 118 ohms, the left -2, conversely, while the Tv was off, the right pin would increase to the point of where I couldn't read it any more, like it was charging up some other component, I guess it's time to order that IC902.

[Dang]

Re: Sharp Aquos LC-37LE320E no back light?

Hi, IC902, is this the same thing?: http://www.ebay.co.uk/itm/2512573176...84.m1438.l2649.

[tom66]

Re: Sharp Aquos LC-37LE320E no back light?

Looks like it.

[Dang]

Re: Sharp Aquos LC-37LE320E no back light?

I now have the part but I do need the required tools for the job, from my research it seems my best option is to use a hand held hot air tool and use solder paste, my question is, what are my best options along these lines, what's a good but cheap solder paste and a usable hot air gun? thanks, Andy.