LG 42LT75 no backlight

[Agent24]

Re: LG 42LT75 no backlight

Maybe the IC or something attached to Pin 8 is leaky?

[budm]

Re: LG 42LT75 no backlight

Check PIN 2 and pin 3 DC voltages to see if there is any activities there. pin 4 is the ground ref point.

[tn245]

Re: LG 42LT75 no backlight

Thanks budm. On pins 2 and 3 on the good board I measure a jump to approx 5V on switch on, then it drops to 0V. (this is out of chassis so protection must be kicking in).

On the faulty board it's only in the mV range.

[budm]

Re: LG 42LT75 no backlight

OK, so it is not driven, if you look at pin 2 and 3, you will see the traces that go to a bunch of transistors which looks like they are driven by transformer T702. There are 2 ICs one is 8-pin SMD and one is 16-pin SMD which looks like it drive the T702. Can you get the P/N of those 2 ICs, see picture #1 on post 15.

[tn245]

Re: LG 42LT75 no backlight

The 8-pin IC is S393, the 16-pin is OZ9925GN.

[budm]

Re: LG 42LT75 no backlight

OK, let me look up the spec sheet for those 2 then we can check the VCC Pin to see if it is getting the voltage or not, it is good that we do have good working board to do the comparison test.

[tn245]

Re: LG 42LT75 no backlight

Great! I looked up the OZ9925 when I first started on this board and didn't find a datasheet, but I did find this page with pinouts: http://www-user.tu-chemnitz.de/~heha...arieren/OZ9925. I also did do some testing on that previously and it is getting 12V Vcc if I remember.

I'll probably have to put it aside till tomorrow now as I've got people coming round in a bit and have to put away all the circuit boards and stuff. Will do some more voltage testing tomorrow.

Feel like we've made some progress. Thanks again!

[stj]

Re: LG 42LT75 no backlight

s393 8pin - sounds like an LM393

[tn245]

Re: LG 42LT75 no backlight

Yes, I think it's the same type of thing. Datasheet for S393 attached.

So I found a chance to take some measurements. S393 is getting 5V VCC on pin 8, and OZ9925 is getting 11.75V on pin 2. This is the same on both boards so looks like the ICs are getting their supply voltages ok.

[tn245]

Re: LG 42LT75 no backlight

On the OZ9925 pins 3 and 5 (push-pull driver outputs) lead to that transformer T702, and on the working board I measure an initial 5V on both pins. On the faulty board it's only mVs.

I went ahead and measured all the OZ9925 pins on both boards. Here are the measurements plus pin assignments from the link. Where it says (then drops), I guess it's because I haven't got lamps connected, and it's going into protection.

Pin Working     Faulty       
1  5        5          VREF	Reference Output +5V
2  11.7       11.7         VIN	Power supply +12V
3  5 (then drops)  0.4         NDR2	push-pull driver output #2
4  0        0          PGND	Power GND 
5  5 (then drops)  0.2         NDR1	push-pull driver output #1
6  0        0          AGND 
7  1        1          RT_CT	Sawtooth generator R and C
8  0.58       0.59         LRT_LCT	(Triangular wave for burst-mode dimming)
9  2.3 (then drops) 1.24         DIM	Dimmer control input  
10  2.3 (then drops) 0          VSEN	Voltage feedback input  
11  2.8       2.8         OVT	(Over Voltage Threshold?)
12  0        0          TIME	Switch-On input/output 
13  2 (then drops)  0          SST-CMP	seems to be a power-good output, H = lamps are bright
14  0        0          ISEN on/off control input, H = ON, L = OFF
15  5        5          ENA	Enable
16  5 (then drops)  0.74 (then drops)  VLS	Voltage Low Sense

[budm]

Re: LG 42LT75 no backlight

So some how there is no drive output to drive the transformer, it should have sent out the drive then goes into shutdown after the error is detected. Do you want to try new IC?
This one example of how it is used:

[tn245]

Re: LG 42LT75 no backlight

Yes, I think I'll try a new IC.

My IC I'm pretty sure is OZ9925GN, this ebay link is for OZ9925CN
http://www.ebay.co.uk/itm/1-piece-Ne...-/190985512156

But on their photo it even looks like it could be GN, not CN. Utsource has both CN and GN listed.

I guess either should work anyway right?

[tn245]

Re: LG 42LT75 no backlight

The ic must have been ok. I just put in a brand new one and there's no change. Measured voltages on all the pins are as before.

So I'm wondering if some of the other voltages on input pins might be suspect causing the ic to not be driven correctly. Maybe pin 9 - Dimmer control input which from a chinese page I found should be in the range 1.5V - 3.5V. Or maybe pin 10 - VSEN or pin 16 Voltage Low Sense?

This page has lots of info on the ic, but it's a little hard to understand the google translation of the Chinese:

https://translate.google.com/transla...-text=&act=url

[tn245]

Re: LG 42LT75 no backlight

I think the ic is not on because of pin 16 being low. On the good board it's 5V at start-up, but on the bad one it's almost nothing.

I tried grounding the base of Q312 causing pin 16 to stay at 5V. Now I get the 5V at pins 3 & 5 driver outputs on the ic, and I get those input voltages on the FAN7382 - though there still doesn't seem to be any oscillation happening at the inverter transformer and no light on a CFL.

I think pin 16 of the OZ9925 is normally kept high by the feedback circuits coming from the output of the inverter transformer (top of second pic) but there's no feedback coming via C402/407 at all.

Any thoughts on what's going on here? What could I check next having ruled out the PWM ic?

[tn245]

Re: LG 42LT75 no backlight

Hi! Any further thoughts on this? Budm??

Otherwise I think I'm going to give up on this one and just replace it. I hate to give up unless I know it's unfixable though. Seems a shame to chuck it.

[Agent24]

Re: LG 42LT75 no backlight

I guess you need to find out why Pin 16 (and maybe 9 and 10) aren't getting the same signals as the working one.

Since you have a working one, you can compare voltages in the circuits associated with those signals

[tn245]

Re: LG 42LT75 no backlight

From what I can tell, Pin 16 on the working board is kept at 5V by voltages tracing back to C402/407 which are connected to the outputs of the inverter transformer. On the faulty board there's nothing coming out of the outputs so Pin 16's voltage is switched off. As I said I can force the ic on by cutting out that feedback signal, but I still get no output on the transformer which leads me to think the problem is elsewhere.

I don't really get how that works though, it seems to be a paradoxical loop in my mind. If the ic needs the feeback from the inverter to power on, but the inverter needs the ic to drive it, how does it ever get started?

[budm]

Re: LG 42LT75 no backlight

Not at this point, comparison test with working board and SCH will help at this point.

[tn245]

Re: LG 42LT75 no backlight

SCH = schematic I guess? Yeah, I haven't found one anywhere. One would definitely be helpful! I've been trying to do comparisons but just not sure where to focus on really. Thanks for all your help anyway. I have learned a bit on this one even though I might not get it going.

Can anyone explain the basics of how the feedback loop I described in my last post works though? I'm interested.

[Agent24]

Re: LG 42LT75 no backlight

I don't know how the feedback works, from what you say it doesn't make sense, so maybe there is some extra 'bootstrap' circuitry or such there you are missing?