LG 55LB5900 Totally Dead!

[budm]

Re: LG 55LB5900 Totally Dead!

If you look at the application circuit:
1) Try to locate the string of resistors connected to VH (pin 1) to see what resistance they are.
2) Check the resistance between VCC (pin 10) and GND to see what it is.

[jason123]

Re: LG 55LB5900 Totally Dead!

Ok, so Pin 1 has 4 resistors and 1 Diode in series. Values of resistors are right on (30.k +2.2k+2.2k+2.2k) and diodes tests ok.
Pin 10 does not seem to be connected to anything. resistance between Pin 10 and Pin 12 (GND) is 10 Ohm

[budm]

Re: LG 55LB5900 Totally Dead!

Are you sure you count the pin 10 VCC pin correctly? The resistance between VCC pin and GND should not 10 Ohms, it should be in the K Ohms range. There should be filter cap between VCC pin and GND pin also.
The trace on VCC pin may have routed under the IC, this will require you to trace out the board, it is part of reverse engineering that is required when there is no schematic available, but at lest you now have the spec sheet of the IC showing how each pin is used.

[jason123]

Re: LG 55LB5900 Totally Dead!

Are you sure you count the pin 10 VCC pin correctly? The resistance between VCC pin and GND should not 10 Ohms, it should be in the K Ohms range. There should be filter cap between VCC pin and GND pin also.
The trace on VCC pin may have routed under the IC, this will require you to trace out the board, it is part of reverse engineering that is required when there is no schematic available, but at lest you now have the spec sheet of the IC showing how each pin is used.

So this is a 1 layer pcb it looks like The VCC is routed underneath the IC and comes out down at the bottom (between pins 8 and 9).. indeed, there is a 10 ohm resistor in series with a diode that goes through a cap to pin 14. pin 10 also goes to couple of diodes and caps.. I'll try to draw the circuit and upload.

[budm]

Re: LG 55LB5900 Totally Dead!

Since you removed glue and able to see the traces better, can you please zoom out more so I can see more traces? It will also help to get high res pictures of the whole bottom side of the board.

[jason123]

Re: LG 55LB5900 Totally Dead!

Since you removed glue and able to see the traces better, can you please zoom out more so I can see more traces? It will also help to get high res pictures of the whole bottom side of the board.

I will shortly.. I'm trying to remove more of the glue so pictures can look clearer. And yes, I should have thanked you for finding this part. I looked for a while with no luck.

[budm]

Re: LG 55LB5900 Totally Dead!

Check the resistance between the two legs of the lytics cap C112, it is the starup/running cap for the SMPS IC.

[jason123]

Re: LG 55LB5900 Totally Dead!

So, C112 is measuring 10.1 Ohm, in-circuit with a regular DMM. ESR is 7.12 Ohm (high) and Capacitance 97.2 uF (100 uF value - so close enough).

I'm sorry, I measured the P10 and P14 resistance and it's 720 kOhm. I must have had the wrong pins before.

Took the cap out of circuit and it measures 11.3 Ohm.

[budm]

Re: LG 55LB5900 Totally Dead!

At this point you either have internal short in the IC or shorted cap (not the C112, but the SMD ceramic cap that is also connected between VCC pin and GND.
Check the resistance of the CAP that is right next to the ZD102 (not installed), also check C108 resistance. Also check the resistance of Zener diode ZD103 (it is connected in parallel with C112).
https://www.badcaps.net/forum/attach...7&d=1531506842
BTW, your board layout is not the same the one I post from shopjimmy.
We are getting close to find the cause of the problem.

[jason123]

Re: LG 55LB5900 Totally Dead!

So once i removed the cap c112, the resistance across pads is up to 746k ohm. Just curious why you don't think the c112 is the culprit?

[jason123]

Re: LG 55LB5900 Totally Dead!

C near ZD102 IS 3.6Meg
C108 849kOhm
ZD103 is 746k

[budm]

Re: LG 55LB5900 Totally Dead!

I hardly see newer lytics cap shorted out, in the old days they do have shorting out problem. So if you check the resistance of the cap right now it shows about 10 Ohms?

[jason123]

Re: LG 55LB5900 Totally Dead!

Yes.. it's a 100uf 50V. The only one that's 50v is a 470uf laying around... should I try it?

[jason123]

Re: LG 55LB5900 Totally Dead!

Ok i am getting 3.5v on connector as labeled! Good sign? Hopefully i didnt cause more damage with such large cap. I think it was the C112 or premature?

[jason123]

Re: LG 55LB5900 Totally Dead!

budm... you're awesome.. thank you. One final stupid question... the glue stuff has the hot glue consistency... can i use hot glue?

[budm]

Re: LG 55LB5900 Totally Dead!

I would get closer value cap to the original, when you go up in capacitance, you will get more inrush current.
That stuff is not really glue, more likely to be conformal coating or electronics grade silicone.
I am glad that it is just bad cap not the IC, what you have learnt from this thread is worth a lot more than $39. Glad to see you got it working.

[jason123]

Re: LG 55LB5900 Totally Dead!

Thanks to you my friend. Much appreciated

[vinceroger69]

Re: LG 55LB5900 Totally Dead!

Glad you have managed too get the tv working there has been some good fault finding tips in this thread im sure it will help a lot of people out

[jason123]

Re: LG 55LB5900 Totally Dead!

Me too! thanks vinceroger69

[sputnikoff]

Re: LG 55LB5900 Totally Dead!

I was attempting to rule out lamp issues as I have replaced all with new and I shorted 18 pin cable, vcc 3.5 and pwr on and drv on,board cap voltage goes to 395 vdc . At this point I think issue is with main board module eax65391003.As lamps come on with 3.5 vdc signal and drv on shorted.