LG 37LC2D No sound, picture okay, remote control and side buttons glitchy

[tom66]

Here's an odd one. LG 37LC2D which has no sound, but the picture is fine. When I first got it, the remote control worked fine, but that's also stopped working, as have the side buttons. The batteries are fine.

Opened the unit up to find a bulged cap on the power supply for the 3.4V line. Cap ESR measures at 0.44 ohms when cold, 0.38 ohms when warm. It's a Samxon. I don't trust these caps, even though others do, I've seen plenty bulge. This 3.4V line measure 1.2V(!!) So something is clearly up. The other voltages measure okay, and the caps test fine, although the 12V measures at 11V, which is quite low. The 12V also varies as the unit goes through power on stages, peaking at 11.8V.

I have a spare AV board for this TV (fitted a plasma but they're compatible) - the audio stuff is on there. If the problem isn't solved after I change this cap, I'll change the boards.

[budm]

Re: LG 37LC2D No sound, picture okay, remote control and side buttons glitchy

That intermittent remote problem is very common in the LG TV, if you look at the back, you will see a small mini-phone jack labeled 'IR input', just spray contact cleaner into it and plug a mini-phone plug in and out a couple times to clean the switch contact inside. When the IR port is used, it will switch over from the front panel remote receiver to the rear panel remote input, since not too many people use the IR input, the switch inside get corrosion to make poor contact, that is why you need to exercise it once in awhile. Always clean that jack before selling it to the customer

I replaced all the secondary side filter caps on mine, the relay was clicking on/off/on/off.

[tom66]

Re: LG 37LC2D No sound, picture okay, remote control and side buttons glitchy

That intermittent remote problem is very common in the LG TV, if you look at the back, you will see a small mini-phone jack labeled 'IR input', just spray contact cleaner into it and plug a mini-phone plug in and out a couple times to clean the switch contact inside. When the IR port is used, it will switch over from the front panel remote receiver to the rear panel remote input, since not too many people use the IR input, the switch inside get corrosion to make poor contact, that is why you need to exercise it once in awhile. Always clean that jack before selling it to the customer

Thanks for your input Bud - this could quite easily be the problem. There is a remote in and out port on it. I will definitely clean that. Do you know if this affects the side buttons too?

I replaced all the secondary side filter caps on mine, the relay was clicking on/off/on/off.

I may end up doing this, caps are cheap and I've got to make an order with Farnell soon, plus the TV was pretty cheap and it's in very good condition.

[budm]

Re: LG 37LC2D No sound, picture okay, remote control and side buttons glitchy

That remote jack is just for remote function. I would replace the caps first, since unstable DC can do funny things.

[tom66]

Re: LG 37LC2D No sound, picture okay, remote control and side buttons glitchy

That remote jack is just for remote function. I would replace the caps first, since unstable DC can do funny things.

Have to agree with you there! Lots of strangeness in the semiconductor mind when the voltages go too low. I'm very surprised it gives a picture at all with the 3.4V line so low. My only guess is that the 3.4V is only used for the audio processor, or maybe the IR and buttons too. The buttons, IR and audio are all handled on the one AV board (which is separate from the main board) so it's possible something glitchy on there is causing problems.

Cheers

[tom66]

Re: LG 37LC2D No sound, picture okay, remote control and side buttons glitchy

Okay, I tried to replace the single bulged capacitor, but the 3.4V is still 1.25V. However, I noticed something strange. If I disconnect the "main power" connector from the main board (which carries 3.4V) the 3.4V line rises to 2.2V, and then it will respond to remote control and side button. It doesn't work of course (no picture) but this suggests to me there's another bad cap somewhere. I suspect the output cap for the buck converter is faulty.

[tom66]

Re: LG 37LC2D No sound, picture okay, remote control and side buttons glitchy

Pulled the 3.4V buck output cap - should be 680uF 0.01 ohms; actually measured 568uF 4.4 ohms! But it's still not working... Hmm, it may have to be another external power supply job.

[budm]

Re: LG 37LC2D No sound, picture okay, remote control and side buttons glitchy

Too bad the service manual dose not have the diagram for the power supply.

[tom66]

Re: LG 37LC2D No sound, picture okay, remote control and side buttons glitchy

Yeah, I know. I've never found a schematic for an LG power supply, when they are the most common failures, it's kinda irritating! The area under the 3.4V supply is very discoloured; one part of the board is blackened from heat, so it could be a faulty controller. I need to decide if a new PSU will be cheaper or an option, or if I should try and replace the SO8 IC, or if I will make my own 3.4V supply.

I took home the remote and tried it on my repaired LG 42LC46 and it works fine, so I know the remote is okay.

[budm]

Re: LG 37LC2D No sound, picture okay, remote control and side buttons glitchy

I will go with DIY since you can make it more robust than the original design, I just hope that it will not happen to mine TV. Do you have the pictures of that section so I can see if my is on its way out?

[tom66]

Re: LG 37LC2D No sound, picture okay, remote control and side buttons glitchy

I will go with DIY since you can make it more robust than the original design, I just hope that it will not happen to mine TV. Do you have the pictures of that section so I can see if my is on its way out?

The PSU is currently at the office, I'll have access to it on Monday, so I'll post a pic then. It's very bad. You can even see the browning on the top. Thinking about it, the one bad cap was very close to this heat, so this cap could have simply been cooked.

[tom66]

Re: LG 37LC2D No sound, picture okay, remote control and side buttons glitchy

Another problem is the 3.4V runs off the 5V standby. I may use a DC-DC converter like one of these. http://www.ebay.co.uk/itm/DC-DC-Conv...item4604572d4c But I need an ~8-23V supply, and the board only have 5V standby. Still, I think the 3.4V is on only when the TV is switched on, so maybe I can use the main 12V. This TV has too many rails!! 3.4, 5, 6, 12, 19 and 24V.

If you look near the heatsink in the pic, you'll see the browning I'm talking about.

[budm]

Re: LG 37LC2D No sound, picture okay, remote control and side buttons glitchy

I just look at the pictures I took 2 years ago, the 3.4V is rated at 5A! no browning sign on my board.
Is that buck regulator a surface mounted, I cannot see that in my picture.

[tom66]

Re: LG 37LC2D No sound, picture okay, remote control and side buttons glitchy

Yeah, I saw that too. 5A is massive! Given it runs off the tiny 5Vsb transformer which looks capable of maybe 10W...

But since the audio DSP and the remote control seems to be the only thing that depends on 3.4V, I think it will be okay to replace it with a supply capable of 2-3 amps. However, I don't have a clamp meter yet to test this theory.

[tom66]

Re: LG 37LC2D No sound, picture okay, remote control and side buttons glitchy

Here's the buck converter position.

I am thinking of using an LM2576T simple switcher. 3.3V at 5A is possible, but 3A is a more reasonable output.

It looks like the 6V might also be generated using a buck converter.

[budm]

Re: LG 37LC2D No sound, picture okay, remote control and side buttons glitchy

15W switcher with copper pad for heat sink? No wonder it runs hot.
Thanks for pointing out.

[tom66]

Re: LG 37LC2D No sound, picture okay, remote control and side buttons glitchy

Yeah, I wonder why it runs so hot, if it draws 5A or if the converter is faulty. I will only find out by testing the LM2576T. I hope it does not draw 5A, that is difficult to provide, I would need two converters in parallel!

I have drawn a little circuit diagram of what I'll build when I get some free time. I have designed it so it runs off the 5V standby in standby (which should draw less than 1W) but then switches over to the 12V supply to lighten the power dissipation in the switcher (which is higher at higher input currents.) A little 1 amp relay I have will do the job fine. I would normally use a p-CH fet for this, but I have lots of these little relays lying around.

When the 12V is on, the relay will switch over to using the 12V run supply. When 12V drops out, the 3.3V output stays high long enough for the relay to change over due to the extra large 2x 3300u cap and the high 2200u input cap. The relay will then deactivate (helped by the diode and resistor in parallel with the coil) and power itself from 5V standby. If this circuit proves troublesome, I will just use the 5V standby, but I'd prefer to use the 12V when it is running. (Probably like LG should have done originally.)

[tom66]

Re: LG 37LC2D No sound, picture okay, remote control and side buttons glitchy

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[tom66]

Re: LG 37LC2D No sound, picture okay, remote control and side buttons glitchy

Okay, well I've decided for now I'll try using an LM317T to power it. The only thing that draws from the 3.3/3.4V as far as I can tell is the system MCU (hence side buttons and remote control don't work) and the audio DSP. The power hungry display processor appears to be powered from the main 6V rail. So, I've built a little LM317T circuit up, this can provide 3.3V at 1.5A with a decent heatsink. I think 1.5A would be very high for this TV. But I will characterise it with an ammeter.

[tom66]

Re: LG 37LC2D No sound, picture okay, remote control and side buttons glitchy

Okay, bodged it in. Result: Audio and remote control/side buttons now work. However, the 3.3V output drops to 2.99V as the whole TV draws about 1.43A, just within the 1.5A limit of the LM317T, and it is clearly in drop out with 5V input. I will need to think of a more permanent solution, perhaps using the 6V output or using a buck converter. Regulator gets quite hot too as it's dissipating about 2W, but I have got a good heatsink on it. It overheated after about 5 minutes.