LG50PK550 Relay Clicking Intermittent Problem

[rahulmadhukar]

Hi Friends, I have an LG 50PK550 Plasma TV. The picture quality and the functionality is all good in this TV but it turns on after so many relay clicking. Sometimes it doesn't turn on and the red led light flashes at a high speed or sometime normal blinking. Once turned on, no problems while its on.

I checked on the net and found that there is a repair kit available on E-bay http://www.ebay.com.au/itm/190886339...84.m1439.l2649 which fixes this. So I changed 3 resistors, 1 Diode and 1 Votage regulator which are marked red in pics. Even after changing them the problem hasn't improved. Can anyone please help me find what could be the problem. Here are the pics http://s1347.photobucket.com/user/ra...rary/LG50PK550 Thanks.

[rahulmadhukar]

Re: LG50PK550 Relay Clicking Intermittent Problem

Hi All... Any suggestions...

[tom66]

Re: LG50PK550 Relay Clicking Intermittent Problem

Do you have a digital multimeter?

Can you post a close up image of the power supply connector going from power supply to main board? The main board is the one with the video inputs like HDMI on it.

[rahulmadhukar]

Re: LG50PK550 Relay Clicking Intermittent Problem

Yes I do have a multimeter. Please find attached the close up images of the connector. Thanks.

[tom66]

Re: LG50PK550 Relay Clicking Intermittent Problem

OK, with the TV in the fault condition (failing to start up)
Please check (any one of if multiple present): STBY, 5.1V, 17V, RL_ON, M_ON, ERR_DET, AC_DET.

To test: red probe on appropriate point (around each pin there is a small bit of exposed metal), meter on DC 20V, black probe on chassis ground (wall mount holes, metal plate of plasma panel, RCA video input ground etc.)

You may need to use a piece of wire inserted in the pin to get better contact.

For the voltages, please hold the reading through a few relay clicks to see if any obvious changes are noted.

[rahulmadhukar]

Re: LG50PK550 Relay Clicking Intermittent Problem

Hi tom, below are the readings... Thanks
17V - It is 17.14V and decreases gradually as relay clicks.
5.1V- It is 5.1V only and drops to 0 as relay clicks.
ERRROR_DET- It is 3.3V and goes till 3.7 and back.
STBY - 5.14V when tv is working and 3.43(constant) when tv is clicking.
AC_DET - 0.0V and changes to 2.3v as relay clicks
M_ON - 3.25V and drops to 0.19V as relay clicks

[tom66]

Re: LG50PK550 Relay Clicking Intermittent Problem

It looks like there is a problem with the standby power supply. Do you have access to any high power, low ohm resistors? (10 ohm 5W for example, somewhere in that range.) Or a 1157-style car brake light bulb?

[rahulmadhukar]

Re: LG50PK550 Relay Clicking Intermittent Problem

Yes I have one 10ohm 5W resistor. Also wondering can we suspect the capacitors as I haven't replaced or checked any. Thanks.

[tom66]

Re: LG50PK550 Relay Clicking Intermittent Problem

OK, put the 10 ohm resistor across the STBY and GND pins. The power connection to the main board should be disconnected. Then, check STBY. If it is significantly less than 5V, there is a problem with that part of the power supply: suspects include bad startup cap, bad output capacitors, bad current sense transformer and leaky diodes on other rails.

[rahulmadhukar]

Re: LG50PK550 Relay Clicking Intermittent Problem

Hi Tom, Thanks you for your valuable suggestions and must say this forum is great. Here are the findings... Firstly I applied a 10ohm resistor across STBY and GND pins and removed the connector going to main board. I switched ON the TV but there was no LED on the TV, I mean I couldn't switch the TV on by remote as main board was disconnected. I checked the voltage on all pins there was nothing. So I connected the connector with the resistor still on. Now I could see 3.43v standby voltage and changing to 5.14V when TV is ON.

I was also going through LG 50PK750 block diagram on your website. I found this "If AC-Det is missing, as a work around test, Jump the STBY-5V to the AC-Det line. If the set works normally, then the SMPS is defective." So I did this and the TV was all working fine. Now my question is, can I permanently short AC_Det and STBY-5V. Will this adversely affect the TV circuit. I thought this is not the right way so in order to check the power board I tested the capacitors, diodes and resistors. Two components I am suspecting but needs your suggestions. One is a 330uF capacitor which when tested on a digital multimeter reads 296uF. And second one is a resistor which is tested 0.5ohm but as per the color code it was 0.36, also wondering if I calculated the value correctly. Please refer the attached photo and please verify the value also. Got confused with a black colored fifth line after the gold... I hope so that I am going in the right direction. Thanks again.

[tom66]

Re: LG50PK550 Relay Clicking Intermittent Problem

It sounds like your power board is defective. The capacitor really needs checking with an ESR meter. A capacitance meter may read fine but the cap can still be bad. Your multimeter probes have resistance so it's OK for that resistor to read 0.5 ohms.

You can jumper it now but if you pull the power cord out while the set is running, things may happen which are not good. It could be as little as a crackling sound from the speakers, common if the TV does not mute before it turns off, or it could corrupt some memory or settings. I have done ac detect jumper before on an older 42PC3D LG, with no bad effect, but I can't be sure for 50PK550.

In addition, standby at 3.4V is too low, unless it uses some kind of low-voltage standby mode, which rises to 5.1V in operation. It might cause problems later on. Check it with the AC-DET removed, and again with the 10 ohm resistor removed, to see if it rises to a higher reading.