LG 47LN5700 No voltages on SMPS only standby voltage present

[anupkg1985]

Hi,

I have smps board which is not powering up. I'm getting 3.5 standby voltage though. After shorting PS ON with ground not getting any other voltages like 12V , 24 V on other pins. Any help would be appreciated.

[R_J]

Re: LG 47LN5700 No voltages on SMPS only standby voltage present

Disconnect the cable between the main board and power supply, and connect around a 1k resistor between standby & pwr-on, it looks like it needs a logic high, not low to turn it on

[dick_barton]

Re: LG 47LN5700 No voltages on SMPS only standby voltage present

With the mainboard disconnected from the power supply, I believe you need to jumper the standby voltage to Power On using a 1K resistor so as not to damage anything.
The power supply circuit is here.
https://www.badcaps.net/forum/showth...ht=LG+47LN5700

[anupkg1985]

Re: LG 47LN5700 No voltages on SMPS only standby voltage present

Thanks for quick reply guys on this. I have shorted the 3.5 standby pin with PWR-ON with jumper. Will that be an issue? Also after jumping power on, I'm getting 14V to IC SSC9527s as an source voltage to it.

[R_J]

Re: LG 47LN5700 No voltages on SMPS only standby voltage present

Yes in this case you can short stb3.3 to pwr-on, there is a 470Ω resistor already there. Have you checked F101? When you say source voltage do you mean VCC? 14~16 volts vcc is about right.

[anupkg1985]

Re: LG 47LN5700 No voltages on SMPS only standby voltage present

Yes all fuse are good and I have basic checks all looks good. I'm getting 172V on Darin of Q101 however its not present on Q102. Not getting any voltage on transformer input EL35 and EL36. Actually SMPS was working fine but I shorted the leads of main capacitor c510 to discharge after disconnecting the power. But then it never turned on after that. I'm getting aroung 173V on main capacitor C510. Any idea whats going wrong?

Forgot to mention that 14V I'm getting on J35 which is Vcc for IC SSC9527s.

[R_J]

Re: LG 47LN5700 No voltages on SMPS only standby voltage present

You likely blew up the pfc circuit. check D601,D602,D603, but you may have damaged ic601, Q501,Q502. IC101 may not operate wit the low B+ of 172v, it needs the pfc boosted voltage which is around 385vdc
NEVER discharge the main cap by shorting it out. If you must discharge it you need to do it with a resistor.

[anupkg1985]

Re: LG 47LN5700 No voltages on SMPS only standby voltage present

What are the expected voltages on pins Q501, Q 502, any idea? I'm getting voltage 173v across all the diodes D601, 601 and 603. Here in US input voltage is 123v so not sure if 350v aexpexted on main caps? I'm getting around 173v on main cap.

[R_J]

Re: LG 47LN5700 No voltages on SMPS only standby voltage present

Q501 is the vcc regulator/switch, the input (Collector) will be around 25vdc, the output (emitter) will be around 15~16vdc. Q501 is turned on by IC503 optocoupler.
Q502 is the regulator for the vcc for ic501, its emitter will be around 20vdc
With 120vac input, the DC voltage out of the bridge rectifier (across C610) will be around 160VDC, the PFC circuit (ic601) Switches the dc voltage at around 200khz boosting the 160vdc to around 385vdc.

With the PFC voltage low (160vdc) the VSEN line for ic101 will be too low (likely less than 1vdc) so the ic will not operate, when the pfc voltage is 385v the VSEN will be higher (around 1.42vdc) and the ic will operate driving Q101/Q102

[anupkg1985]

Re: LG 47LN5700 No voltages on SMPS only standby voltage present

Hi RJ,
Thanks for details. I'm getting feedback voltage on IC 503 also below are voltages on IC501 -

PIN1 0V
PIN2 3V
PIN3 0V
PIN4 0.1V
PIN5 170V
PIN6
PIN7 14V
PIN8 0V

Also I have tested other IC highligted with RED color, not sure what IC is it. Not able to see the part number on it. But I'm getting below voltages on that IC 601 -

PIN1 1V
PIN2 2.6V
PIN3 4.2V
PIN4 2.6V
PIN5 0V
PIN6 0V
PIN7 0V
PIN8 14V

Also as you said there is only 0.6V on VSEN for IC 101. I have tested all the resistors on the area marked in GREEN color and look good to me.
Could you please guide me to make it working

[anupkg1985]

Re: LG 47LN5700 No voltages on SMPS only standby voltage present

Forgot to add photo.

[R_J]

Re: LG 47LN5700 No voltages on SMPS only standby voltage present

When you short out a capacitor with 170 volts on it, you do damage. You can check zd602, if it is not shorted, start by replacing ic601.

[anupkg1985]

Re: LG 47LN5700 No voltages on SMPS only standby voltage present

I tested zd602 and looks good however looks like I should be getting voltage on OUT pin (PIN7 0V) of IC601. Here is the datasheet for that ic but not sure how volts it should read on that OUT pin.
https://datasheetspdf.com/pdf/940453.../R2A20133DSP/1

So may be need to replace IC. Could there be any replacement IC for this one?

[R_J]

Re: LG 47LN5700 No voltages on SMPS only standby voltage present

The ic will output a switching pulse, you would need a isolation transformer and a scope to check the ic's output

[anupkg1985]

Re: LG 47LN5700 No voltages on SMPS only standby voltage present

Might be a stupid question but just want to know if I can measure PIN 7 (OUT) using frequency/duty cycle mode on multimeter?

[anupkg1985]

Re: LG 47LN5700 No voltages on SMPS only standby voltage present

Might be a stupid question but just want to know if I can measure PIN 7 (OUT) using frequency/duty cycle mode on multimeter?

I just tested using my multimeter and I'm getting frequency 4KHz on OUT pin.

[anupkg1985]

Re: LG 47LN5700 No voltages on SMPS only standby voltage present

Replaced IC601, smsp started working again. Thanks all for your support.