LG-42d69u no 24v output from smps E59670

[JesusJones]

Hello
I picked up one of these tv's in uknown condition and have found that everything seems to be working except the 24v supply which powers backlights and audio. The 3.3 and 5 and 12v rails and stdby, ps and backlight ON signals are present when cycling the TV/supply. The 24v section can be heard trying to start, but just wont power on and gives no output (.07v). I have no experience troubleshooting SMPS and have learned a fair bit already but am now stumped on what to try or do next.

So far:
I have found that the electrolytic caps (sus'con HG, MG, SG non bloated and esr within spec) have no extreme faults.

I have found no shorts or faults in any of the through hole components, SMDs or final outputs. (Tests done with multimeter, ESR meter)

I've narrowed it down to after the PFC section I get ~400v to switching mosfets but then quickly the controller IC aborts start up and latches itself off. I probed the optocoupler feedback from the Rfmin pin which is supposed to output 2v reference. This 2v is not present when attempting to power the TV/SMPS on. If I disconnect the 400v supply from the mosfets and power it on normally the 2v reference will be present. Externally powering the Vcc and line inputs of the IC will also get the 2v reference to show up.

I'm stumped on what to try next. Seems like it is shutting down from overcurrent and never getting a chance to even read the feedback from the optocoupler.

Really would appreciate the advise of some experienced techs here

This thread contains schematics of the supply https://www.badcaps.net/forum/showth...ight=lc-42d69u
This is the mosfet/switcher controller https://cdn.badcaps-static.com/pdfs/...8e938828b4.pdf

I can also post pictures of MY board if needed.

Andrew

[budm]

Re: LG-42d69u no 24v output from smps E59670

We do need to see the pictures of your boards and pictures of the whole backside of the TV showing all the boards.

[JesusJones]

Re: LG-42d69u no 24v output from smps E59670

The mosfets and heatsink are currently out. Hence the empty space above the transformer.

[budm]

Re: LG-42d69u no 24v output from smps E59670

So if you run the TV without the Inverter board connected to the power supply board, the PFC and the 24V (the two MOSFETs removed) power supply still go into shutdown? and did the the PS-ON signal also go away?

[JesusJones]

Re: LG-42d69u no 24v output from smps E59670

Previously, without the inverter connected the 24v supply still did not come up. The PFC (makes the 400v right?) would stay working. Just the mosfet switching IC for the transformer primary latches itself off. It still receives Vcc power and everything it would have when running normaly (as far as I can tell) it just aborts start up and becomes latched off and needs a reset to try again.

[JesusJones]

Re: LG-42d69u no 24v output from smps E59670

Sorry, I jumped the gun a bit with my answer. I will try connecting the supply with MOSFETS removed and report back.

Previously with supply voltage (400v) removed from the mosfets the MOSFET switchign IC would not shut down.

[JesusJones]

Re: LG-42d69u no 24v output from smps E59670

I have confirmed that:
ps_on recieves 3.23v consistently when the power is turned on. AND The PWM IC (MOSFET switcher) does not shut down. It outputs the 2v reference from the Rfmin pin with the mosfets removed and the inverter board disconnected. Presence of the 2v signal is what I'm using to confirm it has not latched off.

[JesusJones]

Re: LG-42d69u no 24v output from smps E59670

Had some time and I may have narrowed it down more. Noticed that there is a PFC_stop pin that goes low when the IC shuts down. According to the IC schematic this is controlled by an OR gate with three inputs. Two of those inputs are from disabled features that are not used in this design leaving only the overcurrent (Isen) pin to be tripping the shutdown. Using a multimeter on the Isen pin never shows a voltage near high enough to trip the OCP. But that is just slow multimeter...

Applied 24v to the output of the supply (mains disconnected) and it charged to 24v while drawing a normal amount of current. So I think there are no shorts all the way up to the COLD secondary diodes from the 24v output.

I am (stuck) thinking there is a problem with the start up cycle, transformer and or the mosfets driving it. The mosfets tested normally on my multimeter with no shorts, diode working and gate confirmed to turn on/off. The transformer is harder for me to test. Tried, "ringing it", using my scopes square wave CAL output. Seemed to ring but without a known good reference/experience I am not able to judge it confidently. Is there a way to remove the transformer and insert some load resistor to help confirm the transformer is faulty and causing the overcurrent? The start up circuitry is all surface mount stuff and probing with my multimeter has shown no shorts across any cermaic caps or diodes and no resistors open when testing in circuit. The section of the circuit in question is shown below.

Any suggestions? If I can confirm its a transformer short/fault I'll autopsy it. But I don't want to get destructive....yet

Cheers

[budm]

Re: LG-42d69u no 24v output from smps E59670

What Voltage do you have in that pin 7 line sensing pin of the IC? check those three 1M Ohms to see it the value has gone up.
BTW, c827 (22nf 1KV) capacitance is OK, right?

[JesusJones]

Re: LG-42d69u no 24v output from smps E59670

The line sensing pin gets 1.69v and the 1M resistors test good. C827 also tests well, right at 22nf and no DC leakage up to my testing limit of ~100v. C828 also tested good in the same way.