SONY KDL-40BX440 LCD TV "MAIN_POWER" problem

[embdd]

Hello guys, from a couple of weeks now, I have been trying to fix my SONY KDL-40BX440, but failing miserably.

The TV have been slowly degrading for a year now, it used to turn off/lose picture and needed a few tries to turn on. It was working fine for a month after I replaced one of its power N-FETs (Q800) and its electrolytic caps, but now it won't turn on at all. The standby LED flashes 2 times ("Main Power Over Voltage Protection") and there is only standby voltage (3V3). The power board is DPS-166DP (the closest schematic that I found is of DPS-165GP).

This time 2 of its N-FETs were blown (Q800 and Q901), but replacing them didn't fix it. Component side ceramic caps, resistors, low voltage P-FETs, rest of high voltage N-FETs, diodes and diode bridge are fine. The transformer's windings are not shorted (this much I can tell with my basic knowledge of transformers). What seems suspicious to me is that the secondaries of T901 are shorted.

I haven't tested the transistors and ICs on the bottom yet, I thought it would be wiser to ask first.

At this point I really want to know what the problem is, any help is much appreciated.


https://tel-spb.ru/remont-tv-lcd/sony-kdl-40bx440

[CapLeaker]

Re: SONY KDL-40BX440 LCD TV "MAIN_POWER" problem

When a mosfet blows and it’s shorted between all three pins, you need to check out the gate drive circuit as well. STBY voltage is always present and goes to the main board. When you turn the TV on the main board will turn all the power rails on the PSU and the backlight. This being said you can “force” the PSU on by itself only having the backlights connected, using 1K resistors and jump from STBY to PSon, BLon and you may also have to jump to the Dim pin.
BTW there are tons of bad solder joints on the back of the PSU board. Specifically on the hot parts and while you’re at it resolder all pins on every transformer.
Do some voltage measurements, but be aware to be very careful on the hot side of the PSU. There is mains voltage on it and a PFC. If the PFC a isn’t working the TV wont work either.

[Crystaleyes]

Re: SONY KDL-40BX440 LCD TV "MAIN_POWER" problem

Been working on a very similar Sony TV and PSU which needed verification that it was working, which I did by bridging between the stby and Power_ON and (in my case) the T-CON_ON (as the backlight was already working).

An actual schematic would be handy here, as the one you posted has Q901 as an NPN transistor.

Can't help but agree with Capleaker that you need to clean up the soldering first. Suck off the old solder first, a bit of flux and some fresh solder.

Looking at the pictures, I would also test (what looks like) R901A. That looks like it's been a bit toasty and if Q901 is an N channel FET, then that may well be the Source resistor.


Get yourself a dimbulb or isolation transformer, bridge stby to Pwr_ON and take some readings. At least test the bridge rectifier.
Do you have an understanding of how Switched Mode Power Supplies work?
Be very careful when working on these live.

[nomoresonys]

Re: SONY KDL-40BX440 LCD TV "MAIN_POWER" problem

When a mosfet blows and it's shorted between all three pins, you need to check out the gate drive circuit as well. STBY voltage is always present and goes to the main board. When you turn the TV on the main board will turn all the power rails on the PSU and the backlight. This being said you can “force” the PSU on by itself only having the backlights connected, using 1K resistors and jump from STBY to PSon, BLon and you may also have to jump to the Dim pin.
BTW there are tons of bad solder joints on the back of the PSU board. Specifically on the hot parts and while you're at it resolder all pins on every transformer.
Do some voltage measurements, but be aware to be very careful on the hot side of the PSU. There is mains voltage on it and a PFC. If the PFC a isn't working the TV wont work either.

+10, bad solder joints might be the whole problem.

[embdd]

Re: SONY KDL-40BX440 LCD TV "MAIN_POWER" problem

Thanks for the suggestions.

I redid most of the solder joints and replaced the bridge rectifier.
The MOSFETs' gates are not shorted, at least currently.
R901A reads 75k which matches its color code.

All pins on the secondary side of transformer T901 being shorted seems wrong to me. Based on the PCB traces and the schematic for DPS-165GP, there should be 3 separate windings.