FIXED: TV power supply KPS+L150C3-01

[vinicius.jlantunes]

Hello everyone,

I am attempting to fix a faulty power supply in my friend's TV, but no lucky so far. I recorded what we did so far in a Youtube video:
https://youtu.be/Gtshvifap1w
(I say some stupid things in the video I only realized later when editing... should probably add some annotations)

As you can see, we did find a bad cap that we replaced, but that didn't do the trick. I found the schematic online (http://elektrotanya.com/konka_kps-l150c3-01_sch.pdf/download.html) and attached here to this thread.

As you can also see in the video, I think part of the problem might be the fact that the high power switch (FSFR1700) that drives the 12V rail (and 24V rail as well, according to schematic?) is not being enabled, but to be honest I am not sure, could be a red herring. I don't get how some parts of this power supply work by looking at the schematic (e.g. the enable circuit I mentioned seems to be powered from 12V rail the FSFR1700 itself should be driving?).

I have already verified that the mains gets correctly rectified / filtered, generating a rail called " VBUS"  in the schematic which is then fed into two sub-circuits: one for generating  the (various) 5V rail, and the other for generating the 24V and 12V rails. The 5V circuit is fine.
There are some other ancillary circuits as well that I didn't think could be related to the problem, but I could be wrong.

Any ideas as to what I should look at next? Any help is appreciated. Thanks!!!

[isopropilick]

Can you attach some photos of the boards?

[vinicius.jlantunes]

Sure isopropilick, I will do so when back home tonight.

[mij59]

The fault is probably caused by a temporary short in the capacitor CW911, the replacement capacitor needs to meet the voltage, capacitance AND a low loss dielectric rating of the original.

Check the resistor RW903.
Connect the power supply and power the TV up, check the voltage LVCC and VDL.

[Rasz]

what do you mean by enabled? there is no enable pin, only supply (lvcc pin7)
adblock blocks pdf download, but i found another diagram: http://www.go-gddq.com/html/kj_wx_YeJingCaiDian/2013-07/1154286.htm

is that TL431 regulating ~14v for FSFR1700?

[vinicius.jlantunes]

hey rasz,

You might be right: when I googled FSFR1700 it brought this datasheet (https://www.fairchildsemi.com/datasheets/FS/FSFR2100U.pdf) and I saw in page 3 the following pin description:

Pin 2: CON - This pin is for enable/disable and protection. When the voltage of this pin is above 0.6V, the IC operation is enabled. When the voltage of this pin drops below 0.4V, gate drive signals for both MOSFETs are disabled. When the voltage of this pin increases above 5V, protection is triggered.

Well, due to your comment I decided do check again and found this other datasheet (https://www.fairchildsemi.com/datasheets/FS/FSFR1700.pdf); both are VERY similar and mention part FSFR1700 - but this one does not mention pin 2 being an "enable / disable" pin indeed! 

The pin labels in the schematic seem to match the first datasheet though... so I'm not sure at this point.

Anyways, that seems to be a red herring. I checked the supply to the LVCC pin 7 as you suggested and indeed, nothing there. The control portion of the circuit is not being powered up (I measured it between pins 7 [LVCC] and 5 [control circuit ground).
What powers this LVCC pin is a rail called VCC1, which in turn is generated via the TL431 based circuit you also mentioned - I probed there and niente... I continued to trace it going backwards and found this VCCP bus that powers the TL431 circuit not to be working!

This VCCP rail being generated by a little circuit in the first page of the schematic, left-bottom corner. It's just a transistor and a couple resistors; well, there is no voltage coming out of that QB901 PNP transistor! That's where this VCCP bus should be coming out of. I checked the VCC rail that powers this transistor and there I do get a voltage (17.7 V); so it must be this transistor that is faulty!

mij59: as you can see above, I checked LVCC as per yours and rasz suggestion and you guys are right, there is nothing there... that's a problem!

Does my analysis make sense?

Thank you all for the help so far!!!

[Rasz]

check this transistor with diode tester, measure resistors, check cap for short,  should be one of those

btw which cap did you replace at start?

[mij59]

The failure in the power supply VCCP could be the result of a short in the LVCC power rail, check all components connected to LVCC,  e.g.  ZDW901, NW901..

[vinicius.jlantunes]

Thanks Rasz, mij59, I will check the transistor and surrounding components, as well as the other stuff connected to LVCC when I get home tonight.
Rasz, the capacitor I have already replaced is CW911.

As it's possible that the transistor QB901 is gone, I started looking for a replacement. It seems to be one of these (pending confirmation when I'm back home to check the code):
http://www.nxp.com/documents/data_sheet/PBSS5160T.pdf

It's a "special" low Vcesat transistor, 60V 1A. I will have to check the transistors I have at home but I doubt I will have a suitable replacement, and can tell already it will be difficult to source it here in Brazil (in fact EVERYTHING is difficult to source around here, bar 555's and BC548's).
I will maybe have to try my luck with a more jelly bean transistor there - any suggestions are welcome.

[vinicius.jlantunes]

Just wanted to say thanks once more for those who helped. The TV is fixed!

Here is a part 2 video my friend and I shot of the repair. You will see that the transistor was indeed the faulty part, but it could've been a bit embarassing if it wasn't... turns out there is an "enable" signal that comes from the other boards, so testing / measuring solely at the bench can be misleading... you might think it doesn't work but all it is missing is the enable signal...
https://youtu.be/SOMQNVuQjqw