Uniden LCD TV power supply help

[BigPete]

Hi,

I have a rare 42inch 1080P Uniden TV with a PK-0043CA power supply.
The power supply is full with Rubycon YXA capacitors and Panasonic inductors.
Unfortunately it has failed.
The list of failed parts that I know of are:
2kV 222K disc cap
1000V RU2C Diode https://cdn.badcaps-static.com/pdfs/...5711cea6d2.pdf
Skanken str-x6769 regulator https://cdn.badcaps-static.com/pdfs/...4cf767d514.pdf
Unknown resistor

The regulator IC is on order.
The disc cap has been replaced
I will replace the RU2C with a 1N4007 Diode https://cdn.badcaps-static.com/pdfs/...29330501fc.pdf hopefully this is fine.
The problem I have is the unknown resistor. It is completely fried.
The resistor was connected to pin 7 of the Sanken STR-X6769 IC which is the Overcurrent Protection/Bottom Detection pin.
Pin 7 splits into 2 paths, one path goes to a diode(not the faulty one) the other goes through the unknown resistor to the negative of the main bridge rectifier.

Looking for schematics has been fruitless. Does anyone know what size resistor needs to go in. While i'm picking it will be a guess, where should I start, high resistance or low resistance?
Is there anymore info that is needed?
Cheers

[dick_barton]

Re: Uniden LCD TV power supply help

Here is one application which may help you.

[budm]

Re: Uniden LCD TV power supply help

RU2C is Fast switching diode with 180nS Trr, 1N4007 is general purpose diode.

[budm]

Re: Uniden LCD TV power supply help

The Source Resistor on pin 2 (S) is more likely to burnt out too when the Snubber cap or the Power MOSFET inside the STR shorted out.
How about up loading good clear pictures of this PS.

[BigPete]

Re: Uniden LCD TV power supply help

Thanks Dick for that picture, it's a massive help. The whole circuit is virtually the same. A 100 ohm resistor is what I need.
Budm - I should've known better. The ease of going to the local electronics store clouded my judgement. This diode should be better then: http://nz.rs-online.com/web/p/products/7743600/. The Source resistors are indeed failed. Thanks for pointing that out.
I'll get a pic up later. In a rush at the moment.
Going forward. What will improve power supply reliability? Replacing the source resistors with higher wattage ones and/or the 100 ohm resistor with a higher wattage version? The unknown 100 ohm resistor was only about 5mm long and thin.
Cheers for the help.

[budm]

Re: Uniden LCD TV power supply help

i think what happen when the IC shorted it, the resistor on pin 7 was also got dumped by the HIGH Voltage (>300VDC) feeding the IC.
You want those resistors to go open circuit when the IC failed so in case the main fuse did not blow, the resistors were damaged due to the failure of the IC, so when the resistors go open circuit then the current stop flowing which is what you want to happen when the fault occurs instead of heating up the resistors that may burn up other parts nearby and catch on fire.
The resistors burnt up for reason due to too much current flowing through them.

[BigPete]

Re: Uniden LCD TV power supply help

Thanks Budm. So in short, it was just bad luck that the IC failed?

[BigPete]

Re: Uniden LCD TV power supply help

Or do I have to put a smaller fuse in? It has a 6.3A fuse which seems a lot for an LCD TV.

[BigPete]

Re: Uniden LCD TV power supply help

Just a follow up on this if anyone has a similar issue. Optocoupler NE004 in Dicks pic above was also faulty. Datasheet for it is: