Philips 42PFL3604D/12 - No Power

[sententia]

Hello everyone!

This Philips 42" TV has no power symptoms. No standby led and looks completely dead.

Power supply board only has 4.1V on +5VSB and no other voltages on any other line.

All electrolytics were measured OK.

I am stuck. Any help greatly appreciated!

Thanks!

Here is board and schematic.

[paulstef]

Re: Philips 42PFL3604D/12 - No Power

First check major power semiconductors for faults. Then, are you getting the ON signal on connector CN903?

If you have standby voltage then IC905 works.

Remove the connector CN903.

Connect a 100R resistor from +5VSB to S/B (connector 903)
+5VSB should now increase from 4.1V to probably 5V.
The signal S/B will also power up the two ICs IC901 and IC902.

IC902 being the boosting circuit which creates 395V and IC901 24V and 12V.

Post the results here.

[Alastair E]

Re: Philips 42PFL3604D/12 - No Power

Looks like there's low 5v rail. Thats derived by the 'small' SMPSU part. 4v aint enough, It Has to be 5 to 5.1V or the mainboard doesnt like it, that goes more or less for all sets.

Looking at your picture, there's two transformers at near bottom centre--The Small (STDby 5v) supply comes from the smaller of the two transformers.
--It's driven by that 7/8 legged DIL chip just above it.

For a quick start at repair attempt, I would replace the two small electrolytic caps near/in that chip's vicinity--Prob around 10-47uF they'll be.
I wouldn't bother measuring 'em--They are cheap enough just change on sight, sometimes a cap can measure OK but still cause troubles.

One will be the start-up cap I expect, other will be the chip's supply reservoir--Worth just changing 'em
Other caps that could cause issues are the 1000/3300uF (whatever value they are on your board) below that transformer--usually 10-16V rated parts...

C946 = 47uF 35V, C950 2,200uF 10V. Change those tow for new--see what happens, Might get lucky.

[paulstef]

Re: Philips 42PFL3604D/12 - No Power

Looks like there's low 5v rail.

No.

The TL431 (IC908) has a 2.5V reference. 4.12V is the calculated voltage the circuit will adjust to while the signal S/B is LOW. That looks normal.

When S/B becomes HIGH then 22k (R960) are in parallel with 15.4K (R957) and the circuit will adjust to 5.26V (calculating).

It's more likely the OP doesn't get the S/B signal.

Of course caps can still be bad.

[sententia]

Re: Philips 42PFL3604D/12 - No Power

paulstef that's true! With resistor the psu turns on.

Voltages are
+5v = 5.24V
+12V = 12.5V
+24V = 25V
Dim & ENA = -1.8V

..

Alastair all electrolytics capacitor were tested good including the 47uf..

[paulstef]

Re: Philips 42PFL3604D/12 - No Power

Check the 395V too.

Now you need to find out why the mainboard doesn't send the S/B.

What's the mainboard number? Can we have a schematic for this as well?

[sententia]

Re: Philips 42PFL3604D/12 - No Power

Thank you all for the input!

The 395V line looks good = 398.4V

The board is 715G3299-1A, i did a quick search on the net but didn't find a schematic.

Also the mainboard is full of Capxon caps... not very promising, although they are rated 2000hrs

[paulstef]

Re: Philips 42PFL3604D/12 - No Power

Repairing such aboard without schematic will be difficult.
If you have an oscilloscope you could check the S/B signal, it may just be a brief pulse. (There can still be a problem with the caps in the standby circuit on the PSU.
You need to know if the 4.1V don't drop when you try to turn on the TV.)

If you are sure the 4.1V are steady/never drop below this value then the problem is most likely on the mainboard.
First find linear voltage regulators, I'm pretty sure the +5V line goes to a 3.3V regulator. Find this and see what voltage you get on the output.

Also do a visual inspection first to see if you find any burnt components.

[sententia]

Re: Philips 42PFL3604D/12 - No Power

4.1V does not fluctuate and stays steady with everything connected.

Many caps on mainboard do measure a bit low in capacitance (470uf around 440uf) however....

Measuring input voltage on regulator marked in red circle (which is a LD33) shows 0.1V.

Measuring resistance on D100 and the cap next to it shows 24ohms...
I removed the diode thinking it would cause the problem but short or low resistance still remains.

Also another 3.3V regulator i tested does not get any input...

[sententia]

Re: Philips 42PFL3604D/12 - No Power

I found schematic for the mainboard.
This is the DC-DC pages.

[paulstef]

Re: Philips 42PFL3604D/12 - No Power

Some of these voltages are only switched on when you turn on the TV.

The standby voltage on the board is called +5V_STB.

U152, what is the voltage at pin 3 and 2 (to gnd)?

[paulstef]

Re: Philips 42PFL3604D/12 - No Power

And 24Ohm is not at all "low" when you consider the output voltage of only 1.1V at that point.

[sententia]

Re: Philips 42PFL3604D/12 - No Power

It looks that there are differences in the schematic i provided to the mainboard.
U152 on schematic is a AME8810 but on board has ADI 920 on the SOT-223.

Either way both are regulators. Pin 3 has 4.05V but there is no output on any other pin...

[paulstef]

Re: Philips 42PFL3604D/12 - No Power

If there is no voltage on pin 2 than either the regulator is defective or there is a short on its output. Does this chip get hot or warm?
You could try to slightly lift up pin 2 and see if voltage comes out. Do you read a short on pin 2? (without voltage applied of course :-) )

[paulstef]

Re: Philips 42PFL3604D/12 - No Power

And we also need to know where DV33SB (the output of this regulator) goes to. Must be on another part of the schematic.

[sententia]

Re: Philips 42PFL3604D/12 - No Power

Yes pin 2 is shorted to ground. The ic slowly gets hot.
I will try to lift the pin or remove it to find out whats wrong...

Thanks again paulstef for helping

It goes to a lot of places.... I included one page. Other is IR and Photo resistor circuit.

[paulstef]

Re: Philips 42PFL3604D/12 - No Power

I included one page. Other is IR and Photo resistor circuit.

Where is it?

[sententia]

Re: Philips 42PFL3604D/12 - No Power

ooops... here it is.

I removed the regulator and it is OK. the short is somewhere else on the mainboard....

I get 56 matches in diagram for DV33SB...

[paulstef]

Re: Philips 42PFL3604D/12 - No Power

56 matches. That's .... a lot. :

If you happened to have a thermal imaging camera it would be easy to find out. I assume you don't have one so you will have to go step by step.

Cutting traces, removing components etc.

Have a look if you find ceramic caps (little brown ones) near fixing holes, I have several cracked and shorted when the PCB bends due to too much pressure exerted by the screw nearby.

Other than that it's really a trial and error approach.

There instruments out that could detect though the distance to a short, don't know how they work, my guess is with an AC signal and measuring the impedance.

[sententia]

Re: Philips 42PFL3604D/12 - No Power

No unfortunately i don't have a thermal camera...

I don't see any caps near the screw holes...

Do you think an electrolytic might be shorted? I have not removed any of them to check them off board..

U154 also have its output shorted to ground. Resistance is around 7ohms..

I will try to connect and feel by hand if the are any hotspots...