PSU with FAN8754 won't torn on

[fejlong]

Hello everyone,
Recently I got an LED TV Hannspree HSG1210 with no power symphtom.
The power supply is a 40-P081C0-PWD1XG, I could not find a schematic for it other than from a similar TV, Philco PH32.
The power board has a VIPER17L which makes 3.3V, and a FAN8754 which is not working or won't start.
The VIPER17L is working and 3.3V is present. FAN8754 is making a rattling sound and does not operate the power mosfet.
I have changed the FAN8754 and the power mosfet, thinking they are defective. But the 24V still does not appear.
The four rectifier diodes I did change, still not working, I have 300V on the HV cap. there is near to 0V all the time on the power mosfet's gate. so the fan is not control it.
Anyone knows where can I continue troubleshooting ?
Any info whould be much apreciated

[ReeceyBurger123]

Re: PSU with FAN8754 won't torn on

First of all you need to make sure the motherboard is telling the Psu to come on, what is the Pson voltage at ?

[CapLeaker]

Re: PSU with FAN8754 won't torn on

no boosted voltage on the primary filter cap(s) = no 24V.

[fejlong]

Re: PSU with FAN8754 won't torn on

Pson it is labeled P-ON, and it is about 3.2V

[budm]

Re: PSU with FAN8754 won't torn on

Did you find the spec sheet for the 'FAN8754'? Need to find out if you have VCC to run the IC or not, I expect around 15 VDC.
R112 is less than 1 Ohm?

[fejlong]

Re: PSU with FAN8754 won't torn on

Sorry for that, It's a FAN6754. I am attaching the datasheet.

Resistor R112 is labeld as 0.15 Ohm, with the multimeter it shows 0.2 Ohm.

Supply Pin 7 of IC has 18.5V ,

HV pin 4 should be at least 104V according to the datasheet page 5 high voltage section, and this has only 94.5V don't know what is causing this

[dick_barton]

Re: PSU with FAN8754 won't torn on

Check the components connected to it. D101, R102 (563 = 56K), Rxxx (104 = 100K) in series with it.

Edit
18.5V for pin 7 (VCC) is above the spec for the chip to operate. Vcc min is stated as 15.5V, typical 16,5V, maximum 17.5V

What's the voltage on the feedback pin 2 which should be <4.6V

[dick_barton]

Re: PSU with FAN8754 won't torn on

The mosfet pin needs to be soldered and looking at your photo there appears to be a solder shard between pins 1 & 2 of the FAN chip although it could just be the lighting.

[fejlong]

Re: PSU with FAN8754 won't torn on

I have did a check on those components, everything looks good.

initalIy I have changed the mosfet, the solder is good on the mosfet, may it stil has some flux on that area.

[dick_barton]

Re: PSU with FAN8754 won't torn on

What's the voltage on the feedback pin 2 which should be <4.6V

And is that a small slither of solder between pins 1 & 2 of the IC or just a reflection of the light on the photo?

[fejlong]

Re: PSU with FAN8754 won't torn on

Between pins 1 and 2, It is flux wich remained after replacing the IC.

I will check the voltage on the feedback pin and be back to you.

Attached some close up pics of the IC

[fejlong]

Re: PSU with FAN8754 won't torn on

On feedback pin 2 is 2V

[fejlong]

Re: PSU with FAN8754 won't torn on

There is only 2V on the feedback pin, what does this means?
Where should I look for the problem?

[dick_barton]

Re: PSU with FAN8754 won't torn on

The datasheet says that if FB > 4.6V for more than 55mS then it latches off the PWM.
With 2V at this pin then that is not the problem.

[dick_barton]

Re: PSU with FAN8754 won't torn on

initalIy I have changed the mosfet

Was that with an identical part or substitute?

[dick_barton]

Re: PSU with FAN8754 won't torn on

You are correct regarding the HV pin input voltage being low. I would be tempted to replace the two resistors and diode feeding it and see if the voltage at the HV pin increases.

[fejlong]

Re: PSU with FAN8754 won't torn on

I tried that, no change

[dick_barton]

Re: PSU with FAN8754 won't torn on

What is the voltage at the other end of the diode and resistors?

[fejlong]

Re: PSU with FAN8754 won't torn on

Regarding wich ground ?

[dick_barton]

Re: PSU with FAN8754 won't torn on

The same ground as you used to measure the 17.5V in post #6

Edit
What is your mains supply voltage to the set.