Samsung LN32C350 No Backlight

[Glitcher]

Hey guys, I need some help with this one.

I initially got this set with the problem of no power and no standby light.
I opened it up and found a bad mains fuse and one shorted FET that's labeled 10NM60N. I replaced the fused and the fet with a 10NM60ND.

The standby light is now on and the set turns on with the Samsung chime but there is no backlight. I shined a light on the screen and can see the picture.

How do I know if this is a main board or PSU/inverter problem? What voltages should I check?

[budm]

Re: Samsung LN32C350 No Backlight

Look to see if the main board send the BL_ON signal to this power supply/inverter combo board. Check the DC voltage on the big main filter to see if it has about 370~400VDC right at the two legs.

[Glitcher]

Re: Samsung LN32C350 No Backlight

Hmm I only see one labeled BLU_ON, is that the one? Its measuring 0.05V

In standby the cap measures 162.2V, but once I turn on the set, it drops to about 157.6V. I'm guessing that's a problem in the PSU?

[selldoor]

Re: Samsung LN32C350 No Backlight

If you unplug the lead to the lamps does the voltage at the big cap still drop?
Is there no flash from the lamps at all?

[Glitcher]

Re: Samsung LN32C350 No Backlight

There's no flash at all.

There's two connections going to the lamps. I just tried unplugging both connections and then individually and the big cap would still drop its voltage in all cases.

[budm]

Re: Samsung LN32C350 No Backlight

"In standby the cap measures 162.2V, but once I turn on the set, it drops to about 157.6V" the PFC voltage booster is not working. Need to find out if PS_ON is also present, no PS_ON = No PFC Voltage booster.

[Glitcher]

Re: Samsung LN32C350 No Backlight

While in standby PS_ON measures 0.04V and while the set is on, PS_ON measures 1.98V.

[budm]

Re: Samsung LN32C350 No Backlight

So the PS_ON is OK, but the PFC Voltage booster is not working, that will the next section to work on.

[budm]

Re: Samsung LN32C350 No Backlight

Post good pictures of the bottom side of this PS-Inverter board, in the means time check all the Diodes (especially the two black one next to that main filter cap) on the primary side of the circuits, and look for any burnt/damaged parts.

[Glitcher]

Re: Samsung LN32C350 No Backlight

So far I checked all the diodes on the PSU in circuit. They all seem good with readings in one direction and infinite in the other.

There are no obvious burnt/broken components that I'm seeing.

[budm]

Re: Samsung LN32C350 No Backlight

That MOSFET you replaced is for the PFC circuit, there is a SMD IC close by that MOSFET, it is an PFC Controller IC which is more likely to be damaged when the MOSFET went out, so you need to get the IC number to get the spec sheet and the pin outs, then you will need to check ALL the resistors and diode connected to the Gate and any resistor connected to the Source pin of that MOSFET you replaced.

[tom66]

Re: Samsung LN32C350 No Backlight

I'll just leave this here...


Check DP805 is not short.
Check Vcc across CP809 for PFC controller.
Check Vcc across CM804 for standby voltage/multi voltage.

edit: above might not be relevant if PFC IC is damaged, did not read post fully.

[Glitcher]

Re: Samsung LN32C350 No Backlight

I checked the resistors directly connected to the gate pin and they measure to the correct range. The diode beside is still in question, since I'm not sure how to check that.

I'm guessing that 8 pinned IC (IC802S) to the left of the FET is the PFC IC ? From tom's schematic its a FAN7530MX. I found the datasheet for it but not sure how to check if its still good. Should I check the voltages of each pin, and since its in the primary side, I should check them relative to the main cap correct?

Tom - Thanks for the schematic, I checked DP805 out of circuit and it appears normal. What did you mean when you said to check the "Vcc"?

[budm]

Re: Samsung LN32C350 No Backlight

You can check the resistance of all the pins against the Ground pin, report the reading. Also check the voltage on the VCC pin, main cap negative is the circuit ground ref. for the primary side.

[tieny]

Re: Samsung LN32C350 No Backlight

...deleted *wrong thread...

[Glitcher]

Re: Samsung LN32C350 No Backlight

Measurements are shown below in the photo.

The Vcc pin measures 15.9V while the set is on.

The CS pin has a low resistance reading, but im not sure if it means it shorted or not. Measuring the voltage it reads 0.71V while set is on.

Reading the datasheet
Pin 4 CS: "This pin is the input of the over-current protection comparator. The MOSFET current is
sensed using a sensing resistor and the resulting voltage is applied to this pin. An
internal RC filter is included to filter switching noise."

Not sure what it means.

[tom66]

Re: Samsung LN32C350 No Backlight

CS pin is connected to ground via very low ohm resistor (<1 ohm) so that's normal. If you have got 16V at that IC you should have PFC voltage.

Check DP803, RP803, RP823, RP802 (SMD resistor/diode probably.)
DP803: normal diode check (not open)
RP803: 30 ohms
RP823: 68 ohms
RP802: 10k ohms

Also CP810 - across diode DP805 - should read open

[Glitcher]

Re: Samsung LN32C350 No Backlight

How would you go about testing the smd diode (DP803), the one with 3 pins,

[tom66]

Re: Samsung LN32C350 No Backlight

For now, check to see it is not open circuit. As that is all is important for now to get that FET going.

[Glitcher]

Re: Samsung LN32C350 No Backlight

I checked it with diode mode. It reads 0.095 in both directions. It reads this for both pins.