Powersupply proble, SAMSUNG LA32R71B

[vikingchow]

Hi all, I am now fixing a LCD TV, SAMSUNG LA32R71B. The problem is the PSU does not output right voltage level. PSN model is BN96-03775A, here is the schematics:

http://www.elektroda.pl/rtvforum/dow...7bacedd1274c26

I have done some checks and found stand by voltage is good but +24V and +12V are only +16V and +7.5V with no load (disconnected from the logic board), even worse when load was added(drop to around 9V and 4V). Apparently this is not right. Checked all the smoothing caps, they are all fine. Checked the big cap close to switch MOSFET, was +380V(315V when B/L ON/OFF was off), also seems fine.

Suspected it was the feedback voltage problem but could find any faulty component.

Any idea or suggestion? I had been totally lost.

Thanks so much.

[PlainBill]

Re: Powersupply proble, SAMSUNG LA32R71B

Hi all, I am now fixing a LCD TV, SAMSUNG LA32R71B. The problem is the PSU does not output right voltage level. PSN model is BN96-03775A, here is the schematics:

http://www.elektroda.pl/rtvforum/dow...7bacedd1274c26

I have done some checks and found stand by voltage is good but +24V and +12V are only +16V and +7.5V with no load (disconnected from the logic board), even worse when load was added(drop to around 9V and 4V). Apparently this is not right. Checked all the smoothing caps, they are all fine. Checked the big cap close to switch MOSFET, was +380V(315V when B/L ON/OFF was off), also seems fine.

Suspected it was the feedback voltage problem but could find any faulty component.

Any idea or suggestion? I had been totally lost.

Thanks so much.

The link for the schematic doesn't work; suspects you have to be logged in to download it.

What area are you in? 380 volts is an unusual voltage for the main filter cap.

PlainBill

[vikingchow]

Re: Powersupply proble, SAMSUNG LA32R71B

Thanks for the reply.

Here is the working link:

380V seems because this is a boost converter, based on the schematic, am I right? I here the main voltage is 240VAC 50Hz.

[PlainBill]

Re: Powersupply proble, SAMSUNG LA32R71B

Thanks for the reply.

Here is the working link:

380V seems because this is a boost converter, based on the schematic, am I right? I here the main voltage is 240VAC 50Hz.

Argh!!! I already had the schematic. Someone else had a problem and thought it was the power supply; it was a blown fuse in the inverter.

This design has a PFC front end, I would expect the voltage across CP812 (150uF, 450 volts) to be at a minimum of 315 volts when the PFC is turned off (TV in standby). I'm a little uncertain about the voltage with PFC on, but 380 volts seems appropriate.

The other poster (Sam I Am) had reported that there have been reports that ICM8015 - the F9222L was a source of problems with this power supply; I'm not sure what the exact nature of the problems were.

PlainBill

[vikingchow]

Re: Powersupply proble, SAMSUNG LA32R71B

This design has a PFC front end....

The other poster (Sam I Am) had reported that there have been reports that ICM8015 - the F9222L was a source of problems with this power supply; I'm not sure what the exact nature of the problems were.

PlainBill

Yes, things become complicated where the PFC part added. Worse that I can not find the datasheet of F9222L and have no idea how it works. I had check all pin out resistance vs ground of this IC, no obvious short to ground though. Could you find the link of the reported F9222L problem so that I can have a look please?

Thanks PlainBill.

[PowerAmpFreak]

Re: Powersupply proble, SAMSUNG LA32R71B

Did you change the electrolytes at Vcc pin of the F9222L?
I've never seen the '9222 before but it appears to be an integrated halfbridge driver and controller. I would hook up (an isolated) oscilloscope to pin 19 of the '9222 and check how the primary pulses looks like.

[vikingchow]

Re: Powersupply proble, SAMSUNG LA32R71B

Did you change the electrolytes at Vcc pin of the F9222L?
I've never seen the '9222 before but it appears to be an integrated halfbridge driver and controller. I would hook up (an isolated) oscilloscope to pin 19 of the '9222 and check how the primary pulses looks like.

Vcc Pin of F9222L shows a very clean 17.5V, and those caps are both looking alright.

Just found this chip's block diagram on internet, see attachment1. I measured pin 20 and pin 19 with a scope, see attachment2&3. It seems the IC is modulating output voltage at full duty cycle, but output still too low.

[vikingchow]

Re: Powersupply proble, SAMSUNG LA32R71B

Then I measure the switching component before the PFC module, see attached pics.

Looks like some weird things happening here? It is not modulated properly, the 1st pic is QP801S Gate pin, 2nd one is Drain. The 3rd one is also Drain but in bigger time scale.

This MOSFET is not modulated in full time, just like a "part time" working guy. However CP812 still gets a smooth 380VDC.

Anyone can tell me this is normal or not?

[PowerAmpFreak]

Re: Powersupply proble, SAMSUNG LA32R71B

Connect the fluke across the transformer primary winding, or perhaps one of the secondaries to tell what duty cycle/pulse width the 9222 outputs.
I would also carefully check the error amp circuit (ZDTM851 which is a standard TL431A) and measure across RM855, you should have 2.5VDC here if voltage loop is in "balance", lower voltage indicate too low output voltage and primary controller should increase pulse width to compensate.
Also take a look on optocoupler PC803S and measure both ledside and transistorside. This signal is then fed to pin 11 of the 9222 which seems to be the dutycycle control input.
The above measurements could tell if the fault is in the error amp circuitry or in the primary controller.

[vikingchow]

Re: Powersupply proble, SAMSUNG LA32R71B

Connect the fluke across the transformer primary winding, or perhaps one of the secondaries to tell what duty cycle/pulse width the 9222 outputs.
I would also carefully check the error amp circuit (ZDTM851 which is a standard TL431A) and measure across RM855, you should have 2.5VDC here if voltage loop is in "balance", lower voltage indicate too low output voltage and primary controller should increase pulse width to compensate.
Also take a look on optocoupler PC803S and measure both ledside and transistorside. This signal is then fed to pin 11 of the 9222 which seems to be the dutycycle control input.
The above measurements could tell if the fault is in the error amp circuitry or in the primary controller.

Windings' waveforms are all like full duty cycle... I already checked the feedback circuit and changed KA431 and the opto-coupler as well, not their problems.

Come back to my previous suspicion, QP801S, the main switching MOSFET is not modulated as I expected. I looked at the switching IC, ICP801S, its power supply was a bit noisy, followed this track, found the stand-by power switching IC's power supply was noisy also(IPS801S). Noise ripple was about 5V. Pretty sure its not filtering caps' problem.

[vikingchow]

Re: Powersupply proble, SAMSUNG LA32R71B

Now found that the noise on ICs' Vdd was from PFC circuit, I disconnected this part by desolder the resistor RM801, noise gone.

[PowerAmpFreak]

Re: Powersupply proble, SAMSUNG LA32R71B

Windings' waveforms are all like full duty cycle... I already checked the feedback circuit and changed KA431 and the opto-coupler as well, not their problems.

Come back to my previous suspicion, QP801S, the main switching MOSFET is not modulated as I expected. I looked at the switching IC, ICP801S, its power supply was a bit noisy, followed this track, found the stand-by power switching IC's power supply was noisy also(IPS801S). Noise ripple was about 5V. Pretty sure its not filtering caps' problem.

BUT, did you just replace them without even measuring what's going on? Did you check the feedback voltages applied to '431. Can you actually verify that the voltage loop wants to increase output voltage and stay in regulation?
It appears so, since you say the windings have full duty-cycle but the supply still fails in getting the voltage up. There could be problems in the power switching circuit, like the transformer. I would measure inductance in the primary winding of it. In these topologies, there use to be rather high inductance to minimize magnetizing current in the primary.

[vikingchow]

Re: Powersupply proble, SAMSUNG LA32R71B

Yes I have checked the feedback loop. Reference voltage of '431 was ~1.5V, because that 24V output had dropped to 16V.

I have no way to measure the transformer windings' inductance. But could you look back to my #8 post, the main switch circuit before the PFC switching waveforms doesn't look right, does it?

[PowerAmpFreak]

Re: Powersupply proble, SAMSUNG LA32R71B

PFC's can be rather tricky to measure, I've no idea how the PFC should look like in this specific supply.
BUT, fact remains, if the PFC succeed to provide 380V for the half-bridge supply, even when your secondary outputs drops, I would not blame the PFC.
If the half-bridge is fed with 380V all the time, the failure should be in the half-bridge. And you discovered the error amp and feedback loop appears to do what it's supposed to do, full throttle!! and duty-cycle, since the outputs are dropping. I will take a look in the schematics again.

[PowerAmpFreak]

Re: Powersupply proble, SAMSUNG LA32R71B

Did you check CM802. All primary current in the transformer must go through it. If it's bad, I suspect the supply will loose power.

[Wizard]

Re: Powersupply proble, SAMSUNG LA32R71B

Very important thing capacitance meter does not test ESR that you are after. Change the small capacitors for that power supply if you do not have ESR meter. And the secondary capacitors that has feedback circuit in it. Noisy voltages forces the SMPS IC to cut back on pulses due to big spikes sensed.

opt isolator can go bad once in awhile. Check all resistors just in case.

Cheers, Wizard

[vikingchow]

Re: Powersupply proble, SAMSUNG LA32R71B

Now I think it's because of ICP801S, another switching controller, its drive Pin only offer about 10% dutycycle PWM when modulating, and most of the time doesn't output modulating pulses at all. Base on the first page of schematic, the PSU DC outputs would not affect its duty cycle, only a voltage divider across 380V DC line gives this IC voltage feedback.

Voltage on RP814 now measures 2.5V which is alright, RP819 has right value of resistance also. Vcc(Pin8) is about 15V, very clean and stable(I have disconnected RM801 so no noise from F9222L anymore).

I am going to order this IC(TDA4863G) and will change it then see what happens.

[pauls15silvia]

Re: Powersupply proble, SAMSUNG LA32R71B

Hi All,

I have the same tv set with what appears to be the same or very similar power supply problem that vikingchow has described.

I got to the point of looking at the F9222L as a possible cause of the problem as the caps all seem ok and the standby supply is operating correctly. I wanted to say thanks for the information as I couldn't find anything useful on the F9222L IC until my searching brought me here.

Before I dig further can anyone tell me if this problem was solved?

Thanks, Paul.

[vikingchow]

Re: Powersupply proble, SAMSUNG LA32R71B

Hi all,

Finally this TV was fixed up. The problem was exactly the chip "F9222L". It took me nearly two months to get this chip from China (I asked one of my friends there to order it for me, cheap of cause). Once this IC changed, problem solved.

Thanks for all of you who gave me hints and information.

[pauls15silvia]

Re: Powersupply proble, SAMSUNG LA32R71B

Hi guys,

Same here. After far too many hours of poking around inside this TV I came to the conclusion that it had to be the F9222L IC. It was still running but just didn't seem right.

After replacing the F9222L with an expencive one from ebay all is now well.

Thanks again guys. Very helpful information.