Philips 42PF9541/98 42" LCD TV No 1.2v error

[taxxin]

Dear Members

I have a Philips TV that would not turn on and enter protection mode with error led with 8 blinks, This indicated no 1.2v present.

After long searching I have found that the Logic board has a DC-DC from 12v to 3.3 and 12v to 1.2v. I did some quick probing to find that the 1.2v inductor had lower resistance to ground than the other 3.3v rail, the other inductor would measure over 1k ohm and this one only 28ohms i thought there might be a short somewhere, but injected 1.2v into it and it doesn't seem to be shorting, seems to be taking it up at 1.2v and 300-400ma.

Anyways Ive removed the Dual mosfet package (high and low) and tried to test it, the low side seems to be working fine but the high side doesnt seem to be working, (current not flowing from s to d after powering gate).

What should i do at this point? should i look at anything else before replacing this mosfet? i have the circuit diagram but not every component is marked on the PCB.

[RJARRRPCGP]

Re: Philips 42PF9541/98 42" LCD TV No 1.2v error

248W=Looks like a plasma, but dunno. I dunno if even CCFL LCD TVs would even need to draw that much.

Albeit only roughly 50W more than my old Panasonic 42-inch LCD TV.

[taxxin]

Re: Philips 42PF9541/98 42" LCD TV No 1.2v error

248W=Looks like a plasma, but dunno. I dunno if even CCFL LCD TVs would even need to draw that much.

Albeit only roughly 50W more than my old Panasonic 42-inch LCD TV.

yes its strange, its most likely for their "Ambilight" feature, which are CCFL tubes that surround the entire back side of the tv through translucent plastic shell.

But anyways i unplugged the logic board and noticed that the TV starts up and backlight on, with grey screen. But im not completely sure of replacing the mosfet will fix it.

[dick_barton]

Re: Philips 42PF9541/98 42" LCD TV No 1.2v error

Have you checked the 22uF cap (2U22) is not the issue?

[dick_barton]

Re: Philips 42PF9541/98 42" LCD TV No 1.2v error

Re-reading your first post if you injected 1.2V and a current of between 300 to 400mA flowed, would indicate it's feeding into a resistance of approximately 3.5 ohms?

[taxxin]

Re: Philips 42PF9541/98 42" LCD TV No 1.2v error

Have you checked the 22uF cap (2U22) is not the issue?

Hello Dick, thanks for the input, Yes i suspected it was the cap the colour was different to other caps , but when i removed it, the resistance stayed the same. I also measured it for capacitance and all is well with him.

I believe it might be normal? I suspect that the 1.2v rail is what feeds the microprocessor and would probably be a low resistance to ground. i had a quick look at what else it could be, it seems that there is no other path to ground without a series of resistors or transistors.

I also removed the mosfet and the resistance also stayed the same...

[dick_barton]

Re: Philips 42PF9541/98 42" LCD TV No 1.2v error

Are you able to change the suspected mosfet.
7U01-1 and 7U01-2 supplies +3V3

One of the mosfets 7U03 supplies +1V2
and the other 7U03 supplies +2V5

Have you checked fuse 1U01

[taxxin]

Re: Philips 42PF9541/98 42" LCD TV No 1.2v error

Are you able to change the suspected mosfet.
7U01-1 and 7U01-2 supplies +3V3

One of the mosfets 7U03 supplies +1V2
and the other 7U03 supplies +2V5

Have you checked fuse 1U01

Yes i have checked both fuses and they seem right, I just ordered a pack of mosfets, they should arrive later in the week. Will see how it goes, hopefully well.

What are my other options? Is it possible to replace the logic board with a chinese generic logic board with LVDS connector? I dont really need the fancy audio and speakers.

I saw on ebay some logic boards https://www.ebay.com/itm/38291040446...EAAOSw-HRb9HjJ but the cost plus shipping to Thailand would be rediculous, it would cost the same to buy a whole TV set.

[taxxin]

Re: Philips 42PF9541/98 42" LCD TV No 1.2v error

just found this in the service manual

.2.5PSU Start-up Sequence

1. If the input voltage of the DC/DC converters is around 12 V (measured on the decoupling capacitors 2U17/2U25/2U45) and the ENABLE signals are "low" (active), then the output voltages should have their normal values.

2. First, the Stand-by Processor activates the +1V2 supply (via ENABLE-1V2).

3. Then, after this voltage becomes present and is detected OK (about 100 ms), the other two voltages (+2V5 and +3V3) will be activated (via ENABLE-3V3).

4. The current consumption of controller IC 7U00 is around 20 mA (that means around 200 mV drop voltage across resistor 3U22).

9.2.6+2V5D Linear Stabilizer

•Provides the +2V5D voltage, and is derived from the +5V2-STBY voltage coming from the Main Power Supply.
•The output current is limited to a few tenths of mA.
•Output over-voltage protection is done by zener diode 6U17.9.2.7+12V Switch
•The +12V switch is activated when the POD-MODE signal is "low"
.•The rise time of the output voltage is set by components 2U42, 3U43, and 3U95 at about 30 ms.
•The switch "off" is fast, because there can be fault currents that must be interrupted.
•When the input voltage (+12VS) is higher than 15 V, the switch is disabled via circuit 6U12, 3U52, 3U53, 2U71, and 7U14-2.9.2.8Internal Protection
•Provides a SUPPLY-FAULT signal (active "low"), when the output voltage of any DC/DC converter is out of its limits (±10% of the normal value). In such cases, the Stand-by Processor will immediately stop the supplies by sending a "high" control signal towards the external and internal supplies: ENABLE-xVx, POD-MODE, ON-MODE, and STAND-BY. Note: The SUPPLY-FAULT control signal is "low" when any DC/DC converter is disabled by its control signal (ENABLE-xVx) and +12VSW is present, therefore it is ignored during start-up!

•The internal protection works together with the output over-voltage detector transistors 7U15-1, 7U15-2, 7U29-1, and 7U29-2.9.2.91.2V and 3.3V DC/DC ConvertersIntroductionThe circuit used is a so-called "synchronous buck converter". Some characteristics:
•Switching frequency: approx. 250 kHz.
•Efficiency: approx. 90%.
•Built-in output over-voltage and over-current protections
•Soft start.
•Software controlled “on/off” (via ENABLE line).Block DiagramFigure 9-4 Block diagram synchronous buck converter.The advantage of a "synchronous buck converter" over a "classical buck converter" is its better efficiency (about 90%). The difference between the two is that in a synchronous buck converter the "low -side" diode is replaced by a MOSFET TS2 (item 7U03). This, because the voltage drop across a MOSFET is smaller than the forward voltage drop of a diode. This second MOSFET TS2 conducts current during the "off" times of the first MOSFET TS1 (item 7U01 at the input side). 8V6SWITCH+5V+8V6+8V6-SW+2V5D+2V5+12VSWSUPPLY-FAULTON-MODE+2V5DLINEARSTABILISER+5V2_STBY12VSWITCH+12VSW+1V2ENABLE-1V2VSWVTUNGENERATOR12V/1V2DC/DC CONV.12V/3V3DC/DC CONV.+12VS+5VPOD-MODE+2V5LINEARSTABILISER+3V3VTUNENABLE-3V3F_15400_004.eps280905(Reserved)(Reserved)(Reserved)12V/2V5DC/DC CONV.+2V5ENABLE-2V5F_15400_005.eps090505DSGDVinSGPWM GENERATOR& MOSFET DRIVERGNDVoutGNDC1TS2FBTS1L1
Circuit Descriptions, Abbreviation List, and IC Data SheetsEN 153BJ2.4U/BJ2.5U LA9.The upper MOSFET TS1 conducts, to transfer energy from the input to the inductor L1 and load RL, while the lower MOSFET TS2 conducts to circulate the inductor current (free wheel). The synchronous PWM control block regulates the output voltage by modulating the conduction intervals of the upper and lower MOSFETs.PWM Generator and MOSFET DriversThis circuit is a one-chip solution (item 7U00). It contains all the circuitry for two independent buck regulators (3V3 and 1V2). The MOSFETs T7U01 and T7U03 are the switching transistors, they are conducting alternatively.

•Time sequence 1: T7U01 is conducting; energy is stored in coil 5U00/5U03. The current is flowing from the +12VSW power supply source.

•Time sequence 2: T7U01 is blocked; energy is stored in coil 5U00/5U03.

•Time sequence 3: T7U03 is conducting, and the current circuit is now closed via T7U03, Coil 5U00/5U03, C2U24/2U22, and the load. So the energy stored in the coil during time sequence T1 is consumed during sequence T3. The signal on the gate T7U03 is 180 degrees turned compared with the signal on the gate T7U01.Voltage BoosterThis circuit is build around capacitors 2U11 and 2U26, resistor 3U11, diodes 6U22 and 6U23, and transistor 7U07.It generates the +18 V boost voltage on pin 4 of item 7U00, to drive the "high-side" power MOS-FET 7U01. The voltage is generated only during normal operation of the converter; therefore, any drop in its value means an internal fault condition, which is sensed by the internal protection circuit. The AC component of the voltage on the source of transistor 7U01 is rectified by the diodes and added to the input voltage, resulting into the boost voltage. The resistor 3U11 limits the peak current through the rectifier diodes.Over-current DetectionOver-current detection is done via components 3U07, 3U08, 3U82, 3U83, and 2U18 for the 3.3 V converter and 3U09, 3U10, 3U96, 3U97, and 2U12 for the 1.2 V converter.Under-voltage DetectionThere is an additional circuit (7U10 and 7U11) to switch "off" the 3.3 V converter in case the +12VS drops below 9 V

.Service Tips

When a power MOS-FET is found defective, replace the other power MOS-FET and fuse 1U01 as well.
•For a normal operation of the converter, it is important to check the switching frequency, the value of the boost voltage, and the amplitude of the gate voltage of transistor 7U04 (it should be close to the boost voltage).


So it says to replace both of them and the fuse... How can i check if the pwm controller is defective? the Part no NCP5422ADR2 It looks like it does the voltage checks and detects any faults. so if it is going into protection mode its still good?

[dick_barton]

Re: Philips 42PF9541/98 42" LCD TV No 1.2v error

A fuse doesn't need replacing unless it's blown so since the fuse hasn't blown then the IC may well be OK and only the mosfet needs to be replaced. Was the faulty mosfet open circuit between drain and source?

[taxxin]

Re: Philips 42PF9541/98 42" LCD TV No 1.2v error

A fuse doesn't need replacing unless it's blown so since the fuse hasn't blown then the IC may well be OK and only the mosfet needs to be replaced. Was the faulty mosfet open circuit between drain and source?

I tested the mosfet off the board, no shorts from drain to source, but i tested the operation, only one of the mosfet circuits work as the should, I just used diode check mode (continuity test) on the source to gate and test if the gate turns on and gives continuity between source and drain, one of the gates when powered work, the other one doesn't. Which happens to be the 12v high side.

So it could not be getting the voltage it needs to make the 1.2v

[taxxin]

Re: Philips 42PF9541/98 42" LCD TV No 1.2v error

Ive ordered a pack of new mosfets, they are not the same ones listed, but they have the same pinout and have similar or higher characteristics as the original part. has higher peak values. Hopefully its going to do the job. otherwise im gonna dump the whole thing in the bin

[taxxin]

Re: Philips 42PF9541/98 42" LCD TV No 1.2v error

updated:

The Mosfets came in this morning, I replaced both of the mosfets on the board, but did not replace the fuse.

Put everything back together and the exact same issue, turn on, relays click, 12v gets sent to the logic board through the fuses about 1 second later it all shuts off, relays click shut. then the standby LED blinks 8 times like before.

I tried to measure the voltage out from the mosfets to the coil induction and it looks like nothing. however i do get 12v reading on the high side output on the mosfet.

On standby i only have a single wire coming into the logic board with 5v, but id have to chase it up to see if it powers the mosfet drivers and such.

Its really quite hard to check because it swtiches on then back off because the protection kicks in.

What would be the proper way to go around troubleshooting this? should i manually inject voltages into each rail?

I downloaded the service manual for the model of tv but dont know where to start studying it.

[taxxin]

Re: Philips 42PF9541/98 42" LCD TV No 1.2v error

Heres a revelation, would be useful to people in the future.

A page out of the technical service manual for troubleshooting 1.2 and 3.3v problems

According to the manual, the 12v and 5v get turned on first. then after that the 1.2v is turned on and checked then the 3.3v and 2.5v turn on and get checked. So the problem lies in the 1.2v enable signal to its components, good at least ive narrowed down my problems.

[taxxin]

[SOLVED] Philips 42PF9541/98 42" LCD TV No 1.2v error

Update: I found the PROBLEM!

So I had a look at the troubleshooting in the technical service manual that I found. I discovered that putting the TV into SDM (Service defaut mode) it will bypass the protections and keep the tv on until just before the fauil appears, So I jumped the SDM mode jumper and BOOM, Steady power on ready to diagnose. At first I felt around to see if anything was getting hot, nothing.

Also in the Manual it states that if there is a fault with the 1v2 rail, you must check for 12v input coming into the main 12v fuse (check), into the mostfets (check), lastly into the Mosfet driver. I checked the VCC input on 14 pin on NCP5422A (Dual Out−of−PhaseSynchronous Buck Controller with Current Limit) only 2.3v, Checked the datasheet to see what it needs which was 12v. There is a 10 OHM large SMD resistor in between VCC and the 12v Rail one side was showing the full 12V and the other showing only 2v. I measured the resistance on it and it read about 1mOHMS which is far too much. So i proceeded to remove it. I couldnt find a 10OHM resistor of that size and wattage so i desoldered a 0 OHm resistor from the old GTX 550TI that blew up. Placed that in place of the 10 OHM resistor ( in the Datasheet it doesnt specify that the 10 OHM resisitor is required).

Put it all back together again and BOOM no more error, some strange LEDS lit up on the board that previously did not. and the screen turned on and went blue.

Im so happy that it worked out, I dont think that replacing those mosfets were in vain, because one of the gates wouldnt open, im sure it would affect the 1.2v rail without a low side working.

So the big lesson here is FIND THE TECHNICAL SERVICE MANUAL & READ IT. THE ANSWER IS IN THERE SOMEWHERE!!

[dick_barton]

Re: Philips 42PF9541/98 42" LCD TV No 1.2v error

Glad you got there in the end.

[taxxin]

Re: Philips 42PF9541/98 42" LCD TV No 1.2v error

Glad you got there in the end.

Thank you Dick, wouldnt have made it without you!