Philips 42PFL3605H/12 - Page 1
Forum section: electronics / repair

author: blauerscharik
author_rank: Regular Contributor
timestamp: July 21, 2023, 02:58:58 pm
content: Hi Guys,so the TV above doesn't turn on. After opening it up I found a TNY277PN IC blown (Drain Pin 4). I found a new one on Ebay, soldered it in and after a couple hours the drain pin (4) blew out again. So obviously I didn't fix the cause of the problem but rather the symptom (although just temporarily).I got a data sheet for this TNY277PN although not exactly that one:https://eu.mouser.com/datasheet/2/328/tinyswitch_iii_family_datasheet-1512318.pdfI really would like to learn how this works.I copied the section that should be relevant, from a service manual. The diodes D914/D915 are good. Both have 0,5V.The caps C945 and C926 seem also OK. C926 has like 9nf not 10nf... could that be it?Or am I looking in the wrong spot? Should I be looking what's at BP/M and EN/UV?Why would pin 4 burn out?Could anyone nudge me in the right direction?I don't have a new TNY277 IC yet (it is ordered)Much appreciated.

author: elecdonia
author_rank: Frequent Contributor
timestamp: July 21, 2023, 07:10:10 pm
content: Check D914, D915, C945. This is a snubber network which protects the TNY277 “D” pin from overvoltage.Does this TV operate from 230V mains? If so the snubber network is really important. Here in the USA where mains are 120V the snubber network isn’t quite as important.

author: blauerscharik
author_rank: Regular Contributor
timestamp: July 21, 2023, 08:31:56 pm
content: Hi,I did check those components. The diodes are ok. I de soldered the caps.. they are ok too.Yes it is 230VCould it be anything else?Thanks

author: elecdonia
author_rank: Frequent Contributor
timestamp: July 21, 2023, 08:45:33 pm
content: Are you able to test ESR of electrolytic capacitors?C959 10uF 50V and C946 47uF 50V are very important. If not able to test ESR then replace both.When IC905 failed was the TV on or off?I believe IC905 is the “standby” power source to provide power to MCU while TV is off.When TV is on there may be logic signals which switch off IC905. To determine if this is so we will need to see the entire schematic.

author: blauerscharik
author_rank: Regular Contributor
timestamp: July 22, 2023, 07:48:17 am
content: No I was not able to measure ESR, the meter didn't show anything. So I only measured the capacitance which was 0,0022uF for C945 and 9nF for C926.But you are talking about C946, that I haven't measured yet.Yes the TV was on when it failed.I attached the full schematic.thanks a lot elecdonia

author: blauerscharik
author_rank: Regular Contributor
timestamp: July 22, 2023, 07:56:42 am
content: Quote from: elecdonia on July 21, 2023, 08:45:33 pmSorry I looked at the wrong caps.Yes I can measure ESR and will check those two. Thanks

author: blauerscharik
author_rank: Regular Contributor
timestamp: July 22, 2023, 08:15:42 am
content: Quote from: elecdonia on July 21, 2023, 08:45:33 pmC959 is not even present on the board.And C946 has 49uF and ESR 0,139 ohms.

author: wraper
author_rank: Unknown
timestamp: July 22, 2023, 08:49:52 am
content: Quote from: blauerscharik on July 21, 2023, 02:58:58 pmLikely it was fake. It's quite common that Power Integrations ICs from China are relabeled lower power versions. So it could easily be relabeled TNY274, or not made by Power Integrations at all. Also check high voltage electrolytic capacitor, sometimes it going bad can cause IC to blow up.

author: blauerscharik
author_rank: Regular Contributor
timestamp: July 22, 2023, 09:04:43 am
content: Quote from: wraper on July 22, 2023, 08:49:52 amThat thought crossed my mind as well. That's why I ordered a new one from an actual electronics supplier.Do you mean all the caps before the bridge rectifier?Thanks

author: wraper
author_rank: Unknown
timestamp: July 22, 2023, 09:29:37 am
content: I mean capacitor after rectifier.

author: elecdonia
author_rank: Frequent Contributor
timestamp: July 22, 2023, 03:36:50 pm
content: Quote from: blauerscharik on July 22, 2023, 07:48:17 amThanks! That is a very sophisticated power supply. I was able to view the.pdf on my iPad but will need to get to a real PC to zoom in to the needed level of detail.The power supply architecture appears to contain 3 main sections:1) Active power factor controller located at upper left of drawing2) Main DC-DC switching power supply (upper right)3) Low-power standby switching power supply (lower right).I’ll be posting more after I get a proper view of the entire circuit.As for the new TNY chip failing, I do recommend purchasing 2 or 3 of these chips from a reputable distributor. As stated in another post, IC chips sold on eBay and Ali-express are not always genuine. In your case the eBay chip actually operated for a few hours before it failed. So the chip you received from eBay wasn’t an entirely “fake” part. But it may have been a used chip recovered from a discarded PC board, or a “factory second” which was rejected by the manufacturer.Or it could have been perfectly functional, but there is still some other fault on the PC board which is causing too much stress on the TNY chip.

author: elecdonia
author_rank: Frequent Contributor
timestamp: July 22, 2023, 04:40:54 pm
content: OK, I was able to view the entire power supply schematic in sufficient detail. This is a complicated power supply circuit. There are several interconnections between the 3 main parts (standby power supply, main power supply, active power factor correction).The first step in repairing it is to get the standby power supply to operate in the "TV off" state. In this state the power-factor correction is supposed to be disabled (off) and also the main power supply is off. The output voltage from the standby power supply should be +5V ( +/- 500mV). The DC voltage across the large energy storage capacitor (C907, 120uF/450V) should be no higher than 340V.I suggest supplying mains power through a series connected 230V incandescent lamp bulb no larger than 60W. This is the old "dim bulb" protection technique often used to safely power up devices which might still have issues when operated from a full 230V.I also strongly recommend using a mains isolation transformer when troubleshooting the "hot" section of this power supply. No part of the "hot" section is at earth potential at any time. In fact the circuit common rail of the hot section will be at approximately -160V DC in reference to external earth/neutral at all times.

author: blauerscharik
author_rank: Regular Contributor
timestamp: July 22, 2023, 06:21:25 pm
content: I did order 2 new TNY from an electronics supplier this time.Where can I measure that 5 Volts Standby and can I do that without the TNY in place yet?The Cap (C907) was around 325V.I do have a light bulb in series. When I changed the TNY the first time, I powered the TV through the bulb and the bulb was pulsating (turning on and off).And I power everything trough a transformer as well.

author: elecdonia
author_rank: Frequent Contributor
timestamp: July 23, 2023, 12:58:16 am
content: Quote from: blauerscharik on July 22, 2023, 06:21:25 pmIC905 (TNY277) must be installed in order to generate the 5V standby power rail. The 5V output may be measured between the "cold" common rail (-DC) and the +5VSB rail. This corresponds to the + and - terminals of capacitor C951 (470uF/16V). Note: "Cold" side common rail (-DC) schematic symbol looks like this:__|__/ / /Voltages on the "cold" side are to be measured with the - probe of multimeter connected to this point.QuoteThis is the correct DC voltage for the "TV off" state. In this state the voltage across C907 is directly proportional to the incoming AC mains voltage. It is not regulated. For a nominal 230V input from the AC mains it will be close to 325V. The formula is: DC V across capacitor = 1.4 * AC mains input VHowever, when the TV is switched to "on" then the power-factor control circuitry becomes active. In this state the power-factor controller IC902 actively regulates the DC voltage across C907. According to the schematic switching the TV to the "on" state causes the DC voltage across C907 to rise to 395V. This value of 395V must be confirmed. It is regulated by the power-factor control IC902 and a voltage divider comprised of 4 resistors (R907, R908, R909, and R911). This value of 395V should be stable and always within +/- 5% of 395V (between 375V and 414V). It must not ever rise above 415V. When the TV is switched to "off" it should drop back down to 325V. If out of spec then the voltage divider resistors should be measured and (if necessary) replaced.QuoteIf the wattage of the bulb is 60W it should become very dim when the TV is on the "off" state. However, for the TV "on" state it will be necessary to use a considerably larger wattage of incandescent bulb. My test bench is equipped with several parallel lamp sockets which are individually switched. I can select the total wattage of bulbs over a range from 10W up to 900W depending on the switch positions and the wattage of the individual bulbs I put into the sockets. A large LCD TV with fluorescent backlighting such as this Philips TV will require 150W to 300W of series lamp bulbs in order to operate normally while in the "on" state.QuoteThis is good. I should mention that I once acquired a device labeled "isolation transformer" which actually had an internal jumper between the primary and secondary, thus causing the output to be referenced to neutral of the incoming AC mains. It gave me a nasty shock when I attempted to use it on my test bench. This so-called "isolation transformer" was designed only to reduce RFI interference and to reduce excessive transient voltages. It was not intended to be used for isolating the AC mains power for a test bench. After removing the internal jumper it then functioned properly as a true isolation transformer. Since then I always check every so-called "isolation transformer" before using it in a test bench scenario. There must be no continuity between the primary and secondary side. The resistance should read "over-range" on a multimeter, in other words, 100's of megohms.

author: blauerscharik
author_rank: Regular Contributor
timestamp: July 23, 2023, 08:05:26 am
content: Thanks elecdonia for the detailed explanation.I hope I get my IC tomorrow and then I can test it further.I have three light bulbs, two 60 watts and one 100 watts. I do have more 100 watts so I could do all three 100 watts each.When I turned on the TV with one bulb (60) it was just turning on and off. With two they were both off, so that told me there is no short on the power supply. I then turned the TV on and it worked. But not for very long. And I didn't measure anything thinking the IC was the issue. But blowing pin 4 twice was suspicious and maybe there is more to the board. With my limited knowledge I checked everything connected to pin 4 but didn't find anything suspicious.My transformer had only the ground wire connected coming in and going out. So I had to disconnect that.But a jumper is pretty dangerous.

author: blauerscharik
author_rank: Regular Contributor
timestamp: July 23, 2023, 08:18:05 am
content: Quote from: elecdonia on July 23, 2023, 12:58:16 amSo technically is that ground?And on the hot side the triangle with the circle that is also the common for the hot side? Technically also ground?They wouldn't be both connected or would they?Why the different symbols?

author: elecdonia
author_rank: Frequent Contributor
timestamp: July 23, 2023, 06:13:59 pm
content: Quote from: blauerscharik on July 23, 2023, 08:18:05 amIt could be referred to as "ground" or "earth" for the main PC board and the inverter PC board. It is also the negative return for all of the power supplies which are on the "cold" side. Every place this symbol appears on the schematic represents a connection to this "common" rail.It is the common negative (-) terminal for all of these power supply rails:+5VSB(always on, also referred to as "standby power")+12VActive only while TV is in "on" state. This rail likely supplies power to all of the logic circuitry, and also the video and audio processing circuitry. The main PC board will have several local voltage regulators which reduce the 12V rail to 5V, 3.3V, and perhaps other voltages for specific IC chips located on the main PC board. Power consumption of the electronics on the main PC board is typically from 10W to 50W depending on the number of features the TV has. It is not proportional to size of the screen. 10W corresponds to 0.8A, 50W would be 4A of current consumed from the 12V rail.+24VActive only while TV is in "on" state. This rail likely provides power specifically for the fluorescent backlights via the inverter PC board, which steps up the 24V to several hundred volts for the fluorescent tubes. For TV sets with fluorescent backlights the power consumed by the backlights will range from 25W up to perhaps 100W or 120W. This depends mostly on the size of the screen. 25W requires 1A from the 24V source, 100W requires 4A.QuoteThis symbol is the negative (-) return. It is the common (or "ground") for all power supply circuitry on the "hot" side. Each appearance of this symbol indicates a connection to this "hot" side common rail.QuoteThey aretotally isolatedfrom each other.This isnecessary for safetypurposes.The term "hot" applies to circuitry which isdirectly connected to the AC mains.The term "cold" applies to circuitry which isisolated from the AC mains.QuoteThese two different schematic symbols are required to indicate that these are twocompletely separate"common" rails.

author: elecdonia
author_rank: Frequent Contributor
timestamp: July 23, 2023, 06:35:21 pm
content: Quote from: blauerscharik on July 23, 2023, 08:05:26 amIf you could post photos of your mains receptacles and plugs that would be helpful. It would be helpful for me to see what your mains receptacles and plugs look like.I have not been to Europe for many years but I have visited Ireland several times recently. Ireland uses the large 3-wire (grounded) UK-type receptacles and plugs.Here in the states we use 3-wire grounded receptacles and plugs which are considerably smaller than the UK style.Some mains powered appliances and electronic devices in the USA have only 2 prongs on the plug. They don't need a "ground" connection because they are designed so the user cannot touch anything directly connected to the AC mains.Does the power cable for your TV set have 2 pins or 3 pins?As for the "ground" (earth) connection between input and output inside your isolation transformer, it is safe to keep this connected between input and output.The issue with my transformer was thatthe "neutral" wire was also connected between input and output. I removed that connection.In the USA our terms for mains receptacles are:Hot - This wire carries 120V AC power (generally a voltmeter from this wire to actual earth ground will measure 120V). This wire carries current.Neutral - This wire is the "return" for the 120V AC power (voltage between this wire and actual earth ground is usually <1V). This wire carries current.Ground - This wire is connected to an actual "earth" ground and is provided for safety purposes only. Under normal operating conditions it does not carry any AC current at all.My understanding of AC mains power in Europe and UK is that the "hot" terminal is 230V referenced to actual earth ground.But I believe "neutral" and "ground" (or "earth") each have the same definition as in the USA. Under normal operating conditions the "neutral" wire will have no more than 1V referenced to actual earth ground. Please confirm if I am correct about this.

author: elecdonia
author_rank: Frequent Contributor
timestamp: July 23, 2023, 06:40:24 pm
content: QuoteFor a TV like yours I recommend a single 100W bulb for testing in the "off" state.For testing the "on" state I recommend a total of 300W of light bulbs.

author: blauerscharik
author_rank: Regular Contributor
timestamp: July 23, 2023, 07:05:58 pm
content: Thanks again for the explanation about the common rails and voltages. I've learned something new.I attached pictures of our receptacles and and an example plug.What I like better about your receptacles in the states is that you have a dedicated hot side (left or right, dunno) but here it is not regulated which side the hot side should be. So it's basically up to the installer to decide. So every house or maybe even every room could be different. I don't like that.The TV has two pins, so it's not grounded.As for the transformer, if you leave the earth attached you would get a shock if you touch a hot wire, because you're also grounded.When I disconnect the earth wire, I can touch the hot wire and nothing happens. Only if I touch hot and neutral that would zap me.And yes you are correct about the mains voltage here in Europe.

author: blauerscharik
author_rank: Regular Contributor
timestamp: July 25, 2023, 05:24:53 pm
content: Quote from: elecdonia on July 23, 2023, 12:58:16 amSo I finally got the new IC's. Soldered one in and took some measurements.I got the steady 5V Standby Voltage.QuoteI got 325V in the "off" state and steady 400V in the "on" state.So far everything looks all right, don't you agree? Or can you think of anything else to check?Could it be that it was indeed the "cheap" ebay IC?Thanks elecdonia

author: elecdonia
author_rank: Frequent Contributor
timestamp: July 25, 2023, 08:53:44 pm
content: QuoteTime will tell.Because the source voltage for the TNY is considerably larger while the TV is in the "on" state, it is possible that the ebay IC wasn't rated for as high an input voltage as a genuine replacement.Most TNY chips are used in small "wall wart" power supplies which simply rectify and filter the AC mains. For power supplies operating directly on rectified mains voltage the DC input voltage received by the TNY when operating with 120V AC mains is typically only 180V. And when operating on 230V mains the input voltage is typically 325V.However, the "active power-factor correction circuit" in this TV is configured to provide 400V DC to the TNY chip during the "TV on" state. This is 25% above 325V. This is likely too large of an input voltage for a TNY chip rated only for 325V input.Unfortunately it is common for some of the Ali-express, Banggood, and ebay sellers to "relabel" IC chips as a more expensive version. For example they might relabel a low-cost chip rated for 120V or 230V input as a more costly version rated for a considerably higher input voltage such as 400V. Therefore the chips they sell may work OK for most of the people who buy them, but not for those cases which actually require the more expensive IC which has better specs.

author: blauerscharik
author_rank: Regular Contributor
timestamp: July 26, 2023, 03:50:39 pm
content: Quote from: elecdonia on July 25, 2023, 08:53:44 pmTrue, I'm just suspicious that the original and the ebay one failed at the same place...QuoteThe datasheet says it's an 230VAC IC, is that correct or should it be higher then? Maybe I bought the wrong IC?Is there a way to lower this Voltage and if so would the TV still work then?

author: elecdonia
author_rank: Frequent Contributor
timestamp: July 27, 2023, 05:46:16 pm
content: Quote from: blauerscharik on July 26, 2023, 03:50:39 pmDatasheet specifies maximum drain voltage (at pin D) of 700V. Also the operating input voltage is specified up to 265V AC, which corresponds to a rectified DC voltage of 375V. This is very close to the operating voltage provided by the active power-factor correction in your TV (395-400V). So I do believe the voltage rating of the TNY is sufficient.The TNY portion of the circuit (IC905) includes D914, D915, and C945. These 3 components clamp the maximum peak voltage at the D pin of the TNY to 580V. You said D914 and D915 both tested good as diodes in the forward direction (0.6 volts) indicating they are good. And that C945 tested 2200pF (2.2nF), correct?One fault I have observed is with film capacitors of the type used for C945 gradually losing some of their capacitance. Also capacitors in the C945 position can sometimes have physical signs of failure where the capacitor case develops a burned spot or gets bloated. But if C945 looks perfect and still tests with correct uF, then I would consider it good to use.QuoteThe schematic shows 395V and you are measuring 400V. This is within spec. The other circuits in the TV, including TNY (IC905) were all designed and tested to operate on this voltage. I don't recommend attempting to reduce it.My thought is that your ebay TNY device was probably a "relabeled" item with lower specs.If/when a device like the TNY fails it will most often be a short of the D pin to the common rail. Because this TV has such a large energy storage capacitor C907 (120uF/450V) it is likely that a short of the D pin will blast holes into the TNY case. That's a lot of energy!

author: blauerscharik
author_rank: Regular Contributor
timestamp: July 27, 2023, 09:05:14 pm
content: QuoteYeah I read the datasheet more carefully and saw the numbers then as well. But thanks for confirming.QuoteYes that is correct, the caps and the diodes were fine and looked good too.QuoteI guess it makes sense since it is designed that way. Even though not all designs are designed to last long.QuoteIt looks like it was indeed not a genuine part. And where the IC fails does also make sense when you say it like that.

author: fzabkar
author_rank: Super Contributor
timestamp: July 28, 2023, 12:12:17 am
content: Just to test the idea that the TNY chip is a lowly rated fake, would it make sense to limit the supply voltage to 340VDC by disabling the APFC circuit? One could do this by open-circuiting the gate drive to Q902, or desoldering its drain pin. I think the TV should still function at the lower voltage.

author: wraper
author_rank: Unknown
timestamp: July 30, 2023, 07:12:41 pm
content: Quote from: fzabkar on July 28, 2023, 12:12:17 amSuch experiments in high voltage circuits can easily cause a lot of damage. Also open-circuiting the gate can leave MOSFET in unknown state. It can be OFF, fully on or partially on. In last two cases just blowing a fuse would be extremely lucky.

author: fzabkar
author_rank: Super Contributor
timestamp: July 30, 2023, 07:30:18 pm
content: When I suggested open-circuiting the gate, I expected that the OP would disconnect the series-connected drive resistor and leave the gate-to-source resistor in place. I expect that should be safe.As for disabling the APFC, I have done similar things in the past. But maybe I was lucky?