Panasonic Viera TX-32LXD7 Power Supply Dead - No Standby Light - Page 1
Forum section: electronics / repair

author: paul_g_787
author_rank: Frequent Contributor
timestamp: June 17, 2020, 07:49:37 pm
content: Hi Everyone. I am hoping someone will be able to give me some advice / help on fault-finding this power supply.I just picked this TV up free of charge from a neighbour. They say that the TV was left on standby at night and when they came down for breakfast the LED was off and the TV would not turn on.So I have got it home and indeed it has no power. Although I am certain I heard a small noise from the speakers when I plugged it in.So I have pulled out the power supply board, and I have located the schematic (attached).There is 0V output on P2 / P6 / P4 and P5. Where the schematic says there should be 24.There is no direct short on the LV outputs to ground.I am reading an nice stable 330V on the output of the primary bridge rectifier (D801)However on IC801 I am reading 0V between VCC and GND pins (8 and 6).And on IC802 I am getting a fluctuating 9V to 16V DC between VCC and GND (Pins 7 and.I have managed to find the service manual here:https://www.manualslib.com/manual/1576487/Panasonic-Tx-32led7f.html#product-Viera%20TX-32LXD7In the section 'Setting Inspection' on Page 9 There is a table of Voltage confirmation.It says I should have 24,5V +/- 1V on P4 pins 5,6,7,8 in Normal Mode and Standby mode, however I am getting 0V. Nothing!Can anyone suggest what might be wrong or what I could check next?Thank you

author: fzabkar
author_rank: Super Contributor
timestamp: June 18, 2020, 07:35:50 am
content: IC801 appears to be the APFC controller. IIUC, IC802 needs to start working first, and it then generates the Vcc for IC801 from a secondary winding on T801.Have you checked C822 and C825? Q805 and D813?Does any voltage appear across C822?

author: paul_g_787
author_rank: Frequent Contributor
timestamp: June 18, 2020, 11:59:00 am
content: Quote from: fzabkar on June 18, 2020, 07:35:50 amHi, thank you for getting back to me. I am only just starting to get my head around these switching power supplies and don't fully understand every part of them yet so I appreciate the help.D813 seems OK. Using my multimeter in Diode mode is has 0.754V drop one way and no conductivity the other.C825 has a resistance of over 2MΩ in both directions and has the same fluctuating 9 - 16V across it as IC802C822 has a resistance of over 2M in both directions and has a steady 2.6V across it.Q805 does not appear to have any shorts, however is has a lot of flux surrounding it. From the factory perhaps? I have given it a scrape off but it looks OK I think?However, across D813 I have -3.58VDC (NOTE THE MINUS)From the Anode of D816 to ground I have 3.48VDCHowever, measuring across C822 again I have 4.15VDC now.? Although the power supply was on for 2-3 mins while I was taking the other measurements.Also, it might be worth noting that I have just noticed that R828 and R829 have both been replaced in the past. I have checked these and they are both reading spot on, I have re-soldered them for good measure as they both had dry joints! However, no joy.

author: fzabkar
author_rank: Super Contributor
timestamp: June 19, 2020, 02:01:52 am
content: ISTM that IC802 waits until C825 charges up to 16.5V (start threshold) via R828/R829/R815/D818. When this threshold is reached, the IC transitions from stop mode to start mode. It then begins pulsing its internal MOSFET (and transformer T801). This depletes the charge on C825, so Vcc gradually decays until it hits the stop threshold (8.9V). When this happens, the IC stops pulsing and waits for C825 to be recharged from 330V via the same 3 resistors. This cycle either repeats for a fixed number of attempts, or it keeps cycling until power is removed. You would need to read the datasheet.If there is no overload on the 24V rail, a regenerated supply is developed from the MOSFET pulses at the secondary winding of T801 (pin #2). These pulses are rectified and smoothed by D817 and C822. The fact that you're measuring 2.6V at this point means that IC802 is pulsing the transformer, but the regenerated supply is insufficient to reach the start threshold of 16.5V. ISTM you would expect to measure ~20V or so in a working scenario. Something is either loading the 24V supply, or is preventing C822 from being charged to the correct voltage.If you have a ESR tester, I would test C822.

author: paul_g_787
author_rank: Frequent Contributor
timestamp: June 19, 2020, 06:14:06 pm
content: Quote from: fzabkar on June 19, 2020, 02:01:52 amRight that makes sense, so it is trying to start up, which means the IC is working?.Unfortunately I do not have an ESR tester, however I have and LCR meter.I have removed C822 from the PCB and it was reading 47µF. I have replaced this capacitor with a salvaged one of the same value from an old PC power supply, which was reading 96µF.I then fired up the power supply and it is still not working. I am still seeing that 8.9 - 16.5V across C825.I also remove C825 and tested this, it is giving a reading of 98µF so I would say it is fine.Measuring the voltage across the now replaced C822, I left the volt-meter connected for a few minutes and the voltage was very slowly creeping up from about 1.8V and got to 3.8V before it slowed down and creeped up to around 4.1V.So it looks like it is trying to switch on, as you said, but something is preventing it. You said it might be something loading the output stage?I have tried running the power supply board with none of the cables connected, (apart from the AC input of course) and all the readings are the same. So I think this proves the fault is on this power supply board somewhere.I had a look at some obvious components, I removed C859, C860 and C862. These are all measuring around the 950µF mark. I have fired up the power supply without these three capacitors, and no change.D857 is also testing fine out of circuit. 0.7V forward and infinite in reverse.So I have fitted all of these part back to the board as they seem OK.Upon studying the board more, I have located all the test-points on the output stage and taken the following readings:-0.40V on TP851 - This looks like the 24V output for the main power supply, only switched on when not in standby mode?0.29V on TP852 - I think this is the standby power supply which should be on all the time?0.0V on TP853 - This appears to be the stand-by signal controlling IC802 via PC802. So is this expected to be 0v with the TV on standby mode?0.01V on TP854 - this also looks like something to do with the stand-by switching and is running via PC801??

author: fzabkar
author_rank: Super Contributor
timestamp: June 19, 2020, 08:49:30 pm
content: TP853 (BACKLIGHT_PSU_ON) and TP854 (TV_SUB_ON) control the STB (standby) pin of IC802 via optocouplers PC801 and PC802.According to the datasheet, the standby threshold voltage is 1.1V for VSTBon and 3.10V for VSTBoff. When either of the two test points are high, the corresponding optocoupler is switched on and the STB pin sits at VREF (5V), ie VSTBoff. When both of the two test points are low, both optocouplers are switched off and the STB pin sits at 0V, ie VSTBon.The datasheet doesn't explain what happens in standby mode, but I assume that the IC behaves differently in some way, although I would still expect that the output would remain at 24V.TP852 provides feedback via IC851 and PC803, enabling IC802 to sense the output voltage at its COMP pin.TP851 appears to be the panel voltage and is switched on/off by Q855 under the control of BACKLIGHT_PSU_ON.Edit:Have you checked R832 (6.8 ohm, between D817 and T801)?

author: paul_g_787
author_rank: Frequent Contributor
timestamp: June 20, 2020, 02:31:31 pm
content: Quote from: fzabkar on June 19, 2020, 08:49:30 pmThe service manual says the same as you that there should always be 24V on TP852, which appears to power P4 and P5. In this model only P4 has a cable connected.With the exception of P6 and P7, it looks like these are switched on by P4 pin 15 and they must be the power for the backlights? I will check where these wires go later.I will check out R832 as you suggested and let you know.

author: paul_g_787
author_rank: Frequent Contributor
timestamp: June 20, 2020, 02:48:31 pm
content: Okay, R832 is measuring exactly 7.0Ω in both directions. D819 seems to be fine with a forward drop of 0.783VD817 seems fine with a forward drop of 0.486VI am measuring 3.2VDC at the cathode of D817 where it connects to C822I am getting 0VDC on both sides of R832 but not sure if there is an AC voltage. My meter seems to be playing tricks on me. When I probe either side with the meter on AC mode, I get a quick 4.4V which then goes to 0V. Not sure why this is? I tried swapping the probes the other way and taking the measurements again, with the same results.

author: fzabkar
author_rank: Super Contributor
timestamp: June 20, 2020, 09:08:14 pm
content: I don't know where else to look. You could look at C833/C834 and L804/L805 and L807 and R835/R837, but I suspect they will be OK. IME with these types of faults in CRT TVs, the problem is usually the capacitor at C822.Maybe someone else has an idea. Sorry.

author: james_s
author_rank: Super Contributor
timestamp: June 20, 2020, 09:11:57 pm
content: Most of the time when I find a dead power supply in a TV the problem is bad electrolytic capacitors. Sometimes just replacing the caps will get it going again, other times the bad capacitors cause an IC or other part to fail.

author: paul_g_787
author_rank: Frequent Contributor
timestamp: July 02, 2020, 02:25:33 pm
content: Quote from: fzabkar on June 20, 2020, 09:08:14 pmI have checked all these components and they appear to be, as you predicted all just fine. The capacitors and inductors are measuring spot on and so are the resistors.I have removed all the electrolytic capacitors and a friend took them to work to test on their Fluke capacitor tester and they are all within their marked tolerances with absolutely no DC leakage.I have also tested all the transistors and ICs for direct shorts and cannot find any.I don't really understand how this circuit fully works. Is anyone able to give me some pointers as I cannot see anything obviously wrong?

author: Fixin Stuff
author_rank: Regular Contributor
timestamp: July 03, 2020, 03:08:17 am
content: Hi PaulI had a quick look through your post. I'm trying to fix a 'dumpster find' TV right now too. You can see my post here:https://www.eevblog.com/forum/repair/lg-lcd-led-tv-smps-board-primary-secondary-voltage-pulsing-no-standby-light/(start reading from around page 3 if you wanna get to the stuff which is what I'd suggest you try as well, see below).Reason I'm messaging is (as you'll see from my post), I initially thought the power supply was faulty. But the reason the power supply seemed faulty turned out to be because I was testing the power supply disconnected from the TV's main board. It seems that my particular power supply needed the main board connected to provide a load. Having the main board connected as a load stabilised the power output of the power supply, which meant it started to give normal test voltages.Of course this may not be the same reason why your test results are confusing. Are you testing your power supply with the main board connected? If not, it's worth a try.You'll see the suggestions from other members of how to set this up for testing, and a photo of a test suggestion of how to switch on and test the rest of the voltages/parts of your power supply. You'll also see a photo of how I was able to follow those suggestions and test my power supply last night. It seemed that my power supply is probably working ok, but I needed the main board connected to test this.Now I need to try and figure out how to get my main board workingHope it helps! I'll race you to get my TV working first haha!Good luck!

author: paul_g_787
author_rank: Frequent Contributor
timestamp: July 10, 2020, 12:45:41 pm
content: Quote from: Fixin Stuff on July 03, 2020, 03:08:17 amI did think that at first but the datasheet says it should have 24V on TP852 at all times.I have picked up a replacement power supply off of eBay and this is working great, and indeed there is 24V on TP852 regardless of if it is connected or not. The TV is working great with this power supply.I still would like to find the fault with the original power supply though, although after trying every single capacitor and checking every single diode and transistor, and finding no shorts or weird values I am beginning to think it is a problem with the IC and it might not be worth fixing ans the IC costs more than the whole power supply.

author: Fixin Stuff
author_rank: Regular Contributor
timestamp: July 10, 2020, 02:01:13 pm
content: Hi Paul. Glad you got your TV working! Mine is still just an ornamentIt seems my power supply is ok but my mainboard is stuffed. The LGs like mine are known for BGA issues on the mainboards. I'd love to attempt a BGA reball but I can't justify the expense as I'm moving soon and don't have the gear,ORa use for the TV if it's successful. Loads of people literally oven bake their LG mainboards to reflow the solder, but usually it's at best a temporary fix.I know what you mean about wanting to get your power supply fixed. Not sure about which IC is stuffed on yours, or if that's even the issue. Sometimes you can find them for surprisingly good prices. The one I replaced (which turned out to be fine anyway) was only around AUD$14 for 10! Maybe you can find one on Ali Express/EBay/Mouser/Digikey/RS Components (<-- RS is great especially if in the UK) for a fair price.I'm new to electronics so this is an enigma. Wish I had all the gear etc cos it's so satisfying to be able to get things working again!All the best with it

author: paul_g_787
author_rank: Frequent Contributor
timestamp: October 16, 2021, 03:46:32 pm
content: So I recently got an ESR meter and thought I would re-visit this board.Then new C822 I fitted tests fine with the ESR meter with 0.08Ω, whereas the old one had 1.4Ω!!!! Also original C825 had 1.1Ω!!So I have just replaced both of these with brand new caps as I now have 100µF 50V in my parts left over from another repair.Both of the new C822 and C825 caps have 0.02Ω ESR.So yes these caps were faulty, however replacing them has not fixed the power supply is still not working.I am still getting the same reading across C825 (charging to 16.5V then dropping to 8.9V.I am getting 0V across C822.C859, C860, C862 all have good ESR when out of circuit and in circuit.D857 is not shorted.I am getting 100mV across the output (measured at D857) which is very slowly climbing up and took a couple mins to reach 250mV.Any clue what to try next?