TV does not start - Page 1
Forum section: electronics / beginners

author: Kerkyren
author_rank: Newbie
timestamp: September 09, 2021, 01:42:40 am
content: Hello, I have a problem on my TV, it does not turn on, I think it comes from the psu pcb : 725G3812-P02-H20-003D-Z, because an integrated circuit -Tny227pn- exploded, I replaced it, but nothing happens, the LED indicator of the TV does not turn on, and I measured the output tension and there is nothingCeramic capacitors next to the integrated circuit have black traces but I do not know if it comes from the integrated circuit or capacitorCan I take the ESR measurements of a ceramic capacitor with an ESR70 as on a conventional capacitor? (They're500V and 1KV so I do not know, nor if it's the same process for a ceramics, maybe i can replace them but only find 1kv and not 500v)What will you have tested on the PCB in my place? I put you photos, for the zener diodes I'm not too sure of how to test themthanks in advance

author: golden_labels
author_rank: Super Contributor
timestamp: September 09, 2021, 02:12:29 am
content: Oh, yet anothermember of the familythat decided to explode.If that happened, likely other high-voltage side components are damaged. Starting with the fuse next to the power connector. But do not put a new one, before checking if the bridge rectifier, the three MOSFETs and the diode on the large heatsinks are fine. Or you will likely have another fuse blown.

author: Kerkyren
author_rank: Newbie
timestamp: September 09, 2021, 03:07:38 am
content: Haha, indeed it's recurrent with this integrated circuit. I tested the fuse in continuity with the multimeter, he did not explode.I'm a newbie so thank you for your help, I tested the bridge rectifier and it looks ok, is it the components I surrounded in red that I have to test? There is a mosfet with two legs instead of 3 against the radiator, I do not know if it is the diode you tell me, that I surrounded in blue, but there is 0v in both directions in diode test

author: james_s
author_rank: Super Contributor
timestamp: September 09, 2021, 04:35:59 am
content: If you're a newbie don't overlook safety. The bulk filter capacitor can store enough charge to knock you flat, always make sure it has been discharged before you go poking around. Also be very careful if you decide to take measurements while the circuit is live, and don't be tempted to try to probe it with a scope.

author: Kerkyren
author_rank: Newbie
timestamp: September 09, 2021, 02:26:37 pm
content: Thank you, yes I am careful, the pcb has been unplugged for a few days the time to receive some devices, I tested the bridge rectifier it is good, the chemical capacitors too, are the ceramic capacitors and other types of capacitor can be tested with an esr meter or is it different?

author: Jwillis
author_rank: Super Contributor
timestamp: September 09, 2021, 03:05:44 pm
content: Power surge.Check C907 Solder joint. Looks burned / melted away . Check C907 Capacitor . Negative side of 4 pin rectifier also looks burned . Check rectifier . Quite likely the control chip IC801 is probably toast as well . Also the secondary pins are the high voltage drivers for the back light ! Be very careful with those as they may be as high as 1000V or more !Given the possible severity I would try finding a new board for the make and model of TV .

author: Kerkyren
author_rank: Newbie
timestamp: September 09, 2021, 03:11:31 pm
content: I found the same board with the same part number but the seller says he took it out from another brand of tv and mine is a philips, but normally the PCB should be the same?It was more for a repair challenge

author: golden_labels
author_rank: Super Contributor
timestamp: September 09, 2021, 03:37:32 pm
content: The MOSFETs I was referring to are on the larger heatsinks. I am not sure if I see markings clearly, but it seems those are Q801, Q802 and Q902. The 2-legged component, D901, is a diode.As for safety, a thing easy to overlook: if the capacitor is charged, the voltage is present in various areas of the whole circuit. In particular the radiators. So it’s not only about keeping your hands away from the capacitor itself, because you may as well form a loop by touching some component and a radiator. Keep the capacitor discharged. Test capacitor leads with a meter to be sure there is no high voltage on it.Chances a PSU from a different device will suit your own are low. Unless the PCB is the same model, of course. Otherwise you would need to be sure it offers the same voltages, at the same power, reliably over the same period of time, and fits physically into your TV. I presume that you do not have tools to determine the middle two. And since a TV is expected to work unattended, it’s important to be sure the power supply doesn’t catch fire while you pay a visit to the loo.

author: Kerkyren
author_rank: Newbie
timestamp: September 09, 2021, 04:21:53 pm
content: I unsoldered the bridge to test it works, for ceramic capacitors, it's strange but I tried two on resistance mode and it writes "OL", and also with the ESR70, the model I have I found the same model number as mine,It is possible that it is a surge because I had the same problem on another TV on the same outlet, before repairing the electricity in the room

author: james_s
author_rank: Super Contributor
timestamp: September 09, 2021, 05:19:31 pm
content: Quote from: Kerkyren on September 09, 2021, 03:11:31 pmThere is a lot of "badge engineering" with stuff like TVs, only a few companies make the actual panel which is the bulk of the TV. It wouldn't surprise me at all to see the same power supply in more than one brand of TV.

author: Jwillis
author_rank: Super Contributor
timestamp: September 10, 2021, 12:39:50 am
content: Quote from: Kerkyren on September 09, 2021, 04:21:53 pmCapacitors will show OL or infinite resistance when charged . Small value caps charge very quickly when the Ohm meter is connected . To fast to see . With larger caps the resistance will increase as the charge increases . So the ohm meter will show a increase of resistance from 0to OL(infinite) .It's not really infinite but much higher resistance than your meter can measure.You'll need a capacitance tester to accurately check low value capacitors . Some multimeters have capacitance test built in. Expect capacitors to be up to ±20% the marked value . Especially electrolytic caps and cheap ceramics . In most cases this is not a problem .If your cap tester is showing Open circuit or no component then the cap is no good . Always test caps in a discharged state.