TV SMPS DIAGNOSTICS - Page 1
Forum section: electronics / general

author: kolbep
author_rank: Frequent Contributor
timestamp: October 13, 2012, 08:27:06 am
content: Hi,I am trying to fix an old LG Tv.Was in a garage for some time. Tried it the other night, and it worked well for several hours. Switched it off for about 5 hours (with the hard power switch), then when trying to switch it back on, it does the Tick-Tick-Tick and flashes the power led.I have replaced all the electro caps in the powersupply LV section (Except for the 2 big Filter caps, which I don't have spares), I have also checked all Resistors and Transistors.If I unplug the LV connector between the smps and main board, I still get the tick-tick-tick.Please could somebody look at this schema, and tell me if it is possible to Load this board correctly, so that I can see if the problem is just on this board, or if (when it is plugged into the main board, it is loaded too much, and when it is not plugged in to the main board, that the Unloading causes a problem.)Basically I want to test the SMPS board by itself.The Schema is here:https://sites.google.com/a/fhc.org.za/smps/smpsThanksPeter

author: Psi
author_rank: Super Contributor
timestamp: October 13, 2012, 09:37:41 am
content: Does the tick-tick-tick sound like- High voltage arcing- A relay- Or pulses of audible inductor noise

author: kolbep
author_rank: Frequent Contributor
timestamp: October 13, 2012, 09:52:35 am
content: The Tick-Tick-Tick is definately an inductor noise.There is no arcing, and the relay on the psu board (for the Degauss) is not clicking (it gets its signal from the uC on the main board, which is not starting up due to this Ticking.ThanksPeter

author: SeanB
author_rank: Super Contributor
timestamp: October 13, 2012, 10:59:17 am
content: Look on the mains side for high value resistors and small electrolytics. The resistors go high in value or the caps are open or leaky. R805/6 and C811 are suspects. Cheack C815 and D805 as well. Then check the secondary rectifiers ( preferably out of circuit) and the capacitors across them.Then check the voltages on the secondary side, to see if they are there for the brief perios that the supply is active.Check the replacement caps are the right way round ( Don't ask why I know this fact is so easy to do.......) and try running it disconnected. Check the line driver transistor and associated efficiency diodes are not shorted.

author: tom66
author_rank: Super Contributor
timestamp: October 13, 2012, 01:00:25 pm
content: How old? CRT, LCD or plasma?

author: SeanB
author_rank: Super Contributor
timestamp: October 13, 2012, 01:14:25 pm
content: Likely a CRT, degaussing circuit.......Check as well Q800, R811 and ZD800 for being leaky or short/open. This supply will run in a hiccup mode if unloaded very quietly, so it will give low outputs if unplugged but the 5V from IC804 supply will be correct.

author: kolbep
author_rank: Frequent Contributor
timestamp: October 13, 2012, 04:16:15 pm
content: Hi AllThe Set is a CRTMy Multimeter (A Toptronic Brand - not at all good for this stuff) is too slow to properly catch the voltages,but I did manage to see 5v out of the 7805my tester also shows about 5v into the 12v regulator, but absolute 0 outputI also get about 60v on the B+ outBut my tester is probably too slow to catch the proper voltages.I will check the primary side components of the SMPS as soon as I have recovered (a high chair, and a low table for a workbench really messed up my back after the 4 hours I was working on it last night).I also do not like the beefy cap on the Primary side. Kept on catching me with 160vdc. Every time I got zapped, it smelled like burned hair.P

author: kripton2035
author_rank: Super Contributor
timestamp: October 13, 2012, 04:35:11 pm
content: a step by step guide to fix smps.the clic-clic-clic is detailed ... and the basic tools to have...good luck !http://www.repairfaq.org/sam/smpsfaq.htm

author: LoyalServant
author_rank: Regular Contributor
timestamp: October 13, 2012, 04:57:29 pm
content: I have fixed... thousands of these..This supply has a standby and a run state.The SE130 is used during the run state and in standby the transistors Q802,803, and 804 are used.The first thing I do with these supplies is isolate the B+ supply that goes to the horizontal and check all of the secondaries.Lift L801 and see if the supply will run.If it runs then your issue is back in the horizontal circuit.

author: SeanB
author_rank: Super Contributor
timestamp: October 14, 2012, 07:03:24 am
content: If you are getting 5V out then it is working, and in standby. The issue then is most likely a shorted line driver transistor ( Bu208d most likely or a better rated equivalent) or efficiency diode. Take it out and see if the set tries to start ( will not do so as a lot of LOPT derived supplies are not there, but will flash a fault code on the power led) and you will be closer to fixing it.

author: kolbep
author_rank: Frequent Contributor
timestamp: October 14, 2012, 02:31:31 pm
content: Thanks for all the info.All the parts tested out fine.Lifting B+ Had no change.But, If I bridged +5v to the PWR on the powersupply, it started up fine, with all the correct voltages.Plugged it into the set, and the set operates fine.Now just to figure out why the power signal from the uC to the Psu is not working. Which I can only get to when my daughter finishes watching Mickey Mouse Clubhouse on the set.If anybody wants, it is a MC74A Chassis, and the Schematics are here:https://sites.google.com/a/fhc.org.za/smps/smps

author: SeanB
author_rank: Super Contributor
timestamp: October 14, 2012, 02:44:16 pm
content: If it powers on then and works correctly try turning it off with the remote ( will not actually do so if hardwired on) and then remove the link and try again. Might be corrupted memory in the micro.

author: kolbep
author_rank: Frequent Contributor
timestamp: October 14, 2012, 03:07:33 pm
content: Hi Sean.Just tried that.Pushed Power on Remote, and then cut my jumper on the PWR, but the set did not go off. If I used the hardpower switch to turn off, then when I switched on, it gave the Tick-Tick-Tick.I am beginning to think it may be the NPN transistor on Pin1 of IC12.My understanding of the Schematic is as follows:The PWR pin is normally pulled high (if the uC pin is low, then the transistor is not conducting. when the uC pin is high, the transistor is supposed to pull the line low).Either it could be the resistors or transistor that are causing the PWR to float intermediate, not pulled low or high, and this would cause problems.As soon as the girl's shows are over, I will try again.

author: kolbep
author_rank: Frequent Contributor
timestamp: October 14, 2012, 07:06:48 pm
content: The problem with the remote not switching the set off was grime on the remote contacts. That is sorted out nowAll resistors, and the transistor in that section are fine.Looks like the Uc is not powering up properly (Because that 5v Line is Pulsing with the Tick-Tick-Tick when the set is in standby)If I apply 12v, from a seperate battery, to the 7805 regulator on the powersupply board, then the remote can turn the set on and off fine.I do not know much about how the standby circuit works in the powersupply (how we get 5v through the isolation transformer when the supply is supposed to be in standby),but when in standby, the ticking of the powersupply is also fluctuating the supply to the 7805 regulator, and this causes the uC to hangup. As soon as the set is in running mode, the loads must stabilise the powersupply.Any idea why the supply is not stabilised in standby?This is one of the first projects that I am trying to get done to the end, instead of just giving up at the 80% mark, which has happened with so many other of my projects,Thanks for all your help.Peter

author: tom66
author_rank: Super Contributor
timestamp: October 14, 2012, 07:58:05 pm
content: Dried startup capacitor would be my guess.

author: SeanB
author_rank: Super Contributor
timestamp: October 14, 2012, 08:14:01 pm
content: Q800, R811 and ZD800 for being leaky or short/open is most likely then for not working in standby, along with R805/6 and C811, C815 and D805. Replace the caps and try again, they might measure ok but easier to replace with new. These are all the standby supply, which keeps the micro powered by letting the power controller provide a few pulses every second to keep the power rail that supplies the 5V regulator above roughly 8V, while in running it is at probably 18V. This needs the startup circuit to work perfectly, as the chip then is always storing charge on these capacitors to provide a few cycles of drive before doing an undervolt shutdown again then waiting to power up again.

author: kolbep
author_rank: Frequent Contributor
timestamp: October 19, 2012, 06:15:16 pm
content: Thanks SeanYou hit the resistor on the Head,the 1/2 Watt R811 (20K) was O/CAll good now.Thanks

author: SeanB
author_rank: Super Contributor
timestamp: October 19, 2012, 06:52:29 pm
content: Good, hope it gives you service until Aardvark Park stops broadcasting analogue TV, and there are no more composite/RF output DTT decoders.Did find the fault on the monitor I tore down, one CBB inductor and a totally black burnt MOSFET, which surprisingly enough still works, I think the CRT driver downstream of it ( one of 3 hybrids) or an associated component has gone dead short. The user got a new LED backlit monitor ( and promptly dropped resolution down to 800x600) after she complained that the replacement had "wrong colour" on the display..............CBB - Charred Beyond Belief. Don't know the original colour, but they are now black, including the metal TO220 heatsink tab, and no longer able to see the device markings, though the PCB screenprint identified it as a MOSFET.