Author Topic: Need help identifying faulty component  (Read 2255 times)

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Offline sanchaz12

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Need help identifying faulty component
« on: August 24, 2015, 01:49:03 am »
Hello everyone,

I am trying to repair a faulty power board from a JVC LT-26DC9BH LCD-tv. It does not power on at all. Before the complete failure, sometimes the image would randomly disappear and comes back after a few seconds. The backlight did not dim or stop working while the image disappeared.
Recently I opened it up to inspect the power board, and found that several caps have vented and are bulged. I ordered new ones with the same ratings as the old ones and replaced them (see the red capacitors on the attached image, those are new). Also I desoldered the other capacitors to check their ESR value, but on all of them it seems OK. After installing the new capacitors, the TV would still not turn on. Then I noticed something that I heard from the TV before. There is a clicking noise coming from the power board (see the circled components on the attached image) somewhere from one of these components every few seconds. It's not a relay click, but rather like electricity arcing. It's very quiet but noticeable when listening closely.
Sometimes the clicking doesn't start right away after plugging in the power, and I can see the optical audio output light up and stay on. When I try to turn it on the light immediately dims and the clicking starts again.
Could it be the transformer? I am not familiar with these types of transformers but it may be prone to failure.
 

Offline max666

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Re: Need help identifying faulty component
« Reply #1 on: August 24, 2015, 02:22:26 am »
I don't know what's wrong with your board, but transformers are usually very robust. I would be surprised if that one is defective.
But that big ass CapXon is hurting my eyes. You sure that one is ok?
 

Offline sanchaz12

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Re: Need help identifying faulty component
« Reply #2 on: August 24, 2015, 03:54:37 am »
I don't know what's wrong with your board, but transformers are usually very robust. I would be surprised if that one is defective.
But that big ass CapXon is hurting my eyes. You sure that one is ok?
Just measured that one (150 µF, 450V), but in-circuit. ESR value of 0.45 ohm, capacitance reading shows 139,3 µF
I can't get it out easily without damaging it, because it is held in place by some very strong adhesive.

EDIT: Also measured the voltage. It charges up close to 400V but seems to keep its charge when the clicking happens. The voltage drops about 3-5 volts when I hear the click, to rise back up to 400V.
« Last Edit: August 24, 2015, 04:00:30 am by sanchaz12 »
 

Offline cuebus

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Re: Need help identifying faulty component
« Reply #3 on: August 24, 2015, 04:05:02 pm »
It's possible those sounds are current spikes. Like max666 said I wouldn't think of the transformer being at fault until you've ruled more components out.
Are you powering it up under load? If so, your TV could be pulling too much power.
If not, I would go through the various stages with a multimeter starting at the initial rectifier and checking your voltages after the transformer. An oscilloscope could be useful for checking signals as well to make sure you are getting switching at the primary. If you do this though remember to isolate ground on your scope or use a differential probe because otherwise you may short something to ground!!
 

Offline mikerj

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Re: Need help identifying faulty component
« Reply #4 on: August 24, 2015, 04:39:05 pm »
A regular "ticking" noise from these off-line switchers usually indicates that they have entered an over-current protection state.  This is most often (but not always) caused by a fault on one of the other boards being powered from the supply.
 

Offline tautech

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Re: Need help identifying faulty component
« Reply #5 on: August 24, 2015, 05:42:37 pm »
A regular "ticking" noise from these off-line switchers usually indicates that they have entered an over-current protection state.  This is most often (but not always) caused by a fault on one of the other boards being powered from the supply.
+1
You'll need to do checks on the SMPS controller V+ power supply also.
Identify the IC and get datasheet.
Check IC V+ supply is reaching minimum start voltage, it should be via the dropper resistor from HV DC.
This allows the IC to get sufficient voltage to start, then supply switches to FB from the primary via a diode and sometimes a resistor too. Check both. Also there is normally a small value polarised cap ~22-47 uF and this helps smooth and maintain V+ for correct operation. This should also be checked for correct value however ESR is not critical for this cap, just check it against a known OK one.

If anyting is wrong with the components mentioned the IC will go into UVLO and you might get the symptons you describe.

As many secondary side caps were toast some others further into the TV might be also after being subjected to excess ripple as a result of the others failing. This might put the PSU into overcurrent mode as others have described.
Avid Rabid Hobbyist
 


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