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Old 08-12-2015, 03:55 PM   #1
pillit83
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Default LF501EM5F No Power, Chirping/Beeping Transformer

Preface: I know these walmart black friday tv's are real piles of crap..

Emerson LF501EM5F LED TV
As soon as it's plugged in (to mains) it starts to "chirp" at around 2 second intervals repeatedly and will not power on.

Took the cover off and it seems to be originating in or around the bottom right transformer. I believe I tested all the diodes I could find, including the surface mount ones on the back and any that were 0.3v or lower I removed and retested, but they all seem good. Same with the mosfets. The main filter cap has ~160vdc when it's plugged in, but the 6 output pins on the left side of the board all show fluctuating random voltages. Also I'm not sure what they're supposed to be because it's not marked anywhere that I've seen. The 3 caps near those output pins are all 100v caps so am I right to assume they're probably supposed to be higher than 12v or 24v? I don't know anything about LED tv's or their driver circuits. I haven't replaced any components yet. Any help, guidance, or a hyperlink toward some would be greatly appreciated. Thank you!
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Old 08-12-2015, 04:24 PM   #2
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Default Re: LF501EM5F No Power, Chirping/Beeping Transformer

disconnect the backlight LEDs and try to power it on again.
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Old 08-13-2015, 08:56 AM   #3
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Default Re: LF501EM5F No Power, Chirping/Beeping Transformer

Thanks for the response. I tried that, no change.
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Old 08-13-2015, 09:00 AM   #4
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Default Re: LF501EM5F No Power, Chirping/Beeping Transformer

Do you have a DMM I bet that you have a shorted diode on the output of the psu ?
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Old 08-13-2015, 11:29 AM   #5
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Default Re: LF501EM5F No Power, Chirping/Beeping Transformer

I do have a DMM. I tested most if not all diodes. I just double checked the ones i've boxed in red in this picture, are these what you'd consider output diodes?
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Old 08-13-2015, 11:34 AM   #6
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Default Re: LF501EM5F No Power, Chirping/Beeping Transformer

Yes, Test ALL the Diodes will be best, including the SMD if any on the bottom side.
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Old 08-13-2015, 12:38 PM   #7
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Default Re: LF501EM5F No Power, Chirping/Beeping Transformer

yes of course, I tested all of them. I noticed the SMD diodes that are situated near multiple resistors tested bad in circuit but after removing them i see they're good
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Old 08-13-2015, 01:32 PM   #8
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Default Re: LF501EM5F No Power, Chirping/Beeping Transformer

Ok now that you have tested the diodes you need to check the primary fets and there respective resistors.
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Old 08-13-2015, 01:56 PM   #9
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Default Re: LF501EM5F No Power, Chirping/Beeping Transformer

Usually a tick tick tick in a transformer means that there is a short on the output side off it. Most of the time it is a diode, but also could be a shorted cap or something else causing a short. That is why the input circuit of the transformer tries to start, but it gets shut down again. Hence the tick tick tick.
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Old 08-13-2015, 02:02 PM   #10
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Default Re: LF501EM5F No Power, Chirping/Beeping Transformer

Thats not always the case Capleaker youtube use Maxarcade has a video of repairing a set based off a similar chassis and his had a shorted diode and it made a whining noise so what you are saying is slightly correct but not 100%.
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Old 08-13-2015, 02:57 PM   #11
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Default Re: LF501EM5F No Power, Chirping/Beeping Transformer

The Bottom right transformer is the power supply for the main board, what happen if you remove the flat white ribbon cable, will it is still ticking?

http://www.shopjimmy.com/emerson-a3a...upply-unit.htm

http://www.shopjimmy.com/emerson-lf5...repair-kit.htm

http://www.shopjimmy.com/catalogsear...rson+LF501EM5F

I have no idea how much time you will want to spend on component level repair.

Last edited by budm; 08-13-2015 at 03:20 PM..
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Old 08-13-2015, 03:39 PM   #12
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Default Re: LF501EM5F No Power, Chirping/Beeping Transformer

It stops ticking when the cable is removed. Starts ticking again when the cable is reinserted.
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Old 08-13-2015, 04:14 PM   #13
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Default Re: LF501EM5F No Power, Chirping/Beeping Transformer

So it is either the main board is drawing too much current and cause it to go into startup/shut down mode or the power supply is false and cannot handle the load.
More In depth trouble shooting required, you will have to start checking the load resistance of the power supply outputs to see if any one of them show very low resistance.
You will need to look at the bottom side where the connector CN501 is to see which one are the power pins.
"As soon as it's plugged in (to mains) it starts to "chirp" at around 2 second intervals repeatedly and will not power on. " so it is doing it in standby mode.

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Old 08-14-2015, 04:46 AM   #14
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Default Re: LF501EM5F No Power, Chirping/Beeping Transformer

Quote:
Originally Posted by ReeceyBurger123 View Post
Thats not always the case Capleaker youtube use Maxarcade has a video of repairing a set based off a similar chassis and his had a shorted diode and it made a whining noise so what you are saying is slightly correct but not 100%.
Whatever... I am sure that there is a short somewhere on the output side of the transformer and oops for some reason there is!
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Old 08-14-2015, 08:39 AM   #15
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Default Re: LF501EM5F No Power, Chirping/Beeping Transformer

Quote:
Originally Posted by CapLeaker View Post
Whatever... I am sure that there is a short somewhere on the output side of the transformer and oops for some reason there is!
Ok in this case yes, but what im saying is that I have seen power boards where there is a short make a whine noise and in some cases make a tick noise, it works both ways. I dont see why you are getting that way I was merely pointing out what you were saying is not 100% true mate.

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Old 08-14-2015, 09:29 AM   #16
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Default Re: LF501EM5F No Power, Chirping/Beeping Transformer

if you look at the screenprint under the psu, it's all marked.

i see:
gnd
amp +13v
p-on +12v
p-on H2 <???
all +3.3v

near the invertor is bl-sw

in the center is PFC-Vout

for a piece of shit it's pretty well labelled.
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Old 08-14-2015, 10:15 AM   #17
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Default Re: LF501EM5F No Power, Chirping/Beeping Transformer

Oh wow you're right. I hadn't noticed the labels before but now they jump out at me. A 2nd pair of eyes on a problem is invaluable! Thanks! I'll do some prodding and report back
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Old 08-14-2015, 10:43 AM   #18
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Default Re: LF501EM5F No Power, Chirping/Beeping Transformer

btw, it's probably a shorted ceramic cap on the main board.
i'v had that few times on LG
a bastard to find btw!
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Old 08-20-2015, 12:53 PM   #19
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Default Re: LF501EM5F No Power, Chirping/Beeping Transformer

Going off the labels you pointed out, I have:

(power supply plugged in, nothing connected to it)
amp +13v -> reads 8.18vdc
p-on +21v -> reads 13.53vdc
p-on_H2 -> reads 0.00vdc
all +3.3v -> reads 0.00vdc
bl-sw -> reads 0.00vdc

(power supply plugged in, main board connected to it, transformer beeping)
amp +13v -> fluctuating 3.8vdc~4.1vdc
p-on +21v -> fluctuating 0.00~1.2vdc between beeps
p-on_H2 -> reads 0.00
all +3.3v -> fluctuating 0.00-0.8vdc between beeps
bl-sw -> reads 0.00

Does the fact that all the voltages are low even before the main board is plugged in indicate that a problem is likely on the power supply board?
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Old 08-20-2015, 03:45 PM   #20
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Default Re: LF501EM5F No Power, Chirping/Beeping Transformer

well the psu looks wrong, but dont rule out a fault on both.

so lets start with the psu.
what are you getting on pfc-vout?
it should be a few hundred i imagine.
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