Coby LEDTV5028 - No signs of life

[MKGort]

Greetings everyone.

I am looking for some assistance in troubleshooting a Coby TV for a friend.
The model number is LEDTV5028.

There seems to be no power at all to the set. No standby LED or any signs of life.

When everything is plugged in, there is a "chirping" noise that pulses about every second. It seems to be coming from what I believe is a small transformer (red circle in picture).

All voltages are present at the connectors (green circle).

If I disconnect the main board (not shown) the chirping stops, but I don't get voltage readings from the connectors (I would guess I should see something).

I have tested every capacitor with an ESR meter and have not found anything that stands out as being bad. There is also no signs of stress or damage anywhere. (Also tested the main board)

I have reached the limit of my electronics knowledge (which is relatively small) and I do not know how to proceed.

Power Board - MP128FL-T
Main Board - 002-LT23-7612-00R

Any advice on what to test next is most appreciated.

Thank you,

MK

[tom66]

Re: Coby LEDTV5028 - No signs of life

Chirping power supply: I immediately check the diodes on the power supply, all of them, plus the startup capacitors. (Small electrolytic around 4.7~100uF 25~50V rated on primary.)

[MKGort]

Re: Coby LEDTV5028 - No signs of life

Thank you for the response. I will go ahead and order some diodes and replace them first. All of the caps show OK with the meter, so I'll start triple checking them and see what happens with the new diodes.

Cheers,

MK

[MKGort]

Re: Coby LEDTV5028 - No signs of life

The diodes on the board are a HER103G and a HER107G. If I am reading the datasheets correctly, they seem to be very similar to the 1N4007, of which I have plenty. Would it be acceptable to use the 1N4007?

Thank you,

MK

[MKGort]

Re: Coby LEDTV5028 - No signs of life

Being new to the TV repair hobby, I wanted to make sure I was giving an accurate description of the sound it is making before I start replacing parts:

https://www.youtube.com/watch?v=6ye9SNLG6KA

Thanks again,

MK

[tom66]

Re: Coby LEDTV5028 - No signs of life

These cannot be subbed with 1N4007, they are high speed type. UF4007 or UF2008 works fine.
Definitely sounds like there's a short on that board somewhere with that chirping.

[MKGort]

Re: Coby LEDTV5028 - No signs of life

Well, shoot. I replaced both diodes and all the smaller range caps, but it's still a no-go. Problem still pops up when I plug in the main board.

Could this be a main board problem, or am I still looking for a short on the power board.

Thanks for the help!

-MK

[tom66]

Re: Coby LEDTV5028 - No signs of life

It could be a shorted mainboard, have you done a resistance check between 5V standby and ground on the main board? Also, try with a car tail lamp on 5V. It will glow dimly and provide a few watts load for testing.

[MKGort]

Re: Coby LEDTV5028 - No signs of life

Good call. I checked the 5 and 12 and sure enough there was a short. I could not determine where it was coming from, so I found an inexpensive replacement board. It seems to have fixed it, except I am having a picture issue. Do you have any idea what could be causing the odd "offset" in the picture. This also shows in the menu, even when nothing is attached. Could I have gotten a bad board?

Thank you again for your input and help. It is greatly appreciated.

-MK

[tom66]

Re: Coby LEDTV5028 - No signs of life

Make sure the LVDS cable from main to t-con is fully seated. Turn off any sharpness settings. Is that VGA or HDMI for PC?

[MKGort]

Re: Coby LEDTV5028 - No signs of life

I reseated the LVDS cable a couple of times just to be sure. I was really hoping it would be the sharpness settings but, alas, no. I was using the VGA port, but even with nothing plugged in it still exhibits this fuzzy picture (see pic).

I am going to try to wiggle the LVDS while the set is on and see what happens.

Thanks again

-MK

[tom66]

Re: Coby LEDTV5028 - No signs of life

It may be that the main board for the TV is made for a slightly different panel version for example you have a 2D LCD panel with a 3D mainboard or vice versa.

[MKGort]

Re: Coby LEDTV5028 - No signs of life

That never occurred to me. I think I will send this board back and ask for a replacement. I'll post the results.

Thank you, again, for the advice.

-MK

[MKGort]

Re: Coby LEDTV5028 - No signs of life

This was a response I received from the seller. He seemed pretty knowledgeable.

"This board is very tricky and apparently you must match the model and part number when ordering it. Your tv was a 50" and mine a 39" using completely different software yet the same part number.

Such a major failure on Coby's part but nothing we can do about that."

Well, that stinks. Same part number and version but not compatible.

It doesn't seem that anyone sells the replacement for the LEDTV5028

Thanks again for the input and the help!

It's been a good learning experience for me.

Cheers,

MK