Hitachi LE42H508

[mike123333]

Hello everyone, I have an Hitachi LE42H508 that I recently acquired that does not have a backlight. At first the TV would not turn on or go into standby. I went inside to perform some preliminary evaluation and checked the voltages on the high capacitor and found there to be the ~300V DC. Then pursued with the 12.4 volt marked on the PS and that was fine. While conducting the measurements I noticed Diode D107 ( was not shorted) was smoking and the cooling rail for the transistors was extremely hot to the touch, needlessly to say i turned off the set. Upon ohm measurement I found rectifier diode to be shorted D305. So i removed it from the circuit and powered on the TV and the TV went into standby mode and turned on and there was video( using a flash light I can see it). after pulling off D107 it seemed the smoke was from the heat coming off the diode and melting the coating on the board.

I ordered the diode D107 ( FR107) and D503(UF5408)....The D107 diode ws part of package of many different types. One type was the 1N5408, I decided to install both diodes and check the TV...at first I did not hook up the LED cabling to the board. I tested it and noticed nothign happening I measured the voltage and only seen a spike..So I hooked up the LED cable and turned on the set, the back lights came on then shut off then no TV...after measuring the ohms across the diode D503 it turned out it shorted again.

I went ahead removed it and replaced a new one but did not hook up the LED cable, turned the TV on and there was no standby LED on or the TV would have picture.. I then measured the voltage going to the LED and there would be a spike to 150V DC and then steadily drop to 0..

I removed the diode D503 and the video worked and standby LED was present..

Does this sound liek there is something on the primary side of the tranformer for the LED backlights? why would the primary get so hot and secondary short? I took the transformer out of curcuit and tested it for shorts, there were no shorts between primary and secondary windings.

Any input would be greatly appreciated..I checked out the board components as best as I can with my limited experience and found no diodes shorted or capacitors with issues, I did check with my ESR on the capacitors and found no issues, unless its happening under load. I have included photos of the PS.

[mike123333]

Re: Hitachi LE42H508

I wanted to include some additional information I gained with measurements, at the main DC capacitor filter I am getting around 380V with TV in on position and 168 V in standby..as I probe around the transformer for the LED the diode D107 on the non band side is at 380V and the other side which is tied to resistor R123 a spike of over 600V then drops to between 450v AND 550v i am using the main filter negative lead as the ground on the hot side. one side of R123 is at 380V and the other is in that range of 450v to 550v ..does this possibly question the transformer???

[R_J]

Re: Hitachi LE42H508

D107 must be a Fast recovery diode, D503 is ultra fast diode, if you put in the general purpose 1n5408 it will get hot and most likley short out.
Without a schematic it's hard to tell (from the pictures) where those diodes go.

[mike123333]

Re: Hitachi LE42H508

Thanks for the reply..I am awaiting the delivery of my UF5408, I just wanted to test the circuit to determine if anything else was at fault....I do have a question regarding the transformer, why are they using the DV voltage on the primary side of the transformer? that is all I can measure, when I put my meter in AC I do not get any measurement across the transformer primary. all the reading I have done was that the primary is AC and secondary is DC with a rectifier diode. am I missing anything here?

[R_J]

Re: Hitachi LE42H508

On the primary side the raw dc is converted to high frequency ac to drive the transformer. (it is more complicated than that but thats basicaly it)

These transformers rarely go bad, It looks like one transformer is used for standby and one is for the back light, Once you have the correct diodes replaced it might work fine.

The ac is at a high frequency maybe around 50Khz your meter may not read this.

On the bottom of the board it looks like the primary circuits are within the black circle, look up the 8 pin ic's data sheets, they may give you more information about how they operate.

[mike123333]

Re: Hitachi LE42H508

I received the correct UF5408 diode soldered it in above the board like I have in the picture in a previous post (to reduce heat buildup, the originally was flat with the board) and all is well....I am wondering whether there may have been a mnfr defect, the TV ws manufactured in March 2014.

[rustytierod]

Re: Hitachi LE42H508

Just came across this exact problem, did you mean D305? I cannot find D503 on the board...

[dskall]

Re: Hitachi LE42H508

Looks like D305 from picture.

[rustytierod]

Re: Hitachi LE42H508

Trying to find alternate for this UF5408 diode. What's the important spec besides "ultra fast," does anyone know if UT3060 qualifies?

[piernov]

Re: Hitachi LE42H508

UT3060 is not a fast recovery diode. UF5408 is a very common diode so you should be able to easily find it. Most important specs are: similar or higher reverse voltage, similar or higher forward current, similar or lower reverse recovery time (which is why it is called "ultra fast"). Try to have a forward voltage not too far off too.
If it's not a fast recovery diode, they usually don't give the reverse recovery time in the datasheet. It does matter only when it's in a fast switching circuit (for example SMPS secondary rectifier like here).