Hitachi LE42H508

[Fisher77]

Ok this started as a no power issue. I replaced diodes D107, and D305 that were blown. That solved the no power issue. Now when I power the tv on T201 is buzzing.

Any suggestions as to what might be causing it to buzz?

[budm]

Re: Hitachi LE42H508

The output of that section looks to go inside the panel where the backlights driver circuit board is, you need to look at that board, I cannot see really well in your pictures.

[Fisher77]

Re: Hitachi LE42H508

Thanks budm. Which picture do you need clearer, I will get it for you?

[Fisher77]

Re: Hitachi LE42H508

Well I took the lcd off and looked, there is no other board inside the panel. Wire off the psu goes straight to the leds. I now have no backlights at all. The pins for CON301 were loose. I resoldered them, and no change. Even with the backlights not working, the transformer still buzzes. When I disconnect the wires for the leds, the buzzing stops.

Could it be some bad leds causing this problem?

[budm]

Re: Hitachi LE42H508

OK, next test:
Leave the LED connector in place and then check the resistance between chassis GND and the White wire of the LED connector, check the resistance between chassis GND and the Red wire of the LED connector.
Report the resistance reading.

[Fisher77]

Re: Hitachi LE42H508

Here are the readings:

Between GND and the white wire is 21.18k
Between GND and the red wire is 20.81k

[budm]

Re: Hitachi LE42H508

No short GND.
I just look at you soldering works, very bad cold solder joints on the diode you replaced, you should also check the other diode you replaced also.
The transformer legs can also use some re-soldering too but you need to use flux with it.
Did you use flux and 40W Iron?
Once you fix the soldering works, then you need to report the Voltage reading between chassis GND against white wire, then against red wire each time you turn on the TV.

[Fisher77]

Re: Hitachi LE42H508

Ok, redid the solder joints on both diodes, and the transformer.
I used a 65W, and flux.

Voltage between red wire and chassis GND is 21.9vdc.
Voltage between white wire and chassis GND is 9.6vdc.

[budm]

Re: Hitachi LE42H508

That is way too low.
What is the Voltage rating as printed on the body of that black cap right by the LED connector?
Is it still squealing?
And if you do not have the LED connector in place, what Voltage do you get on the LED connector pins of the board?

[Fisher77]

Re: Hitachi LE42H508

The voltage rating on the cap is 200vdc.
Yes it is still squealing even though the LED's are not coming on.
The voltage across the LED pins with the connector unplugged is 185vdc.

[budm]

Re: Hitachi LE42H508

OK so no load Voltage is 185V, with LED connected it drop down to 21.9V on the red wire which is the plus Voltage is fed to the Anode of the LED string.
Something is not adding up, something is heavily loading down the power supply and causes it to squeal and the diode was damaged, and there is no low resistance to GND on the red wire (it has 20K Ohms).
How many LED's are there when you took the panel apart?

[Fisher77]

Re: Hitachi LE42H508

Your right budm, something is jikky.
When I ohmed from red to chassis GND last night it was 20k ohms. This morning is is reading OL. I tried 2 different meters, and they both read OL.

Ohming from the red to white wires I am getting .5k ohms. Don't know if that will tell you anything or not.

I checked the diode again, it was reading .5, so its good.

I didn't count the leds when I took it apart. I will take it apart again and see how many there are and report back.

[budm]

Re: Hitachi LE42H508

OL is fine, it mean open circuit for sure.
"Ohming from the red to white wires I am getting .5k ohms. Don't know if that will tell you anything or not." that is bad for sure, that is only 500 Ohms.
So you do have something bad going on on the LED strip to give you 500 Ohms reading between the Red and the White wire.

[Fisher77]

Re: Hitachi LE42H508

Ok, I will get take it part, and see what I fine.

[budm]

Re: Hitachi LE42H508

Please take good picture of the LED strip assembly and upload them for the record.

[Fisher77]

Re: Hitachi LE42H508

Ok there is 40 LED's total.
There are 6 strings that have 1 blown LED each.
There is a Zener diode on each string, and they are all blown. I measured them in diode mode, is that the correct way to measure a Zener?

Question is what blew the Zener's?
Zeners have N2 on them, any idea what they are?

The LED's were working before I took the tv apart yesterday. I checked for anything shorted as I took it apart today, and did not find anything.

[budm]

Re: Hitachi LE42H508

OK, the ZENER is connected in parallel with each strip of LED strip for protection due to wrong power supply reverse polarity connection so the Zener will blow up, and it also clamp the Voltage down if the feeding Voltage has correct polarity but too high of the Voltage is applied to the strip.
Most LED strip do not have that Zener diode.
Did by any chance you try to power up the LED strip when you trying to test each LED strip?
"The LED's were working before I took the tv apart yesterday." So how did you find out the LED's were working since the LED power supply was dead due to blown diode?

[Fisher77]

Re: Hitachi LE42H508

I tested each led individually with diode mode on my multimeter. It lit them dimly, except for the blown ones of course.


After I replaced the two blown diodes on the PSU yesterday, the tv powered on with the backlights, but the transformer was squealing. After I took it apart, and put it back together, the LED's never lit again. It's weird. I looked at all the wiring, nothing was shorted.

[budm]

Re: Hitachi LE42H508

So right now if you do the resistance between the two pins on each LED strip individually, how many of them are showing low resistance?

[Fisher77]

Re: Hitachi LE42H508

I tested the resistance on each strip.

4.7 ohms
3.8 ohms
37.5 ohms
51.6 ohms
235.1 ohms
138.3 ohms
13.9 ohms
137.8 ohms

I assume the 2 with less than 5 ohms are shorted?