Sharp LC-32D43U Dead

[kevinm34232]

Hi, this TV was dead on arrival. Thought it may be Q7602 and Q7603 so replaced those. In mean time I noticed D7907 was shorted, it's the large diode next to the small transformer T7901. I cut one leg to check it because of how it's soldered into the board. I replaced the 2 transistors and the tv fired right up.

Then I remembered I never soldered the diode back in. After doing that the TV was dead again. When I went to desolder it again I blew the resistor R7002 with a spark.

My question is, what is the purpose of that lone diode, a surge protection? And can someone decipher the value of that resisitor R7002? Thanks!

[randtek]

Re: Sharp LC-32D43U Dead

Resistor R7002 is .22 Ohm 5% flameproof, probably used as a fuse.

It is impossible to tell what that diode does without seeing it on a schematic. Surge supression would be unlikely.

[kevinm34232]

Re: Sharp LC-32D43U Dead

Strange thing is TV worked fine without diode in circuit... Do you know what the wattage of the R is?

[budm]

Re: Sharp LC-32D43U Dead

Is the Cathode (band side) of that lifted Diode connecto the Cathode of the Diode closed to it, if it is, that diode that you lift is part of the snubber diode to protect the MOSFET that drive the transformer, without having that diode in the circuits, the kickback voltage from the transformer when the MOSFET is turn off can be high enough to cause the MOSFET to short out. What is number printed on that diode, 6PKEXXX? The circuit will run for awhile without the Diode before the MOSFET shorted out.

[kevinm34232]

Re: Sharp LC-32D43U Dead

Yes it seems it is connected to anode of D7904 directly to left, showing very low ohms between the 2.
Whats printed on the diode is:

PK
IN
5383B
0842 or OS42

[budm]

Re: Sharp LC-32D43U Dead

OK, it is the 142V 5W Zener diode for the snubber circuit, so right now more likely that you now also have damaged MOSFET which is why it blew the fusible resistor. We need to see more closed up pictures on the bottom and the top side to see where that MOSFET is, and hopefully it did not take out the SMPS IC with it.

[kevinm34232]

Re: Sharp LC-32D43U Dead

OK. the MOSFETS do not show to be shorted so I will try replacing the fuse and that diode.

How do I determine the wattage on that resistor (or any resistor)?

[randtek]

Re: Sharp LC-32D43U Dead

To accurately determine the wattage, you must look at the schematic, and hope it has that information on it. There is really no reliable way to determine wattage of a resistor by looking, unless it is printed on the resistor body.

[tom66]

Re: Sharp LC-32D43U Dead

Randtek is correct, but it is often possible to make a close enough guess. It doesn't matter if you use a 3W resistor in place of a 2W for example.

[randtek]

Re: Sharp LC-32D43U Dead

That resistor looks like it could be 1/2 to 1 watt, but it cold be as low as 1/4 watt. The only concern with guessing at the wattage is that when a resistor is used as a fuse, the wattage becomes a critical part of the equation. If you replace it with too low a wattage, it will probably fail again in short order. If you choose too high wattage, you lose the fuse capability. For example, if the circuit calls for 1/4 watt, and you use 1/2 watt, it will take twice as much current to blow the fuse if something is drawing too much current on that line. So, you can use a higher wattage value, and the circuit will work, but if something else is still bad, or fails in the future, the damage to the circuit could be more severe, and in rare cases, could pose a fire hazard. For this reason, these types of components are often marked as "Critical Safety" components on the circuit diagram.

[tom66]

Re: Sharp LC-32D43U Dead

Ah, I didn't realise we were talking about a fusible resistor, sorry.

[budm]

Re: Sharp LC-32D43U Dead

You can measure the dia and the length and then go to digikey and serch for fusible resistor that will be close enough, it looks like 1/2w.
http://www.digikey.com/product-detai...2BCT-ND/614253

[randtek]

Re: Sharp LC-32D43U Dead

budm, That is really not a reliable means for determining wattage. I have seen 1 watt resistors the same size as 1/2 watt, 1/4 watt same size as 1/2 watt. There really is no standardization to resistor bodies that are accurate, especially on flameproof ones. It was much more consistent on the old carbon resistors.

[tom66]

Re: Sharp LC-32D43U Dead

The most common failure mode a fuse has to protect against is a short circuit.
I see no hazard in 1/2W vs 1W etc., a short circuit will burn the resistor either way.

[kevinm34232]

Re: Sharp LC-32D43U Dead

I ordered the new resistor (the one Budm pointed to), a new diode, and 2 new transistors. I looks like the diode was the initial failure so hopefully that'll do it. Thinking back if I would have know to just remove one leg of the diode the TV probably would have came on since the 2 transistors looked OK. The biggest pain of this PS is those 2 mosfets and the bridge rectifier are screwed to a large heatsink that is soldered in and covers the whole area..

[budm]

Re: Sharp LC-32D43U Dead

@rantek, I know that you can get same wattage resistor with more than one sizes, I used them in our product all the time, for now that is all the info we have to go by on what we have in hand, if there is shorted circuit, the 1/2w or 1w will burn open anywway when you try putting 170VDC across that 0.22 Ohm resistor.

[kevinm34232]

Re: Sharp LC-32D43U Dead

Replaced the diode D7904 (zener 150v 5w) and resistor R7002 (.22 ohm), and TV is back to life. Thanks for the help ID'ing these parts!! I had also initially replaced the 2 transitors Q7602 and Q7603 but I think now they were probably still good.

[budm]

Re: Sharp LC-32D43U Dead

Thanks for the feedback, you were lucky that nothing else got damaged while running the TV without the snubber in place.

[user8802today]

Re: Sharp LC-32D43U Dead

Replaced the diode D7904 (zener 150v 5w) and resistor R7002 (.22 ohm), and TV is back to life. Thanks for the help ID'ing these parts!! I had also initially replaced the 2 transitors Q7602 and Q7603 but I think now they were probably still good.

I think you meant to say "Replaced the diode D7907 [with a](zener 150v 5w)", right? I was a confused for a bit before i figured out what diode you were talking about.

[hapollo]

Re: Sharp LC-32D43U Dead

I think you meant to say "Replaced the diode D7907 [with a](zener 150v 5w)", right? I was a confused for a bit before i figured out what diode you were talking about.

Yes, I believe he meant D7907. I followed this PDF posted on the net and got the TV working again after replacing just the diode.

http://preher-tech.com/documents/sha...c32d43unopower

Recently picked up a Sharp LC-42D62U which has same no power issue. Suspect it's a diode issue as well but diode in that area is a larger size. Need to figure out the diode rating and order from digikey.