Sharp LC-20AV1U

[NxB]

I have a sharp LCD tv that makes a high pitch squeal when plugged in. The relay on the inverter also clicks. I replaced a zener diode that had continuity and now it tries to start up. It still squeals but it clicks less. The diode also glows and I assume will burn if I leave it going. I have no way to tell the value of the diode so I replaced it with one that was similar. Any ideas?

[PlainBill]

Re: Sharp LC-20AV1U

A glowing diode is a bad sign. Similar?? What was your criteria? A 5.2V 500mw zener is similar to a 5.1V 350 mw zener and can probably be used as a substitute. Just because two zeners are supplied in the DO-5 package doesn't mean they are similar.

I'd say the proper starting point would be to find a service manual, or at least a schematic.

PlainBill

[Toasty]

Re: Sharp LC-20AV1U

>>makes a high pitch squeal when plugged in<<
Just sitting there, not turned on it squeals? Does it turn on? Any picture or OSD?

>>The diode also glows and I assume will burn if I leave it going<<
If it glows, either you have an LED there or it's already burnt.

>>replaced a zener diode that had continuity<<
Do you mean shorted? It read the same when you connected it both ways *out of circuit* on the ohmmeter?

What are the numbers on the original?

Toast

[NxB]

Re: Sharp LC-20AV1U

Can't measure the dead one. No markings other than 8.

[NxB]

Re: Sharp LC-20AV1U

When you plug it in, the status light starts up amber and the squealing starts.

[Toasty]

Re: Sharp LC-20AV1U

>>Can't measure the dead one. No markings other than 8.<<

Huh? How did you arrive at the "continuity" statement then?

[NxB]

Re: Sharp LC-20AV1U

DMM beeps, both ways. Completely shorted. I took one leg up because I know diodes can seem blown in circuit, it still did the same thing. I thought I found another one on the hot side but that one worked once I pulled one side up.

[PlainBill]

Re: Sharp LC-20AV1U

DMM beeps, both ways. Completely shorted. I took one leg up because I know diodes can seem blown in circuit, it still did the same thing. I thought I found another one on the hot side but that one worked once I pulled one side up.

Good job of testing. I suspect the replacement is an incorrect part.
Does this particular set have an internal power supply, or does it have an external 'power brick'?

PlainBill

[Wizard]

Re: Sharp LC-20AV1U

Also measure the output side of the diode (one leg lifted) to the ground with mainboard connected and screwed to the chassis to make sure there is no shorts. Shorted load kills diode.

Cheers, Wizard

[NxB]

Re: Sharp LC-20AV1U

Internal power supply. Will check the other diode that measured dead in circuit to chassis ground and the one that keeps getting burnt. I wish there was some way to check the value of this thing. It could just be the value is too low or its another short somewhere. All of the ICs in the inverter test fine too as do the ones in the power supply. Any way to tell if the transformers are screwed up or shorted?

[PlainBill]

Re: Sharp LC-20AV1U

Internal power supply. Will check the other diode that measured dead in circuit to chassis ground and the one that keeps getting burnt. I wish there was some way to check the value of this thing. It could just be the value is too low or its another short somewhere. All of the ICs in the inverter test fine too as do the ones in the power supply. Any way to tell if the transformers are screwed up or shorted?

A ring tester may tell you that. I'm a little vague on just where this diode is. Is it part of the power supply? One trick I have used is to hook up a variable power supply to each of the outputs of the power supply. If you can successfully output 1-2 volts into each output it's pretty certain there is no major short.

As far as determining the value of the shorted diode, the schematic (or parts list) is the best bet. The other approach is to analyze the circuit to see just what it does. At least we can be certain it's no x-ray protection diode.

PlainBill

[Toasty]

Re: Sharp LC-20AV1U

What makes you think it is a zener diode? Where is it located?

If in fact it is, then the 8 means most likely it's an 8 or 8.1 volt zener. Current capacity can be guesstimated by diameter and length.

Check the caps coming off the cathode end (banded end) in the circuit. See what voltage ratings they have. May lend a clue to determining value also.

Pics of:
1) the original diode with some size reference (ruler)
2) the problem area and surrounding components
3) pics of the circuit trace side of the board

Depending on what area this is in, the high pitched squeal and the burning of the diode could be due to a HV capacitor, probably ceramic or poly, that may be open or shorted.

It also may be a Schottky type diode that is typically used in switching power supplies and in the horizontal section of TV's. It needs to be capable of high speed switching. A regular diode can't switch fast enough and will exhibit the overheating you're seeing.

Hmm. More I think about this, if you saw a symbol on the board that looks like a zener, you may actually have a Schottky. The 2 are VERY similar and can be confusing.

Look here at symbols:
http://en.wikipedia.org/wiki/Zener_diode
http://en.wikipedia.org/wiki/Schottky_diode

Toast

[Krankshaft]

Re: Sharp LC-20AV1U

It's very possible that the reason the original diode blew is from an overload on the PSU or through one of it's outputs.

Maybe a shorted inverter driver transistor drawing too much current from the PSU?

As for the zeners value the best way to find it out is shell out a few bucks for the service manual that way you won't just be guessing.

When I'm working if I can't find one for free with a bit of Goggling I can get it for under 12 bucks.

No doubt it will also make tracing out the circuit and troubleshooting easier.

I have also traced many singing inductors or transformers to high ESR caps in circuit with them so you should check that out too.

As for the ring tester it's about 46 bucks from Anatek unassembled unless you're constantly probing electronics like I am it's pretty pricey. But like my ESR meter I would never give it up.

[fzabkar]

Re: Sharp LC-20AV1U

LC-13AV1U | LC-15AV1U | LC-20AV1U Operation Manual:


"... models are different in size and shape, but operate identically."

Since the one user manual covers all three models above, then maybe the service manuals are similar ???

Here are some possible candidates:

sharp lc13_15b4e LCD Colour tv part1:
http://www.eserviceinfo.com/download...c13-15b4e.html

sharp lc13_15b4e LCD Colour tv part2:
http://www.eserviceinfo.com/download...c13-15b4e.html

SHARP LC15B2EA Part 1:
http://www.eserviceinfo.com/download.php?fileid=15413

SHARP LC15B2EA Part 2:
http://www.eserviceinfo.com/download.php?fileid=15414

SHARP LC15B2EA Part 3:
http://www.eserviceinfo.com/download.php?fileid=15415

Sharp LC-30HV4E (19MB):


Here is an extensive parts list for your model:

http://bestbuy.partsearch.com/Model/.../LC20AV1U.aspx

A search for "diode" finds 16 candidates.