26" Sharp LC-26DV22U LCD power problem.

[budm]

Re: 26" Sharp LC-26DV22U LCD power problem.

OK, since you apply a kick start 12V to the Switched 24V output terminal then the power supply will start running. It may be seindinf 24V not present when the power supply try to start up then it goes into shutdown, or something else is not running 100% since you are getting only about 8V at the 24V output.
I look at the circuits and see that it uses 'FAILED POWER DETECT' circuits (this method of monitoring the power supply circuits is used a lot in Sharp TV). If you look at the redlined diagram attached, you will see that it uses Diode OR GATE circuits setup to detect output voltage of the power supply.

D449, D456 is for detecting Switch Sound B+
D446 is for Switched +24V
D447 is for Switched +12V
D448 is for Switched +5.5V

If the output voltage is not present at any one of those output, the Diode will go into Foward Bias mode and conduct (normally it will be off due to reverse Bias by the outputs of the power supplies), then it will turn on Q415 which will trigger protection circuits (Q412. OPTO, etc.).
So it either that the Diode D446 is leaky or the power supply is not on long enough (takes too long for the output voltage to rise up to the point that turns of the pratection circuits, the detection circuits act faster than the build up of the output voltages) to produce high enough voltage to keep the Diodes in off stage.
You can lift one end of D446 (Switched +24 detector Diode) and see if it will start up. Since I am not sure if your board part designators are the same as your board, you hsould verify that we lifting the correct Diode. Please note that it can be anyone of the outputs that trigger the shut down.
I still think the problem is in the primary side that causes the output to rise up slower than the detection circuits.

[Davegerg]

Re: 26" Sharp LC-26DV22U LCD power problem.

Hi, The one I put in was from an old CRT board. Both tested good but, I put the original back in and cleaned up the solder job. The joints were close but not touching. All the caps on this board are Rubycon.

[Davegerg]

Re: 26" Sharp LC-26DV22U LCD power problem.

hi budm, I tried lifting D456 first and no change. I ended up lifting all the ones you suggested and still no change. Still the 8v. I was looking at the trouble shooting flowchart (pg 23) in the manual (attached) and it says if no voltage, check IC401, T401, D437 and peripheral circuit. What do you think?

[budm]

Re: 26" Sharp LC-26DV22U LCD power problem.

Ok, so we verify that it is not protection circuit fault tripping, so as I indicate, some thing is in the primary side. At 8V output, it sounds like the IC401 is not fully driving the circuits to full output and some how when you apply the kick start, then it will run OK, and you have already replace the start up cap.
So if you lift all the Diode, wil it start runnung with the 12V kick start?
I will look furthe to see if the kick start at soem other point in the circuits will make it runs also.
I will be bring home the LC-26SH12U this week end to fix the low audio problem (I have to add the pre-amp circuits to boost the audio level, the factory release the Harware and firmware update to fix the problem but I not doing it their way), may be I can do some experiment with it.

[Davegerg]

Re: 26" Sharp LC-26DV22U LCD power problem.

Yes, after lifting all of the diodes you indicated, the kick start still works.

[budm]

Re: 26" Sharp LC-26DV22U LCD power problem.

What do you get at PCON+5v? It should be 5V when power switch is activated.

[Davegerg]

Re: 26" Sharp LC-26DV22U LCD power problem.

p.con+5v OK, as soon as I power on, it's 5.19v. In about 10 seconds it drops to 4.98v and holds steady. When I kick start, it jumps back to 5.19v.

[budm]

Re: 26" Sharp LC-26DV22U LCD power problem.

Can you check the between Emitter-Collector voltage of Q412 when TV is running, I expect it to be low.

[Davegerg]

Re: 26" Sharp LC-26DV22U LCD power problem.

OK, this one is a little tricky for me. I'm not quite sure how to do it. Here's what I did. BEFORE the kick: with the neg meter lead on ground, I touched the pos lead to the following points in the picture and got: 1) 4.6v, 2) 0v, 3) 0v. I'm guessing #1 is the collector? AFTER the kick, #1 went to 25v.

[budm]

Re: 26" Sharp LC-26DV22U LCD power problem.

You have it right, pin 1 is the Collector, 2 = Base, 3 = Emitter.
This reading is strange becuase BEFORE the kick, you have 0V at the Base which means the Transistor should be OFF so the Collector voltage should be high, but it is reading 4.6V. After kick start, the Collector voltage gose up to 25V which means the Transistor is turn off. What you are see ing does not make sense you you do not have 0.6V at the Base of the transistor, the Collector should always be high.
So that transistor should turn off when you turn on the power switch for the TV to function but some how it stuck partially on, if it is fully on the voltage would be less than 1 volt. if you kick start by short Base to Emitter (this will turn the Transistor off) temporary instead of using the kick start voltage, the TV should kick on also.
There is a small cap C445 by Q415 in the diagram (bottom right), I would like to have it changed.

[Davegerg]

Re: 26" Sharp LC-26DV22U LCD power problem.

I tried shorting base to emitter but with no luck. Also changed C445 but no luck there.

[budm]

Re: 26" Sharp LC-26DV22U LCD power problem.

So when you short the E to B with set on, it dose not start up. and the Collector voltage did not rise up to 8V?
The secondary side is isolated from the primary side by two OPTO and the transformer, when you apply the kick start, there are only two Opto that will see the 12v kick start, it is not making sense yet why the kick start will cause the power supply to run properly wnd why it is only putting out 8V on the 24V when it is not running properly.

[selldoor]

Re: 26" Sharp LC-26DV22U LCD power problem.

How about C456 being faulty 24v>gnd just before the connector where you apply boost?

[budm]

Re: 26" Sharp LC-26DV22U LCD power problem.

I looke at my red-lined sch, but I just cannot find this C456.

[selldoor]

Re: 26" Sharp LC-26DV22U LCD power problem.

Its on righthand side of p47 of the manual in post 23 and in the top left corner of the pic in post one ( unless I am mis reading the board no)

[Davegerg]

Re: 26" Sharp LC-26DV22U LCD power problem.

OK, getting different results now. I had been trying to read the voltages on Q412 with the white connector detached and the board up on end. Figured out where to read it from the top. I plugged everything back in and now when I turn it on, Q415 reads like this: coll +4.26v, base +2.06v, emit 0v. I think I started getting 2.05v at the base AFTER I tried shorting the emit & base. Can't be sure, though. One thing is sure - my kick start no longer works! When I kick it, the voltage on pins 1-5 of CP406 goes to .3v and the Q412 reads 0v. Is it toast now????

[budm]

Re: 26" Sharp LC-26DV22U LCD power problem.

If you are getting 2.06V at the Q415 Base and Emitter (which is connected ground), it means the Base-Emitter junction is open. Are you sure you had the Base-Emitter short together, not Base to Collector? It should show about 0.6 ~0.7V if the Base Emitter junction is OK.
The Collector should go up to 24~25V for the TV to turn on again when using the kick start.
When Base is shorted to the Emitter, it will turn off the transistor so the Collector Voltage will rise close to the VCC (in this case, its the 24V supply), this will turn off the LED inside the OPTO IC409 which will also turn off the Opto transistor, this opto transistor when it is on it will short Base/Emitter of transistor Q406 which causes Q406 to be off which control the regulator transistor Q405. Q406 has to be on to regulate the 19V power supply for IC401 VCC.
You can also see a 30V zener diode D439 which will drive the Base of Q415 on if the 24V supply gets up to 30V which will be high enough to turn on D439 and provide bias to turn on Q415.
So basically, if Q415 is on, the TV will be force off.
There are so many protection circuits in this power supply section. If you replace Q415, it will work with kick start again. You can lift one end of R434 which provide the current for LED inside OPTO IC409, this will allow the 19V power supply to stay on.

[Davegerg]

Re: 26" Sharp LC-26DV22U LCD power problem.

Well, after spending 20+ hours on this project, I've determined it's way over my head. I should have jury rigged it to begin with and I might have gotten x days/months/years out of it. It's now FUBAR so I've screwed it back together and am putting it in the garage sale pile. Thanks for your help. Dave out.

[budm]

Re: 26" Sharp LC-26DV22U LCD power problem.

I still believe if you lift R434, you may be able to use kick start again.

[Davegerg]

Re: 26" Sharp LC-26DV22U LCD power problem.

OK, I relented and put it back on the bench. Lifted R434 and got the kick start working again. I will rig something up with a little switch and see how long it works. Thanks for your help.