Sharp C-1410 no power (smps issue)

[ostlar]

I received a Sharp C-1410ND a couple of days ago, it was working fine for about two hours with my Wii connected, I was getting some snacks when I came back to a dead TV. Didn't hear any unusual like a pop or crackle.

Opened it up to check if anything had gone fubar, but no sign of that. Plugged it in again, was met with a whine that started high in pitch and went down until it halted. I have uploaded a YouTube video
with the sound of the power supply issue. The sound is coming from the power supply area.

The issue is most likely in the switch mode power supply, I am suspecting the primary side.
After reading up on these power supplies, I started troubleshooting.
Started with checking for cracked solder joints and reflowed some suspicious ones. Checked the diodes, most of the resistors, capacitance and ESR of the electrolytic capacitors. Everything checked out fine. Checked the voltage on the main filter capacitor C706, got around 335V. On C710 i get 72V.

Seems like resistor R709, R710 and R711 is the only thing getting hot on the board according to my thermal camera, around 35-50°C. They are marked in yellow here:



What could this issue be? It sounds like there is either something in the feedback circuit that has gone haywire.
I only get about 15V for a brief moment on C635 on the secondary side, so something is happened.
Maybe a short that kicks the PSU into overload?
I have not checked the ceramic/film capacitors as they are usually okay, but might be worth a shot to check them too?

The schematics for the TV is available on ElektroTanya.

Appreciate the help.

[ostlar]

After some more diagnosing I've concluded that there is something shorting out the secondary 115V side. That makes sense with the voltage rapidly dropping and the sound the power supply makes when it cuts into protection.

I measure 20,7 ohms over the 115V filter capacitor, it is not supposed to be that low. I have checked the filter capacitor C635 with an ESR meter and the over voltage protection diode D601 with a multimeter, both are okay.
At 115V and 20,7 ohms, the power draw would be 5,5A or 632.5 watts, definitely not right.

I realized that his has an identical chassis to my Sharp C-3700, so I've been using that as a reference if I find something suspicious.

I have systematically measured most of the 115V circuit and followed wherever the resistance gets lower, and I am at 9,5 ohms at the lowest around the horizontal output transistor Q602.
Removed the transistor and tested it, no issue there.
I have also tested C618 that sits right by the HOT and is referenced to the ground.

Here is a screenshot with my measurements, the numbers show resistance to ground measured over the component that references ground, mostly capacitors:



I have hit a dead end, can't seem to find any resistance lower than 9,5 ohms to the ground.
I have tested most of the components that are referencing ground in the area with the lowest resistance.

What am I missing? Could it be an internal short in the flyback? I hope not.

[vol.2]

Originally Posted by ostlar

What am I missing? Could it be an internal short in the flyback? I hope not.

Definitely measure the legs of the flyback, and also across the yoke.

Assuming those are okay, and that you have a low-resistance short, then it should be possible to isolate it by removing components and testing points to ground with those specific components out of circuit.

[Username1]

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Hi! - You may not have a short, but instead you may have a problem related to one
of the hold down circuits. Low voltage, or high voltage hold down circuit. You can
see on the schematic there is a small winding off the flyback that goes to that
5 pin chip in the power supply. I think it's labeled Impulse, or Trigger.
It's a safety circuit, and this problem is made for service with a
Variac. Set it low and turn the set on and see if it fires up.
Theory is the tv does not develop voltage high enough
to trigger the hold down circuit, and you find the
fault with a volt meter.....

Keep in mind that if part of the tv does not start, with no load the B+
May run artificially high....

The other thing you can do is look at the tutorials on YT for this problem
Search Sharp Hold Down Circuit Troubleshooting - or - similar. For some
reason the Horiz Osc. is starting and shutting off. This tv must have
been made before the Jungle IC. that controlled everything, because
the hold down IC is stand alone by the power supply.......

I don't remember ever having to work on a Sharp Hold Down Circuit
before, so best I can offer is to look at the tutorials on YT, and do
plenty of Google searches on the Sharp Hold Down Circuit tutorials
as there seems to be a few on YT, so there may be some on G.

This tv does not have a switching power supply, but does make
some of the supply voltages off the flyback, A short lived
design.

In this video you can see he finds a bad cap, and talks about
the Protection Circuit and appears to have full B+.

So also search Sharp Protection Circuit maybe add your
model number etc.

https://www.youtube.com/watch?v=7CndoU7Qg-w


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And in this video you can see he found a bad resistor near items labeled D604
which is in the Horiz Output area, Same numbering system as your set.
Open resistor may let some voltages run high if in the feedback to
the hold down circuit.

https://www.youtube.com/watch?v=_Y4G1e7AZGY

And again this title of the video is for the hold down circuit.


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Be sure to check the fusable resistors, and high value resistors,
Diodes, and Zener Diodes as well, because a Zener diode can
be used as a trigger.


Hold Down circuits are very tricky.......



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