Sharp LC-32sb28ut Power Supply has Pulsing DC Voltages

[Paul678]

Ok, when plugged in, there is the normal red LED
on the front, but when you try to turn it on, the Blue
LED comes on, but starts blinking at about a 1Hz rate.
And then, you cannot turn the unit back to the red LED,
as it becomes stuck in the blinking Blue condition, and there
is totally black screen, and no sound. Nothing.

So I checked the voltages, and even with the inverter
disconnected, ALL the DC voltages will pulse IN SYNCH with
the Blue LED, at the 1Hz rate. The +24V will dip to about
+20V, and then back to +24V. The +5V will dip to about
+4.1V. The +12V will dip to about +9V or so.

Then I checked the +5V when I first plug it in (Red LED standby
mode), and it was at +4.1V. So it only goes up to +5V when
you attempt to turn it on, but it doesn't stay there solidly like it should.

This picture is exactly like mine:

http://www.google.com/imgres?um=1&hl...9,r:1,s:0,i:77


I used my Russian ESP cap meter, and all the electrolytics appear
normal. I'm about to check the diodes, but if someone has
a suggestion, I'm all ears......

[budm]

Re: Sharp LC-32sb28ut Power Supply has Pulsing DC Voltages

Did you replace that start up cap for the standby power supply, it is next to the transformer in the hot side of the board close to D981 and the 4-pin Optoisolator.
How about the DC voltage on the main DC filter caps, do you get steady 165V if your AC is 120VAC, and when TV is on, it should then kick up to about 390~400VDC due to PFC Voltage booster kicks in.

[Paul678]

Re: Sharp LC-32sb28ut Power Supply has Pulsing DC Voltages

Did you replace that start up cap for the standby power supply, it is next to the transformer in the hot side of the board close to D981 and the 4-pin Optoisolator.
How about the DC voltage on the main DC filter caps, do you get steady 165V if your AC is 120VAC, and when TV is on, it should then kick up to about 390~400VDC due to PFC Voltage booster kicks in.

If you are talking about C981, 33uF 50V, then that was
a cap that I tested as good already, so I didn't try to replace it.

Ok, the schematic link I gave earlier is NOT the exact same
as this board. It's close, but no cigar, and I couldn't find
a schematic for this board so far: 715G3905-P1B-H20-003U.

You meant for me to measure across the big 100uF filter cap, right?

It was a steady 168VDC in standby mode, and
295-395VDC when turned on, pulsing with the Blue LED
just like the DC voltages are.

[budm]

Re: Sharp LC-32sb28ut Power Supply has Pulsing DC Voltages

295-395VDC, that is big pulsing, this voltage should be quite steady, so it is either the Main DC filter cap is not good or the PFC circuit is not stable.
The standby power supply is always on and it will be running off the 165VDC when the TV is not turn on, at this condition, is the 5V steady? When the 165v get kick up to 390VDC, this voltage will also be used for the standby and for running the 12v/24v POWER SUPPLY section.
How is the ESR and the capacitance reading of that 450VDC cap?

[Paul678]

Re: Sharp LC-32sb28ut Power Supply has Pulsing DC Voltages

295-395VDC, that is big pulsing, this voltage should be quite steady, so it is either the Main DC filter cap is not good or the PFC circuit is not stable.
The standby power supply is always on and it will be running off the 165VDC when the TV is not turn on, at this condition, is the 5V steady? When the 165v get kick up to 390VDC, this voltage will also be used for the standby and for running the 12v/24v POWER SUPPLY section.
How is the ESR and the capacitance reading of that 450VDC cap?

KIDS REMEMBER TO ALWAYS DISCHARGE CAPS BEFORE USING
YOUR ESR/CAP METER!!!

Ok, that cap was about 0.48 Ohms ESR, with 110 uF, so it looks ok.

As I said before, the +5V is at about +4.1V in the standby mode,
and it only goes up to +5V when you try to turn it on (Blue blinking
LED), but it doesn't stay at +5V solidly, but pulses.

[tom66]

Re: Sharp LC-32sb28ut Power Supply has Pulsing DC Voltages

There's two other caps on this power supply. One of these will be for the PFC and is likely the problem.

BTW, very disappointed at this quality for Sharp -- is it just a rebadged model?

[Paul678]

Re: Sharp LC-32sb28ut Power Supply has Pulsing DC Voltages

There's two other caps on this power supply. One of these will be for the PFC and is likely the problem.

BTW, very disappointed at this quality for Sharp -- is it just a rebadged model?

Unfortunately, I've tested all the electrolytics, and they
all look good. Unless one of the smaller, non-polarized
caps is the culprit? It makes sense that it's a filter cap
though, because if you remember, I had a Proview NS-32LCD
that had a similar problem, and it was a small electrolytic on the
primary side that was the problem. It doesn't help that I don't have a proper schematic.

All the diodes, top and bottom, appear good as well.

Another funny thing: The +5V standby voltage was not
there when I disconnected the main board.....not sure if
this is normal....

Not sure if this is a shoddy model re-packaged by Sharp.....

[tom66]

Re: Sharp LC-32sb28ut Power Supply has Pulsing DC Voltages

It is a re-packaged TV, possibly Unihan OEM.

Did you test just ESR or capacitance? These caps usually fail due to leakage current. They are cheap enough to warrant complete replacement.

[Paul678]

Re: Sharp LC-32sb28ut Power Supply has Pulsing DC Voltages

It is a re-packaged TV, possibly Unihan OEM.

Did you test just ESR or capacitance? These caps usually fail due to leakage current. They are cheap enough to warrant complete replacement.

I tested both ESR and capacitance with my russian ESR
Micro v4.0s. All values appear normal.

You think I should just replace them anyways?

[tom66]

Re: Sharp LC-32sb28ut Power Supply has Pulsing DC Voltages

They are so cheap, it is worth a shot anyway.

[Paul678]

Re: Sharp LC-32sb28ut Power Supply has Pulsing DC Voltages

They are so cheap, it is worth a shot anyway.

Ok, I replaced C981, 33uF 50V, and the other two
smaller electrolytics on the primary side, and the
pulsing problem is still the same.

I didn't replace the huge 100uF 450V cap, but
it did measure just fine.

Maybe I should start looking at the smaller, non-polarized
caps? They are pretty small values, like 0.22uF, so
I'd had to take them out of circuit, and measure them
with my other cap meter.

This is starting to look like the needle in the haystack
syndrome, mainly because I don't have a proper schematic.

The $50 PS replacement on Ebay is looking better by the
hour.......especially since I got this unit for $10!
A $60 total investment, and I should be able to get
at least $120 for this unit working, and I wouldn't
be pulling anymore of my hair out!


haha!

[budm]

Re: Sharp LC-32sb28ut Power Supply has Pulsing DC Voltages

"295-395VDC" That is 100V swing which I can see why the 5V or other DC output is not stable when TV is on, the PFC is not stable.

[Paul678]

Re: Sharp LC-32sb28ut Power Supply has Pulsing DC Voltages

"295-395VDC" That is 100V swing which I can see why the 5V or other DC output is not stable when TV is on, the PFC is not stable.

Ok, I found the PFC chip. It's IC901, a FAN7529MX:




So it turns out this older or newer schematic is not completely
wrong (notice the big filter cap C907 in upper right). I'm positive it's the correct IC, because the reference designators for the surrounding
resistors are the same. Switching MOSFET Q901 does not appear to be blown, and the surrounding resistors measure good.

But here's the kicker: IC901 on the actual board appears to be
slightly melted, enough to make the part number
difficult to read! You know how ICs have slightly rough plastic
texture on their bodies? Well, this one is ever so slightly shiny
for some reason...hopefully heat? Anyways, the part # is
difficult to read, but looks something like "PA7AA 961", which
I was not able to find on the web, but I'm willing to bet a
FAN7529MX from EBAY would be a drop in replacement:

http://www.ebay.com/itm/1pcs-FAN7529...item27c9f2d896


Sounds like I should order this $4 part first, before I give up
and buy the $50 full power supply, right?

What do you all expect the Vcc output on pin 8 should
measure? I just measured it, and it went from about +13V
to zero Volts, pulsing with the Blue LED.....

[budm]

Re: Sharp LC-32sb28ut Power Supply has Pulsing DC Voltages


Per spec the typical start voltage is 12V, SHUTDOWN @8.5V, so you are OK at 13V.
The VCC for this IC is generated by the AUX winding of the standby power supply transformer.
That C907 is the typical connection for the PFC power supply setup.

[Paul678]

Re: Sharp LC-32sb28ut Power Supply has Pulsing DC Voltages

Per spec the typical start voltage is 12V, SHUTDOWN @8.5V, so you are OK at 13V.
The VCC for this IC is generated by the AUX winding of the standby power supply transformer.
That C907 is the typical connection for the PFC power supply setup.

Do you think this chip
is the likely culprit in this case?

[budm]

Re: Sharp LC-32sb28ut Power Supply has Pulsing DC Voltages

Is the 13V steady? And if the voltage is swinging at 100V range, I would expect to hear some sort of noise coming from the power supply. I would also try parallel another cap with the main cap and see if the swinging stop. You can try new IC also but make sure is the same as original.

[Paul678]

Re: Sharp LC-32sb28ut Power Supply has Pulsing DC Voltages

Is the 13V steady? And if the voltage is swinging at 100V range, I would expect to hear some sort of noise coming from the power supply. I would also try parallel another cap with the main cap and see if the swinging stop. You can try new IC also but make sure is the same as original.

The +13V is not steady, but swings to 0 V, in time with the
blue LED.

I just paralleled another similar cap to the main big cap, and
although the voltage appears to swing less, it still swings,
and the output voltages still pulse.

You are correct about the noise. The last time I had this
problem, the PS was making a loud clicking noise, in time
with the pulses, but for some reason, this unit is pretty
silent.

I'm not sure what is up with this hard-to-read part
number, but it looks like the FAN 7529 might work.

I'm gonna see if my local supplier has it.....I hate waiting
for parts from China! haha!

[budm]

Re: Sharp LC-32sb28ut Power Supply has Pulsing DC Voltages

That makes sense since that 13v is generated by the same transformer for the 5V STANDBY, so all the power supply outputs are swinging.

[Paul678]

Re: Sharp LC-32sb28ut Power Supply has Pulsing DC Voltages

Update:

I got another power supply from Ebay, and.....

.....SAME GODDAMNED PROBLEM.......shit!!!

Exactly the same voltage pulsing as the other
power supply. I really doubt that I got two dud
power supplies in a row with the exact same problem!

So it doesn't look like it was a power supply issue
at all.....something in the main board (i disconnected
the inverter). But what in the main board could make the
power supply voltages swing like that, so that even the
big main filter capacitor's voltage is swinging?

I guess I can sell BOTH these power supplies as working,
but with all the time I have spent....I'm about to throw this
TV out the window!!!!

FUCK!!!!!

[budm]

Re: Sharp LC-32sb28ut Power Supply has Pulsing DC Voltages

One of the biggest load for the power supply is the inverter board. What happen if you disconnect the power to the inverter board.