Sharp LC-32S7E dead power supply (no STBY).

[Dannyx]

Good day folks. Got this Sharp LC-32S7E TV which is pissing me off: can't possibly figure out what's wrong with it, although I have some ideas. I've attached the schematic for the power supply, a 715g3467, although for some reason it doesn't include the PFC circuit, even though the board has it. When I got it, IC931 was blown apart (to the right of the schematic - you can also use CTRL-F). It's a TNY277 according to the schematic, but I decided to put in a TNY280 just to increase the maximum current a bit. I'm not sure if it was the right thing to do though, because the board still doesn't work: I get nothing on the secondary. Have a look at the datasheet for those TNY chips, perhaps there's some major difference between 277 and 280 which prevents it from starting up that I'm not seeing. One theory of mine is that transformer T901 has gone open circuit: I noticed that I get no continuity across pins 1-2 of transformer T901. USUALLY, on other supplies, the resistance of such a primary winding is low enough to cause my meter to beep, but here I get nothing, which of course doesn't necessarily mean the transformer is bad. If you look closely at the schematic, there's no protection for IC931 - the S pins are tied directly to GND, meaning that when that chip shorted D-S and blew up, it might've taken out the transformer with it, since the 2 mains fuses were also OK when I got it, which only makes me wonder exactly what happened. I get around 316v on the main cap, but nothing on the AUX winding after D981. Interestingly, if I lift the cathode leg of the D981 from the board, I do see 11-12v on it. Solder it back in and I get nothing, so I'm still thinking perhaps the IC itself is not the correct one....I HATE PSUs that think they're being cutting edge by employing all-in-one sh*t chips like these- just have some damn FETs that blow and can be replaced, not this 1-piece bullshit that never seems to want to work...damn....Anyway, enough of my rants. Has anyone repaired this type of Sharp before ? Cheers.

[budm]

Re: Sharp LC-32S7E dead power supply (no STBY).

I do not see the schematic. What DCV do you have on the Drain pin of the TNY SMPS IC? The OPTO for the feedback may be damaged also when the IC blew up.
"just have some damn FETs that blow and can be replaced," but 99% when MOSFET shorted out with Drain to Gate, it will take out the SMPS IC with too, and if you take a chance by not replacing the IC then you may take out the new MOSFET due to failed SMPS IC.

[CapLeaker]

Re: Sharp LC-32S7E dead power supply (no STBY).

I do not see the schematic. What DCV do you have on the Drain pin of the TNY SMPS IC?

x2

[Dannyx]

Re: Sharp LC-32S7E dead power supply (no STBY).

Pff...yeah, epic fail. Forgot to upload the schematic too...overworked....anyway. As far as I remember, I get 0v on the D pin (with respect to my negative probe on primary GND). The opto IC doesn't appear to be shorted (i.e, I only get a voltage reading in one direction with my meter in diode mode).

[budm]

Re: Sharp LC-32S7E dead power supply (no STBY).

The receiver side (Photo Transistor) should not show any conduction, that Transistor will conduct when LED sender side is ON.
If the Drain pin shows 0V then that confirm there is no continuity on the primary winding (open circuit) of the transformer.
Q931 could also be damaged also since it is in parallel with the OPTO.

[CapLeaker]

Re: Sharp LC-32S7E dead power supply (no STBY).

that sux... maybe there is a thermal fuse inside the transformer?

[Dannyx]

Re: Sharp LC-32S7E dead power supply (no STBY).

that sux... maybe there is a thermal fuse inside the transformer?

Doubt it - it's too small IMO to have a fuse. Ok, looks like it's hack time I was thinking of getting rid of the whole STBY circuit and feeding 20v from another supply where the voltage coming off D981 is supposed to go. If you look closely, you'll see a VCC_PI bus which powers the PFC IC one (not depicted in the schematic) and IC951 (towards the bottom left). As far as I can tell, those two ICs are the only ones the supply needs in order to power up the other two rails (12v and 24v for the inverter)....sounds crazy enough to work

[dick_barton]

Re: Sharp LC-32S7E dead power supply (no STBY).

Puzzled. You said you lifted one end of the diode and you have a voltage. If this is the case then the transformer primary must be OK. If it was open circuit there would be no conduction Pins 1 - 2 so you would not have any voltage induced in the winding between pins 4 & 5
Are you certain it's the same schematic for the power supply or is the transformer wired up differently?

[Dannyx]

Re: Sharp LC-32S7E dead power supply (no STBY).

Puzzled. You said you lifted one end of the diode and you have a voltage. If this is the case then the transformer primary must be OK. If it was open circuit there would be no conduction Pins 1 - 2 so you would not have any voltage induced in the winding between pins 4 & 5
Are you certain it's the same schematic for the power supply or is the transformer wired up differently?

I know ! Stuff like this is what ticks me off the most also >_> I'll try swapping the IC once more, this time using the exact one the schematic calls for. If you look towards the bottom of the datasheet where all the numbers are, some of the values differ between the different models of TNY chips so perhaps they're not entirely interchangeable and upgradable like I had in mind.

[budm]

Re: Sharp LC-32S7E dead power supply (no STBY).

" if I lift the cathode leg of the D981 from the board, I do see 11-12v on it." So you are seeing the Voltage at the Cathode of the diode which is lifted off the board, correct? or you are seeing the Voltage at the pad where the Cathode WAS connected to?

[Dannyx]

Re: Sharp LC-32S7E dead power supply (no STBY).

" if I lift the cathode leg of the D981 from the board, I do see 11-12v on it." So you are seeing the Voltage at the Cathode of the diode which is lifted off the board, correct? or you are seeing the Voltage at the pad where the Cathode WAS connected to?

At the cathode - after the diode that is.

[budm]

Re: Sharp LC-32S7E dead power supply (no STBY).

At the cathode - after the diode that is.

Well, that made no sense period if the Transformer has open primary circuit and you also verified that there is no Voltage at all on the Drain of the IC. When something does not make any logical sense then it has to be an error in measurement some where.
And for sure you also DO NOT see any Voltage at all on the Output side?

[Dannyx]

Re: Sharp LC-32S7E dead power supply (no STBY).

Well, that made no sense period if the Transformer has open primary circuit and you also verified that there is no Voltage at all on the Drain of the IC. When something does not make any logical sense then it has to be an error in measurement some where.
And for sure you also DO NOT see any Voltage at all on the Output side?

No output voltage at all and no voltage present at D (though of course I will recheck tomorrow). Yes, having an open primary would make it impossible for the AUX winding to put out anything, since it relies on the field generated by the primary to operate - correct me if I'm wrong (that's how I interpret the schematic at least). If that's not it, then it's probably the IC itself not matching the original specs. I also find it strange how the optoisolator pulls DOWN to GND that ENABLE pin of the TNY chip instead of UP.

[dick_barton]

Re: Sharp LC-32S7E dead power supply (no STBY).

Can you check with your meter that you have continuity between the Drain pin and transformer Pin 2

[skynetronics]

Re: Sharp LC-32S7E dead power supply (no STBY).

Hi Dannyx, first post here.

First of all, forgive my bad english to the forum. I wish to add that this power supplies have much fails on this transformers.

In my experience i had a 715T3150-3 from a Sharp LC-42SB45UT with similar failed components (TNY277PN) and the stand-by chopper transformer T904, P/N: 060TL42T916L.

Symptom: Dead supply, even stand-by voltage. Just having 310v on the main capacitor.

Visual obs: IC903 dead (exploded as you can see).

Procedure: IC903 replaced, and for pracautions primary recaped. After plug in, make no differences.

In a depth observation, i saw that T904 has open circuit (see the red line in the schematic).

From this diagnose, i decided replace the complete board, and now works great.

I hope that this can guide your diagnose and solve your problem.

Greetings from Chile.

[Dannyx]

Re: Sharp LC-32S7E dead power supply (no STBY).

Hi Dannyx, first post here.

First of all, forgive my bad english to the forum. I wish to add that this power supplies have much fails on this transformers.

In my experience i had a 715T3150-3 from a Sharp LC-42SB45UT with similar failed components (TNY277PN) and the stand-by chopper transformer T904, P/N: 060TL42T916L.

Symptom: Dead supply, even stand-by voltage. Just having 310v on the main capacitor.

Visual obs: IC903 dead (exploded as you can see).

Procedure: IC903 replaced, and for pracautions primary recaped. After plug in, make no differences.

In a depth observation, i saw that T904 has open circuit (see the red line in the schematic).

From this diagnose, i decided replace the complete board, and now works great.

I hope that this can guide your diagnose and solve your problem.

Greetings from Chile.

That's exactly what I'm getting as well (though the schematic is ever so slightly different from yours - mine is T901 instead of T904, but the circuit is identical). I presume you no longer have the functional board on hand to measure pins 1-2 of T904/1 to see if you're indeed getting continuity, or perhaps you remember taking such measurement and telling me the results...guess I'll have to swap the board also. That's what you call bad design, when the power IC takes out the transformer, one of the most crucial components to any SMPS since it's not like you can run to your radioshack and get a bunch of these - once it's gone, the whole board goes in the trash.....dumbasses.....:|

[skynetronics]

Re: Sharp LC-32S7E dead power supply (no STBY).

That's exactly what I'm getting as well (though the schematic is ever so slightly different from yours - mine is T901 instead of T904, but the circuit is identical). I presume you no longer have the functional board on hand to measure pins 1-2 of T904/1 to see if you're indeed getting continuity, or perhaps you remember taking such measurement and telling me the results...guess I'll have to swap the board also. That's what you call bad design, when the power IC takes out the transformer, one of the most crucial components to any SMPS since it's not like you can run to your radioshack and get a bunch of these - once it's gone, the whole board goes in the trash.....dumbasses.....:|

Yep, i remember the measurement. I had no continuity between 1-2, but between 1-3, was ok (on bad transformer). On the good one, i remember that have continuity between 1-2, and 1-3.

[Dannyx]

Re: Sharp LC-32S7E dead power supply (no STBY).

Yep, i remember the measurement. I had no continuity between 1-2, but between 1-3, was ok. On the schematic that i attached, the red line represents the open of the coil on that chopper.

Yes, that I understand, but I was asking about the functional replacement board. Did you measure that one as well and indeed got continuity between 1 and 2 as well ?

[skynetronics]

Re: Sharp LC-32S7E dead power supply (no STBY).

Yes, that I understand, but I was asking about the functional replacement board. Did you measure that one as well and indeed got continuity between 1 and 2 as well ?

Yes, on the functional replacement board i remember that have continuity on that chopper (on 1-2, and 1-3).

[Dannyx]

Re: Sharp LC-32S7E dead power supply (no STBY).

Yes, on the functional replacement board i remember that have continuity on that chopper (on 1-2, and 1-3).

Yeah, that pretty much settles it - it's going in the bin....thanks for the info though - would've probably wasted an awful lot of time and possibly money on this thing.