Sharp Aquos lc-37ra1e random power cycles

[Dannyx]

Good day folks, I got this here TV which randomly power-cycles. It turns on alright, but it turns off after a random interval, then comes back on...the relay clicks an uneven number of times during this cycle. After it's come on, it may work for 5 minutes or it may just cycle again. I think the green LED that's supposed to come on when it's working normally is dead, because I can't see it turning green - it only goes red when it's off and that's it, so I don't know if it flashes any error code using the green LED.

Honestly, I don't even know where to begin with this one and the lack of an ESR meter further accentuates the issue. To be fair, I think it's the main board that's causing this, but I started with the power supply anyway. Couldn't find a good schematic, but I struggled to follow the one I uploaded. I replaced the 3 2200uF caps on the 5v bus (C752, C756 and C768 on page 14 after the diode after the transformer) just for good measure and also two little 47uF ones (C723 and C735 on page 13) since they've sat near those heatsinks this whole time, but the problem's the same: can't isolate the fault no matter what. I get 5v, 13.6v and 35v alright....stumped.

I've also added a photo of the main board, though I don't think that's the actual "main board" which has the brains on it, since all I see are the scart and RCA connectors on it. There's another board right above it and I think THAT is the actual CPU and whatnot, but it's just a metal plate so no real point in taking a picture of that. The only reason I'm showing it is because I see some caps there too....probably something for the audio amp, but who knows...maybe someone knows something I don't :| Has anyone repaired this model ? Not too much information out there either...Cheers

[Dannyx]

Re: Sharp Aquos lc-37ra1e random power cycles

By some freak accident I managed to find an uncut schematic here.

[dick_barton]

Re: Sharp Aquos lc-37ra1e random power cycles

Are there any poor solder joints on the power supply or mainboard? Are the two fans freely spinning?

[Dannyx]

Re: Sharp Aquos lc-37ra1e random power cycles

Are there any poor solder joints on the power supply or mainboard? Are the two fans freely spinning?

Oh yeah, I forgot to mention: the schematics show some fan connectors as well, but mine doesn't have them or the fans - the PSU just fits different models. Solder joints: I've already reflown (hope it's spelled right ) a few pads which seemed slightly dry, but the problem's the same sadly What I'm thinking of doing next is pulling out the PSU again, turn it on on the bench and loading it with some lamps and let it "suffer" like that for as long as possible to see if it crashes. I HIGHLY doubt it will, which would confirm that the main board is faulty and it's randomly removing the "PSU_ON" signal...

[Dannyx]

Re: Sharp Aquos lc-37ra1e random power cycles

I made another discovery: the 30v line is running too low (even under no load). It only reads around 23v which I don't think is appropriate. It comes out at PL703 on the right side of the schematic. Haven't yet replaced the caps on that bus...perhaps it's a simple issue after all.

[Moreno83]

Re: Sharp Aquos lc-37ra1e random power cycles

Could be indeed some bad caps , maybe backlight asking to much from the PS?

[Dannyx]

Re: Sharp Aquos lc-37ra1e random power cycles

Could be indeed some bad caps , maybe backlight asking to much from the PS?

Test was done with PSU on the bench and nothing connected, so there was no power draw. The 13.5v bus is also a bit low: only 12v and drops to 11 something when I load it with a lightbulb. Replaced caps.

[Dannyx]

Re: Sharp Aquos lc-37ra1e random power cycles

Another thing I noticed, which didn't make sense at first is those little numbers next to some of the pins of some components. Apparently they indicate what value that pin should read, as many of them match my reading, except for the one at shunt regulator IC705: pin 3 of that should read 4.4v, but mine reads around 8v (almost double)...the resistors around it read fine...could it be a bad optocoupler ? Funny thing is that the values at the optocoupler also match the ones in the schematic....

[dick_barton]

Re: Sharp Aquos lc-37ra1e random power cycles

Another thing I noticed, which didn't make sense at first is those little numbers next to some of the pins of some components. Apparently they indicate what value that pin should read, as many of them match my reading, except for the one at shunt regulator IC705: pin 3 of that should read 4.4v, but mine reads around 8v (almost double)...the resistors around it read fine...could it be a bad optocoupler ? Funny thing is that the values at the optocoupler also match the ones in the schematic....

IC705 (MR4020) pin3 is labelled GND = 0V?

[Dannyx]

Re: Sharp Aquos lc-37ra1e random power cycles

I think I managed to fixed it, more through freak luck than anything else: I replaced those two small caps at the upper-left hand side of the PFC coil (right above where it says "PRI" on the circuit bord) and the TV hasn't power-cycled anymore up until now. Will have to let it run for a full day before I can return it, but I think that was the issue - it gets VERY hot in there, but funnily enough, if this is indeed what was causing the reboots, those caps are out in the open and not next to any hot surfaces, so it's interesting they failed but none of the others did. Can't remember exactly what those caps are for...they followed on the primary side somewhere after the optocouplers (the transistor side of the optocoupler) and also did something with those TXA chips which nobody's heard about and there's no datasheet for them, so I don't know what they are. So anyway, if someone's having the same issue, consider replacing those two caps. Cheers.