Sharp LC-42SB45U Powers on at Random Intervals after Cap replacement

[pilotguy251]

I have an old Sharp LC-42SB45U set that went out in late April of 2019. In reading other threads here on the forum about this specific set, I decided to go ahead and replace a majority of the capacitors on both the PSU and Control boards. When plugged in, the red standby light does not come on, however, the set will power on randomly after a few hours, and stay on until it times out if not powered down intentionally.

Following the time out, the set will not power back on unless it is unplugged for a few hours, then plugged back up, only powering on when it gets ready, and not responding to the power button. If the unit is allowed to time out, then the red standby light comes on, however, if it is powered down using the power button on the side or on the remote, the light goes out completely with no further response from the set. Seems as if it has to go through some sort of reset period here.

Right now, the red standby light is on because the unit powered on at about 2 am this morning, 7.4.19, and was allowed to time out overnight, but if I were to go and press the power button, the standby light would go off without the unit powering up. However, I have noticed that when I attempt to power the unit up with a red standby light, the capacitors are warm to the touch after a few minutes as if the set wants to power on, but nothing happens.

I replaced all but the large 450V capacitor on the PSU board (excluding the turquoise ones) and all but two of the green 470uf 16v CapXon capacitors on the control board (again, excluding the turquoise components). Are the turquoise components capacitors as well? Neither of the capacitors I replaced was bulged or leaking, although the ones removed were tested using the continuity function on a multimeter and a few of them didn't conduct. The two remaining green CapXon caps are warm when the power button is pressed following a red standby light, along with the majority of the ones I replaced, so it seems as if they still conduct fine.

Attached are pictures of both boards and a picture of the screen when the unit does decide to power up. That photo does the screen no justice, but the picture is much clearer after replacing the capacitors I did.

Since I have determined that the set still works and displays a nice picture, I would like to see if I can fix whatever the issue may be and squeeze as many more hours out of it as possible. I'm probably looking over something simple.
Referenced threads:
https://www.badcaps.net/forum/showthread.php?t=18894

https://www.badcaps.net/forum/showthread.php?t=44996

[yohnsee]

Re: Sharp LC-42SB45U Powers on at Random Intervals after Cap replacement

Hello,
the turquoise components are also capacitors. This question leads me to ask you, have you checked your work, I mean, haven't you soldered any caps backwards? Every new cap has the uF value the same as the old one? Are there any shorts where you soldered? No offense, just asking.
Second, you should check the voltages when the set is in standby mode (or should be), and when powered on. Good start would be with CN902. Report back the voltages, then we can move on.

[pilotguy251]

Re: Sharp LC-42SB45U Powers on at Random Intervals after Cap replacement

Thanks for your response. First time doing board work, but the ones I replaced seem to be on snugly and have a good connection with no shorting. uF values are the same as the old ones on each new cap. I'll have to wait until the set decides to come up again to do the voltage test. Hopefully that won't be too long, but I will report back at that time.

[pilotguy251]

Re: Sharp LC-42SB45U Powers on at Random Intervals after Cap replacement

OK, the set eventually came on, but I was out of the house when that happened. Did catch it in standby mode though. The only two leads reading something while the set is in standby are the "inverter on/off" and "bright control". With the multimeter set to the 20V setting, "inverter on/off" reads 0.14 while "bright control" reads 0.12. Could be possible that I wasn't making a good connection with the other leads, but I did them all the same way, so that shouldn't be an issue.

[pilotguy251]

Re: Sharp LC-42SB45U Powers on at Random Intervals after Cap replacement

Hello,
the turquoise components are also capacitors. This question leads me to ask you, have you checked your work, I mean, haven't you soldered any caps backwards? Every new cap has the uF value the same as the old one? Are there any shorts where you soldered? No offense, just asking.
Second, you should check the voltages when the set is in standby mode (or should be), and when powered on. Good start would be with CN902. Report back the voltages, then we can move on.

Finally caught the set while it was on and tested the leads on CN 902. Inverter on/off and bright control both read 3.6V. The two 12V leads did read 12V. The two 24V leads did read 24V. Nothing read on any of the grounds. Power on/off read 1.6V, and 5V on the bottom 5V lead. I will allow the set to go into standby and test these leads then, as I mistakenly tested CN 905 the first time in standby.

[R_J]

Re: Sharp LC-42SB45U Powers on at Random Intervals after Cap replacement

The one voltage that always needs to be there is standby 5 volts, (CN902, pins 11,12) if it is missing, nothing else works
You can easily check it on R943 (the lead near cn902)

[SLK001]

Re: Sharp LC-42SB45U Powers on at Random Intervals after Cap replacement

The only two leads reading something while the set is in standby are the "inverter on/off" and "bright control"...

If there is no voltage around +5V on the STANDBY pin, then you aren't powering the necessary standby components, like the side controls and the IR receiver. Concentrate your efforts on the standby area of the PS. Double check your work there.

Of the three transformers on your PS, the STANDBY XFORMR is the smallest one.

[pilotguy251]

Re: Sharp LC-42SB45U Powers on at Random Intervals after Cap replacement

Thanks for the replies. When the set comes on and times out to Standby mode as mentioned above, there is a steady 4.45 volts on the 5V leads of the PS. I disconnected the mainboard from the PS as suggested in another thread and it doesn't seem that the mainboard is dragging down the power supply, so I will study the service manual diagrams and proceed from there. Thanks again.

[nomoresonys]

Re: Sharp LC-42SB45U Powers on at Random Intervals after Cap replacement

Kind of seems like a mainboard issue, possibly eeprom? Mainboard there for 37 bucks, if you get it make sure you triple check to make sure it's a match: https://www.ebay.com/itm/SHARP-Main-...0AAOSw-otdDHNb

[nomoresonys]

Re: Sharp LC-42SB45U Powers on at Random Intervals after Cap replacement

When it times out try heating the mainboard with a hairdryer, small section at a time be sure to hit all the capacitors, see if it comes back on.

[nomoresonys]

Re: Sharp LC-42SB45U Powers on at Random Intervals after Cap replacement

Maybe try freezing eeprom U708 with freeze spray or upside down duster, can also try some heat on it, see if tv acts any differently.

[pilotguy251]

Re: Sharp LC-42SB45U Powers on at Random Intervals after Cap replacement

Thanks. I'll look into this as well.