View Full Version : Apex LD4688T - no power

04-20-2014, 11:31 AM

I rescued this set from the trash and get no power either with remote or pressing manual power button. I would like to try and repair it. Any idea on where to start? I will look for bulging/blown caps, but what do I do if I don't see anything obvious?

Any help is greatly appreciated!:thmbsp:

04-20-2014, 01:59 PM
I am not familiar with that unit. If it's a modern set, it will have a switching power supply. First determine if the power line connects to the input common mode choke, and that the fuses are not blown. Then see if the power line gets rectified, probably an integrated bridge or full wave rectifier diode.

04-20-2014, 02:29 PM
Thanks for the reply. Can you recommend a book or web site that might explain your advice? I am a newbie at this. Do I need an ESR meter?

04-22-2014, 01:21 AM
Does the standby LED light? If so is it flickering or steady?
Does the standby LED change color when you press the power button? If so does it change back if you press the button again (after say 30 seconds, some sets take ages to "boot")?

A typical LCD TV power supply consists of a power factor correction stage (in sets over 26" or so, not usually found in small sets drawing less than 60W), the standby supply (which supplies the always on 5V @ 0.5A or so), the main supply itself (which usually supplies an additional 5V rail, 12V rail and a 24V rail for the backlight inverter/driver) . In some sets the backlight inverter/driver is integrated into the power supply.

Long list of possibilities (all of which I have experienced in various sets) as I may not be around to answer replies for a few days

If the mains fuse is open you will most likely find shorted FETs and/or diodes in the PFC stage. If so check the resistors surrounding them and hope they haven't taken the IC with them - often they blow apart leaving you to guess the guess part number.

If the PFC stage is working you will have rectified mains on the big reservoir capacitor (around 360VDC for us with 240V mains, 180VDC for 120V mains).

The standby power supply will usually be based around an IC with an integrated switching FET. Most of these are DIP-8 (sometimes with a missing pin). Vcc for these ICs is usually 12V-18V derived from a winding on the primary side of the transformer via a diode and a capacitor (often 47uF).

When these smaller capacitors fail they are less likely to visibly bulge. I generally wave the ESR meter over all of them and replace any that are suspect. Without an ESR meter replace it if you're not getting the voltage there.

If the standby power supply is running, but you are getting less than 5V (or whatever it should be, it is usually marked on the board near the connector) this could be the capacitor on the secondary side (sometimes they fail without bulging) or it could be a failure on the mainboard loading it down. The 5V will be regulated down to 3.3V on the mainboard. On the most common 3 terminal regulators the tab is the output, the rightmost pin is the input.

If you have standby 5V and 3.3V at the regulator, but still no standby LED it is probably an EEPROM or Microcontroller fault which often means a board swap.

If the standby LED does light, but does not change color when you press the button check the power on signal. This is a pin on the power supply that is either pulled to gnd or to 5V to turn on the main supply. It can be labelled switch, stby, pon, etc. If it doesn't change state when the power button pressed it is most likely a EEPROM or Microcontroller fault on the mainboard.

A shorted FET on the backlight inverter/driver (common) will stop the 24V from coming up. A shorted audio IC (less common, but happens) will stop the 12V from coming up. If the 12V and 24V share a transformer either will take out the other. These are often on separate connectors so it is easy to isolate them.

If they are not on separate connectors it may be easier to disconnect the mainboard entirely and tie power on line to gnd (or 5V as the case maybe) via a ~220 ohm resistor to force the power supply on for testing.

If the standby LED changes color when the button is pressed, but does not change back when it is pressed a second time it is either not starting up completely or freezing. Check the voltages at the rest of the regulators on the mainboard. Some will likely be 3.3V, 2.5V, 1.8V. If all the voltages are correct this is likely an EEPROM problem.

If the standby LED changes color when the button is pressed and changes back when it is pressed a second time and all voltages are correct then it is likely the set is "working", but you can't see anything because the backlight is off - either because the inverter/driver is bad or because the CCFL/LEDs inside the panel are bad. In some panels they are wired in series - if one fails open none will light.

If this is the case you may be able to see "something" on screen under a bright enough light - easier with some panels than others.

Something that has caught me out before has been child lock. If it on (and often it seems to get put on unintentionally) and the remote dies (say from leaking batteries) it becomes impossible to turn the set on even though there is nothing actually wrong with it. On most cameras the IR from the LED of a working remote shows up as white.

04-22-2014, 09:01 AM
Thanks for your detailed reply.

No standby light, nothing happens with remote power on or manual button.

I'm just trying to figure out if I need to replace caps, power board or main board. Don't want to spend $60 - $100 before narrowing down the problem.

Thanks again!

04-28-2014, 10:19 AM
Have you checked the mains fuse? Does this set have physical power switch ahead of the power supply? Check that you have continuity from the mains plug/connector to the connector on the power supply board.

If the fuse is open check for shorts as mentioned above. If you don't find any shorts you may have gotten lucky and a surge may have just taken out the fuse. Replace the fuse and try it. If the fuse blows again you've missed something. If the fuse holds, but you still have no power look for open resistors, PTCs, etc. Essentially check that you have continuity from mains input, though the filtering to the rectifier.

At this point you could get a replacement power supply board if you didn't want to continue trying to diagnose/repair the existing board.

04-28-2014, 03:10 PM
I did a physical inspection yesterday and did not see any fuses. The board looks intact, no bulging caps, no stains or burn marks on the board. Attached is a picture of the PSU board LK-OP422001A. If I wanted to test for standby power, where would I put in the MM leads? There is no power whatsoever when it's plugged in - no standby light on set front, and nothing happens with remote or power button push on side of set. I ordered a replacement PSU and hope this is the solution to the problem.


04-29-2014, 11:15 AM
I added some annotations to your picture. The fuse is there in the bottom right corner. It's a PCB mount type rather than a usual glass fuse and they like to hide them in heatshrink tubing for some reason.

5V standby will be on one of the pins of the top left connector, but I can't read them in the picture. You can measure it from diodes to the left of the standby transformer. Negative lead to ground, positive lead to the striped side (bottom in the picture) of the diode.

04-29-2014, 12:09 PM
Okay, so if I peel the heat shrink off the fuse, how do I test it? If it's bad, where do I find a replacement?

Also, I assume the standby test on the diodes requires the set to be plugged in so it is getting current? If so, is this safe?

Thanks again!

04-29-2014, 12:48 PM
One more thing -- if you go to this link you can see the PSU with zoom in:

Also, I downloaded the service manual from here for free:

Thanks again, again! :thmbsp:

04-29-2014, 01:04 PM
Disconnect and remove the power supply from the TV. Set your multimeter to continuity/ohms. Probe the pins of the fuse from the bottom of the board. If the meter beeps/reads zero ohms the fuse is okay.

If the fuse is open ciruit you'll need to unsolder it and replace it, but check the various things mentioned in previous posts. Fuses generally blow for a reason and it is likely it will blow again if whatever caused it to blow isn't dealt with.

An electronics components store should have suitable fuses, though I don't know which ones if any are in your area. Of course you could order them online, though minimum order and/or shipping charges could be painful.

To measure the standby voltage it needs to be plugged in. Put the board back in the TV and reconnect the cables. Screw it down so it can't slip off the mounting points and short out against the metalwork. The cold side of the power supply (where you are measuring) is not dangerous. The hot side is dangerous. The large capacitor on the hot side will hold charge even after the power is disconnected.

04-30-2014, 06:05 PM
I measured the diodes as you said and got 5V standby. Does this mean the power supply is okay? If so, is it the mainboard that's bad? How do I test it?

05-01-2014, 08:49 AM
I downloaded the service manual. While it has more detail than I expected it also appears to cover several variants. The power supply in the manual is not the power supply that you have.

Can you verify that the mainboard in the manual is what you have? If not than the following may not apply.

J2 on the main board is the power connector. As you have 5V standby on the power supply you should have 5V on pin 2. If not there is a problem with the power supply. As there is 5V on the diodes you measured the only obvious thing I could think of would a dry joint at the connector.

Pin 1 on the J2 is the power on signal. If there is 5V on this pin the main power supply should be on. For testing you could force the power supply on by temporarily bridging pin 1 and 2 (ideally with 100-220 ohm resistor, but you usually get away with just shorting them). If the 5V, 12V and 24V rails come up the power supply is fine and the problem on the main board.

Next thing to check would be U10 on the main board. This is a regulator that should have 5V on its input and 3.3V on its output.

05-02-2014, 12:53 PM
I looked and the mainboard in the service manual is not what I have either. Attached is my MB:

I expect to receive the PSU today so I'll try installing it and hope for success.


05-03-2014, 12:39 PM
Great news! I replaced the PSU and it now works just fine! I will take a look at the old board fuse to see if that was the problem.


05-04-2014, 06:47 AM
That's good. Though as you had 5V standby the fuse, standby supply and probably PFC stage are all in the clear. That leaves the main supply to diagnose, though as the set is now working it isn't really worth the effort unless it's for your own curiosity. In which case start looking around the main FETs which I highlighted in the picture above.