LG 42PC56 fault

[netdent]

Hi I'm new to the forum, glad I found it, right up my street

Ok I have the above plasma, it switches on green light blinks as normal then stays solid for a few seconds then shuts down, no picture is displayed, but sounds like as soon as the HV kicks in it shuts down, I replaced a few caps on the psu even though none were bloated. any help apreciated

thanks Ronnie

[PlainBill]

Re: LG 42PC56 fault

Hi I'm new to the forum, glad I found it, right up my street

Ok I have the above plasma, it switches on green light blinks as normal then stays solid for a few seconds then shuts down, no picture is displayed, but sounds like as soon as the HV kicks in it shuts down, I replaced a few caps on the psu even though none were bloated. any help apreciated

thanks Ronnie

The service manual is here.

The standard troubleshooting technique is to disconnect the power cable to the Z-sustain (with power off, of course) and try to turn on the TV. If it comes on, the Z-Sustain is probably responsible. Turn the TV power off, reconnect the power cable to the Z-sustain and disconnect the power cable to the Y-sustain. Now try to turn it on. If it turns on, again, the power supply is most likely responsible. A last test is to disconnect both Y-Sustain and Z-Sustain and see if it turns on.

If the TV will turn on only if the Z-Sustain is disconnected, the Z-sustain is almost certainly the problem.

If the TV will turn on only if the Y-Sustain is disconnected, the Y-sustain is almost certainly the problem.

If the TV will turn on only if the both the Y-Sustain and Z-Sustain are disconnected, the power supply is probably the problem.

If the TV refuses to turn on with both Sustains disconnected, the power supply or the main board is the problem.

This chassis has a history of problems caused by a failed hybrid (used on both the Y and Z sustains).

If you require additional assistance, attach (using 'Manage Attachments', below the text entry area) of the TV.

Also, if you add your location to your profile, it allows us to make meaningful suggestions as to where to buy parts.

PlainBill

[netdent]

Re: LG 42PC56 fault

Hi plainBill, thankyou for all the imformation, I did try to add my location etc but would not let my in (do not have relevent privilages) will look again now

I did unplug the larger left sustain board ( I get confused which is z or y!) it did power on ok, my next question would be can these boards be repaired at component level, I am in the UK

many thanks Ronnie

[PlainBill]

Re: LG 42PC56 fault

Hi plainBill, thankyou for all the imformation, I did try to add my location etc but would not let my in (do not have relevent privilages) will look again now

I did unplug the larger left sustain board ( I get confused which is z or y!) it did power on ok, my next question would be can these boards be repaired at component level, I am in the UK

many thanks Ronnie

The left is the Y-sustain. Did you do all the combinations I listed. Recently someone had a similar problem. He only unplugged the Z-sustain, the power supply stayed on, so he ordered a replacement z-sustain. He had the same problem after installing it. Then he discovered the power supply would stay on if EITHER sustain was disconnected - he had a bad power supply.

Yes, the boards can be repaired to the component level. The problem is often the hybrid. Unsoldering it is a major pain in the posterior.

PlainBill

[netdent]

Re: LG 42PC56 fault

The left is the Y-sustain. Did you do all the combinations I listed. Recently someone had a similar problem. He only unplugged the Z-sustain, the power supply stayed on, so he ordered a replacement z-sustain. He had the same problem after installing it. Then he discovered the power supply would stay on if EITHER sustain was disconnected - he had a bad power supply.

Yes, the boards can be repaired to the component level. The problem is often the hybrid. Unsoldering it is a major pain in the posterior.

PlainBill

Thanks plainBill, sorry I gave wrong information! I'm working on 2 LG screens both have the same fault, ok this one the power supply shutsdown whatever combination of y or Z is connected so guessing power supply fault, have changed a number of capacitors already

thanks Ronnie

[netdent]

Re: LG 42PC56 fault

Ok an update, been poking around the psu, found fuse F851 open, replaced it, no change fuse again open, checked some diodes in the area then 2 mosfettes near the fuse Q851 and Q852 marked W20NK50Z these seemed to read almost short allways, 1 to 2 ohms, I removed them from the pcb they both still reading short in all directions, thought this was wrong? but would 2 of them go short? maybe this is what they should read, stuck now so any advice PlainBill or anyone then can help

thanks Ronnie

[retiredcaps]

Re: LG 42PC56 fault

2 mosfettes near the fuse Q851 and Q852 marked W20NK50Z these seemed to read almost short allways, 1 to 2 ohms, I removed them from the pcb they both still reading short in all directions,

If they read 1-2 ohms out of circuit, they are definitely bad and need to be replaced.

[netdent]

Re: LG 42PC56 fault

If they read 1-2 ohms out of circuit, they are definitely bad and need to be replaced.

Thankyou for confirming this, was what I thought, will order some, farnel and digi key seem to have them but described as STW20NK50Z 500 volt N Mosfette

thanks Ronnie

[netdent]

Re: LG 42PC56 fault

Mosfets on order, will report back, fingers crossed

Ronnie

[netdent]

Re: LG 42PC56 fault

Ok mosfets fitted today power supply still shutting down! the fuse is now remaining intact, I'm now stuck, psu does not power long enough to read any voltages

[netdent]

Re: LG 42PC56 fault

Some pictures of the offending psu, the flying capacitor has extended legs as the same value I had to hand were fatter!!

[Colin21958]

Re: LG 42PC56 fault

Hi,

I have exactly the same problem described here. Switch it on, green light flashes 5 times, stays solid for a couple of seconds then goes red. In the process you hear a loud click from the set somewhere. I have already replaced the 7 main capacitors on the PSU, even though I couldn't see any sign of bulging or leaking and therefore had little hope that this could resolve the problem. I have also followed the troubleshooting technique described on this page of disconnecting the X and Y sustain cards in the orders stated. The set still fails to turn on with either or both of these disconnected, so according to your explanation here it points at either the power supply or the main board? Is there a test for each of these boards to identify which one is causing the problem and are they repairable? Thanks.

Colin

[PlainBill]

Re: LG 42PC56 fault

Dang!!! Two in one day. The problem is PROBABLY the power supply. There are three ways to verify this

1. Buy a replacement power supply. This causes unhappiness if it's not the problem.

2. This works best with a helper. Check each output voltage of the power supply in the few seconds it is on. If any are missing, it's probably the power supply.

3. Rig a 3 volt source using Radio Shack 270-408 or an equivalent battery holder. Disconnect all outputs of the power supply. Locate the control line from the main board to the power supply. It may be labeled PS_ON, or something similar. Attach the black lead of the battery to ground.

4. Connect the power supply to line voltage. Connect the 3volt power source from chassis ground to the PS_ON pin. The power supply should turn on. Measure the output voltages.

PlainBill

[Colin21958]

Re: LG 42PC56 fault

Hi PlainBill,

I did measure all the voltage outputs (in the few seconds it was on each time) from the PSU board and they all matched the values stated on the board beside each pin; there were none missing or outside of spec. So it would appear that the PSU is OK. (I did replace the main capacitors already in the hope that this would cure the problem but it didn't; to be honest I didn't really expect it to because they all looked fine.). So, it is beginning to look like the main board? Is there anything else I should check before replacing that? Is there any test I can do to verifiy that the main board is the problem? Thanks,

Regards,

Colin.

[PlainBill]

Re: LG 42PC56 fault

Hi PlainBill,

I did measure all the voltage outputs (in the few seconds it was on each time) from the PSU board and they all matched the values stated on the board beside each pin; there were none missing or outside of spec. So it would appear that the PSU is OK. (I did replace the main capacitors already in the hope that this would cure the problem but it didn't; to be honest I didn't really expect it to because they all looked fine.). So, it is beginning to look like the main board? Is there anything else I should check before replacing that? Is there any test I can do to verifiy that the main board is the problem? Thanks,

Regards,

Colin.

I always come back to the same theme. Without pictures showing exactly what you have, any advice must be of a general nature.

In some TVs a defective tcon will cause problems. If disconnecting that results in the TV staying on, it should be replaced. Likewise, there is a line of master-slave inverters that is notorious for causing problems.

The main board probably contains a number of LDO (Low Drop Out) analog regulators. Verify the output of those. Check for open fuses. Of course, any smoke stains indicate a problem.

PlainBill

[Russ300]

Re: LG 42PC56 fault

Hi guys, hope its ok to re-ignite an old thread ( seemed smarter than opening a new on)

I have a problem with one of the LG 42PC56, as i far as i can remember it went like this, I take the tv off a friend with a whole load of spare boards from an identical set with a supoosedley smashed screen, he had swapped out the PSU, z sus and Y sus, the Y sus did make a difference, on inspection the Y sus had a b,lown fuse and the hybrid chip is shorted out, now i put the original Y sus in and changed the Main signal board, the set then powered up and shut off, i tried again and now it stays powered up but no image, seemed like a lot of noise the first time.

After consulting the service manual page 23, it says to test the high and low volatges - are these Va and Vs ?

[steves7839]

Re: LG 42PC56 fault

Safe to assume that, based on my own limited experience. No Vs, no backlight.