LG 42PC51 no video.

[Trevtron]

Another antique, but well-made looking set.

As title, audio OK, but no picture. I have checked the o/p voltages from the PSU, all seem OK, a couple of them about 1V high.

Which drew me to the LVDS panel (6870QCH007B) which has 5 surf-mount LED's- only 3 of which light up. D1 & D15 are dark.

Any suggestions gratefully received :-)

TIA.

[Hamie]

Re: LG 42PC51 no video.

Probably the modules on the ysus.
Have you check the fuses on the y and z?

[Trevtron]

Re: LG 42PC51 no video.

Probably the modules on the ysus.
Have you check the fuses on the y and z?

Yes, fuses all OK.

Is there a way of checking the modules other than substitution?

Trying to upload pictures but it's not happening. I'll try again later...

[ReeceyBurger123]

Re: LG 42PC51 no video.

Probably failed Zsus Ipm repaired this same set with failed ipm on the Zsus. If you diagnose faulty Zsus also consider replacing the ipm on the ysus. To test the ipm remove the board and check for any shorts to ground like VS to ground it will be labeled on the bottom of the board.

[Trevtron]

Re: LG 42PC51 no video.

OK, I pulled the Y-sus board & checked underneath for shorts, and found two very low resistance paths (2~3 Ω) to earth on pins SUS_DN and SUS_UP. I desoldered the pins, and the (almost) short was on the board, not the module.



Then I had a poke around the top and found a s/c transistor, (Q408) under the smaller heatsink.



Sadly this wasn't the cause of the earlier low-z problem, I followed the tracks and they seem to lead to a small surf-mount chip which is hidden away underneath the large heatsink.

I have to go out now on a (paying!) job so I'll pull the module off later & see what the chip is.

[ReeceyBurger123]

Re: LG 42PC51 no video.

If my notes are correct that should use a stk814 chip desolder it and let us know. They are cheap IPMs are so be prepared to pay about £20 for each module on Y and Xsus.

[Hamie]

Re: LG 42PC51 no video.

It's also worth noting that they are pretty tough to desolder because the heatsink on the modules takes all the heat away from your iron.

A cheap iron and solder sucker won't cut it.

Even with a desoldering station I still find them tricky to get off.

[Trevtron]

Re: LG 42PC51 no video.

It's also worth noting that they are pretty tough to desolder because the heatsink on the modules takes all the heat away from your iron.

A cheap iron and solder sucker won't cut it.

Even with a desoldering station I still find them tricky to get off.

Yes, it is proving a bit sticky! At least if I know the module is duff I can be a bit more "robust" with the heat. I started it before lunch & got about half-way round- the easy bits-, will have a go at the rest later on.

Thanks for all the help so far, I would never have tackled it otherwise

[Hamie]

Re: LG 42PC51 no video.

Using a cheap electric hob works well.
Lay the heatsink on the electric hob and heat it up to around 150c (or as low as it will go).

Then desolder the legs, this way the heatsink won't be a problem.

[ozy]

Re: LG 42PC51 no video.

I always use chip qwik for ipm removal , prefer it to pre-heating tbh.

[Trevtron]

Re: LG 42PC51 no video.

Latest update: The board I mentioned earlier was the Ysus panel. (The Zsus IPM didn't show any shorts to earth). I managed to remove the Ysus IPM (STK 795-821) from the board without too much drama. A heavier tip, temp maxed out at 420°C and decent solder sucker did the trick.
With the heatsink out of the way I am now able to see a small 14 pin IC, IC103 (MM74HC) which is low resistance (4R6) between pins 11 & 13. Its identical meighbour IC104 does not exhibit this.
So my thinkng at this stage is this: Could the IPM in fact be OK, and the fault is simply IC103 having been damaged by Q408 shorting every way?

[Trevtron]

Re: LG 42PC51 no video.

I always use chip qwik for ipm removal , prefer it to pre-heating tbh.

That stuff looks good- is it as easy to use as it looks on the promo video?