LG 42PX5D - no picture, replaced IPM

[tn245]

Hi there, back for your advice once again.

This one's an LG 42PX5D. Original fault was sound but no picture. Ysus fuse was blown and IPM shorted so I replaced the IPM. It was a pain but I wanted a challenge. Used the heat-gun method and got it off eventually.

Now the TV comes on with a brief flash of static on the screen but then goes black with just a few random sparkles for a few moments. I can't detect any glow. Ysus and Zsus are buzzing faintly and fuses are holding. No shorts detected on either board nor the buffer board.

Voltages:
Va: 59V
Vs: 186V
Vsc: 119.8V
Vy: -268V
Vzbias: 99V
V across large Ysus poly caps: 90.7V

What do you think?

[tn245]

Re: LG 42PX5D - no picture, replaced IPM

Ah! I thought that Vy looked a bit high so just went back to re-test and the TV is working now with Vy at -200V. In between measuring the voltages and posting I had taken the boards out again to re-check my soldering and test the buffers but hadn't tried turning it on since then. Something must have not been quite seated properly the first time I put it back together.

Sorry for wasting your time - nothing to see here! (couldn't find a delete option)

Happy to have it working anyway!

[tom66]

Re: LG 42PX5D - no picture, replaced IPM

Your -Vy is much, much too high. Try setting it down, but if it is that high, buffer boards might have fried.

[tn245]

Re: LG 42PX5D - no picture, replaced IPM

Thanks Tom, did you see my reply to myself just before you posted?

[tom66]

Re: LG 42PX5D - no picture, replaced IPM

Looks like I was 2 seconds behind. -Vy too high just means the image remains more or less erased continuously.

[tn245]

Re: LG 42PX5D - no picture, replaced IPM

There is one more issue with this TV I might need help with.

When in standby there's a high pitched squeal coming from the power board. It goes away when the TV is on. I've attached pics. The second pic shows the rough area where I think the sound is coming from. The caps seem fine visually. What might be causing the noise?

[tn245]

Re: LG 42PX5D - no picture, replaced IPM

Update: After hunting around with a pen casing stuck in my ear, I *think* the squealing is coming from the smaller transformer at the bottom of the second pic. It's really hard to tell exactly though. Any ideas what might be making it sing? Would love to get this one wrapped up. It's working really nicely except for the annoying noise when it's off.

[tom66]

Re: LG 42PX5D - no picture, replaced IPM

Sometimes this noise is normal, with the case on, you should not hear it from viewing distance. But if it is abnormal, I would try applying varnish to that transformer.

[tn245]

Re: LG 42PX5D - no picture, replaced IPM

I can hear it from upstairs! But maybe I have sensitive ears. I had heard that bad caps or diodes around the area could cause the frequency to lower into the audible range. I may try the varnish on the transformer though. Thanks for the suggestion.