LG 50PC55-ZB stays in stand-by

[tom66]

Re: LG 50PC55-ZB stays in stand-by

Do they show as shorted? Wow. Those IGBTs should be <£5 each... if you can source them!!

[laszlo31]

Re: LG 50PC55-ZB stays in stand-by

Yes, 3 out of 4 show shorted Collector-Emitter, only 1 is properly insulated and can be opened or closed with applied Gate voltage.
Would you recommend me checking the remaining 2 IGBTs under the other heatsink?

[tom66]

Re: LG 50PC55-ZB stays in stand-by

If you can. Though usually only the FETs in one group will go at once.

[laszlo31]

Re: LG 50PC55-ZB stays in stand-by

I got lucky as a guy is selling RJH3047 IGBTs at 1€/piece in my city so I can pick them up tomorrow. I hope the TV will get out of protection mode and the replacement transistors will not go blown right away. Keep your fingers crossed.
If fuses are OK on the boards, would you say that the IGBTs just got too old or would you recommend some extra precaution before I solder the new components and switch the set on?

[tom66]

Re: LG 50PC55-ZB stays in stand-by

I got lucky as a guy is selling RJH3047 IGBTs at 1€/piece in my city so I can pick them up tomorrow. I hope the TV will get out of protection mode and the replacement transistors will not go blown right away. Keep your fingers crossed.
If fuses are OK on the boards, would you say that the IGBTs just got too old or would you recommend some extra precaution before I solder the new components and switch the set on?

One probably failed and took out the other three. These TVs run hot, accelerating failures.

[laszlo31]

Re: LG 50PC55-ZB stays in stand-by

I got the replacement IGBTs, replaced them but unfortunately the TV still clicks back to stand-by as the Z0_SUS_UP pin doesn't go up to 5V as (I guess) it should.
The chirping sound became louder from around the upper heatsink. Being afraid of shorting another set of transistors I tried only few times powering up but I got no luck. Desoldered the components again and checked. At least the transistors and the rectifiers are still OK.
I try to upload a short videoclip so somebody might recognize what would give such a distinctive chirping metallic noise.

[vinceroger69]

Re: LG 50PC55-ZB stays in stand-by

could it be ic701 as in this kit
http://www.ohmsupplies.co.uk/Repair-...55-EAY32957901

[laszlo31]

Re: LG 50PC55-ZB stays in stand-by

I will check IC701, thanks for the tip.
Here's a link to a short clip that has the mentioned sound. Please listen to it and let me know if this sounds familiar or typical.
http://videa.hu/videok/tudomany-tech...Qv1L9apH4OXLFX

[laszlo31]

Re: LG 50PC55-ZB stays in stand-by

Unfortunately Youtube uploading is unbearably slow so I had to upload the short video to the Hungarian version video sharing site.
Please listen to the sound and try to guess what makes it...Thank you.
http://videa.hu/videok/tudomany-tech...Qv1L9apH4OXLFX

[laszlo31]

Re: LG 50PC55-ZB stays in stand-by

I still haven't found the culprit.
What are the chances that if all PSU voltages are OK with connectors removed, under load these voltages are off specs? Should I build some test device with light-bulbs?

[tom66]

Re: LG 50PC55-ZB stays in stand-by

Bad capacitors could cause the issue. However, you should be able to test the voltages with the boards connected and see if any drop out.

[laszlo31]

Re: LG 50PC55-ZB stays in stand-by

Due to the nature of the problem (TV doesn't power up but goes back to stand-by) I have difficulties measuring voltages under load. My window of opportunity is few second every time that should be enough but I am afraid of ruining components as a quite harsh sound is coming from the upper part of the Z-SUS. Few posts earlier I posted a link to a short video, I would appreciate if you could listen to it and at least share your opinion if this sound is a sure sign of dying components or not...

[laszlo31]

Re: LG 50PC55-ZB stays in stand-by

Checking all components on the Z-SUS I came across a shorted diode next to the IGBTs that were previously replaced due to short. Desoldering the diode, it tested OK but both empty pads were measured shorted to the GND of the Z-SUS. See attached picture. Following the printed wires it turned out that 3 pins of a IRS21064S driver IC cause the short. See attached data sheet. Having limited experience in IC testing could you please advise whether this IC seems faulty.

[tom66]

Re: LG 50PC55-ZB stays in stand-by

The IC shouldn't be able to short from Vs to Ground but I suppose it's plausible. In theory, the high voltage and low voltage sides are separate. The shorted IGBT could zap the driver, potentially.

[laszlo31]

Re: LG 50PC55-ZB stays in stand-by

Hi All,

After several evenings on component level troubleshooting and isolating the issue at a faulty 14 pin IRS21064S driver IC I decided to order a new Z-SUS that arrived today. After replacing the board TV works as a charm. Family happy. I learned a lot about plasmas. Thanks for the support and advice.
László from Hungary

[tom66]

Re: LG 50PC55-ZB stays in stand-by

I'm glad you fixed it, sorry that I couldn't offer much help. Enjoy the TV.

[laszlo31]

Re: LG 50PC55-ZB stays in stand-by

Sometimes giving hope is just as important as giving a solution...Personally I even consider more important if someone wants to learn. And I do. Thanks.