PN50B450 - Turns on, Sound good, I think Backlight is good, No Picture

[brnrubbr]

Re: PN50B450 - Turns on, Sound good, I think Backlight is good, No Picture

No test patterns were observed.

[tom66]

Re: PN50B450 - Turns on, Sound good, I think Backlight is good, No Picture

OK, show me a picture of your control board with this set up, I need to check it is all correct.

[brnrubbr]

Re: PN50B450 - Turns on, Sound good, I think Backlight is good, No Picture

Ask and ye shall receive. Still no Test Pattern, the included photo is what I did per instructions.

I received the Y-Main repair kit and have begun replacing sets of FETs. So far, I have replaced both IRFP4332 and the one in U5407 still gets ridiculously hot.

[tom66]

Re: PN50B450 - Turns on, Sound good, I think Backlight is good, No Picture

OK, looks good

With this setup still connected as is, please try to check the Vsc voltage on the Y-main.

Also, we need to check Vsch, for this, we will need to see a clear picture of around the scan drive ("buffer") chips on the Y-main.

[brnrubbr]

Re: PN50B450 - Turns on, Sound good, I think Backlight is good, No Picture

X-Main
VCC = 15.1VDC
Vss = 209

Y-Main
Va = 54.7VDC
Vc = 15.1VDC
VS = 209VDC

Logic Board
D5.3 = 5.2VDC


BIG UPDATE - After replacing the IRFP4332 FETs on the Y-Main, now I do not even get a flicker on the screen.

- Neither IRFP4332 FET is getting warm now.

- Found bottom-most Buffer U5407 @ 115 degrees F. Hot, but not hateful

[tom66]

Re: PN50B450 - Turns on, Sound good, I think Backlight is good, No Picture

Can you post a pic of the long board running along the bottom of the set. We need to check the Va fuse.

Also, do you have any access to an oscilloscope?

[brnrubbr]

Re: PN50B450 - Turns on, Sound good, I think Backlight is good, No Picture

All fuses on X-Main, Y-Main, Logic Board, Z-Buffer(?..... along bottom of TV) are all good.

Yeah, I have access to an O-scope. Where shall I check?

[tom66]

Re: PN50B450 - Turns on, Sound good, I think Backlight is good, No Picture

That's the address buffer
OK, scope is good. Show me a pic of what you got.

[brnrubbr]

Re: PN50B450 - Turns on, Sound good, I think Backlight is good, No Picture

Ask and Ye shall receive!

[tom66]

Re: PN50B450 - Turns on, Sound good, I think Backlight is good, No Picture

Sorry...meant the scope!

[brnrubbr]

Re: PN50B450 - Turns on, Sound good, I think Backlight is good, No Picture

One O-Scope of varying sizes and degrees in the workshop.

[brnrubbr]

Re: PN50B450 - Turns on, Sound good, I think Backlight is good, No Picture

A bump to keep it fresh

[tom66]

Re: PN50B450 - Turns on, Sound good, I think Backlight is good, No Picture

Sorry, got delayed...

See this

Scope probe tip (using hook) on red area
Ground on black area. Make sure it's secure.

Caution: Sustain output is up to 400V DC! Keep hands well away during probing, and make all probe changes with TV power off.

Scope probe on 10X.

Scope V/div on 10V/div, make sure scope is also set to 10X.

Timebase 50ms/div, trigger AUTO, type EDGE or LINE, source LINE (if EDGE).

Connect to Ch1 and enable and run the scope then stop it when you see something.

Show me what you get with the set powered on in test pattern mode.

[brnrubbr]

Re: PN50B450 - Turns on, Sound good, I think Backlight is good, No Picture

A whopping 0VDC (zero VDC) at both test points. It is verified with a multimeter.

I also found U5008 and U5011 hot as hell too (190 degrees F) after a couple minutes. So a few more IRFP4332 FETs are on order from Mouser.

[brnrubbr]

Re: PN50B450 - Turns on, Sound good, I think Backlight is good, No Picture

Oops, Q5008, Q5011.

[tom66]

Re: PN50B450 - Turns on, Sound good, I think Backlight is good, No Picture

Are Vs, Va and Vg present during this fault? And Vsc?
Maybe you should go back to the original parts - a glow from the screen indicates the Y-main was likely working.

[brnrubbr]

Re: PN50B450 - Turns on, Sound good, I think Backlight is good, No Picture

X-Main
VCC = 15.1VDC
Vss = 209

Y-Main
Va = 54.7VDC
Vc = 15.1VDC
VS = 209VDC

Logic Board
D5.3 = 5.2VDC


Do we know what IC or logic is controlling the FETs and what input/output is expected at the FETs?

[tom66]

Re: PN50B450 - Turns on, Sound good, I think Backlight is good, No Picture

There is a lot of control on the Y-main boards but it is all ultimately controlled by the control board. That controls all of the power electronics signals.

You need to find Vsc as well may be marked as VSCAN.

[brnrubbr]

Re: PN50B450 - Turns on, Sound good, I think Backlight is good, No Picture

Update after a long while.

VSCAN was acting really wonky. It initially looked like I had a signal on startup, but then the IRFP4332 would escalate to 190 degrees F and then VSCAN would shut off.

This is when the investigation really went sideways on this project. I replaced all the IRFP4332 components on the Y-Main and all the results were the same. Fun fact, I think I damaged the TO-247 Heat-transfer pad. One of the IRFP4332 shorted out like a shotgun........ so that was fun.

Anyways, this whole time I was able to find another Y-Main online and ordered it since it seemed most of the issues were on the Y-Main (hot FETs..... and now shorting). Replaced the Y-Main, all is good now.

I wish I could have troubleshot the original Y-Main, maybe again in the future or if I can get my hands on another PN50B450. Thank you all for watching the saga and offering assistance. I've got another few Plasma/LCD TVs in the garage, I'm sure I be posting about those soon. Cheers!

[tom66]

Re: PN50B450 - Turns on, Sound good, I think Backlight is good, No Picture

No problem...glad you sorted it and thanks for feedback.