EEVblog 1388 - Dumpster Diving TV Murphy's "Repair"

[EEVblog]

Dave fixed this Hitachi dumpster diving 4K TV so well that he broke it!

[fable]

Here is schematic

[eblc1388]

Should have used a wired 5A fuse for replacement. PFC switching MOSFET draws high instantaneous current through the inductor following the AC wave profile. This might have caused the PTC resettable fuse to go into higher than normal resistance, and PFC controller IC doesn't like it a bit.

[BrianHG]

Ohh, the modern version of when in the old days, I would have a good working CRT, with an intermittent problem which occasionally required me to whack the display to fix.  I open it up, find a few cold solder joints.  I re-solder cleaning them up.  The display works for a day, then literally blows up the next time I power-on.

I was told by an experienced repair tech that some of TVs become dependent on the cold/poor contact solder joint and doing repairs may destroy the display.

[ivb]

Ohh, the modern version of when in the old days, I would have a good working CRT, with an intermittent problem which occasionally required me to whack the display to fix.

I'm reminded of a similar problem my family had when I was growing up. Something went goofy in the tuner section of our CRT TV where it would just suddenly go out of tune despite being "digitally" tuned. My dad being a machinist and definitely not an EE or even electrician decided the best remedy was to drill a hole in the case directly above the RF shielded tuner and use a brass rod inserted to regularly hit the RF shield it until it came back in tune. We called it the "bonger" as in "get the bonger the TV's gone out again". Oddly enough no one ever managed to electrocute themselves despite what I now realize were our best efforts .

[David Hess]

I am surprised that the PFC controller IC was not damaged.  In my experience when the power transistor fails, usually high voltage gets applied to the gate or base destroying the drive circuits as well.

[dcac]

Yeah, everything can be shorted in a failed power transistor. Not just Emitter to Collector - but Emitter/Collector to Base too, or in this case Drain/Source to Gate. I don't know if Dave measured the Gate - I don't think he showed it in the video.

Still bit of a mystery why something else seems to have broken by just installing the new mosfet. If the PFC IC is damaged and its output failed open it shouldn't do anything at all - if it failed short as in sending a constant voltage to the mosfet - the mosfet and/or the fuse would blow again.

[EEVblog]

Yeah, everything can be shorted in a failed power transistor. Not just Emitter to Collector - but Emitter/Collector to Base too, or in this case Drain/Source to Gate. I don't know if Dave measured the Gate - I don't think he showed it in the video.

Still bit of a mystery why something else seems to have broken by just installing the new mosfet. If the PFC IC is damaged and its output failed open it shouldn't do anything at all - if it failed short as in sending a constant voltage to the mosfet - the mosfet and/or the fuse would blow again.

Yes, that was my analysis too.
The gate was about 10ohms, so the chip might have been taken out.

[dcac]

Yeah, everything can be shorted in a failed power transistor. Not just Emitter to Collector - but Emitter/Collector to Base too, or in this case Drain/Source to Gate. I don't know if Dave measured the Gate - I don't think he showed it in the video.

Still bit of a mystery why something else seems to have broken by just installing the new mosfet. If the PFC IC is damaged and its output failed open it shouldn't do anything at all - if it failed short as in sending a constant voltage to the mosfet - the mosfet and/or the fuse would blow again.

Yes, that was my analysis too.
The gate was about 10ohms, so the chip might have been taken out.

And likely then the ZD101 zener too as it sits directly between Gate and GND. If that zener is shorted it kinda rules out any involvement from the PFC IC together with the new mosfet causing the new fault.

[Robert Smith Eco Warrior]

I wasn't sure where to put this reply but since it was a dumpster dump find I thought I would put it in a dumpster dive thread...

I helped my mum move a wardrobe from a house that was being cleared. There was a skip outside full of alsorts of rubbish plus various old hifi (mostly low-fi crap) and these two little boxes.
I asked the guy there and he said take what I want. I thought these might fetch £10 on ebay or pull them to bits and add them to my scrap metal or just put something in the nice wooden boxes.

Bloody hell.... these things go for mega bucks on ebay 
Vitavox GP1 Pressure units...

[Pinkus]

Life is a give an take. Just thinking about how my future heirs will give away many of my valuables for less than they are worth or even throw them away, makes me feel sick already.

[Robert Smith Eco Warrior]

I think we should all make a equipment and tools list to tell relatives what all our 'junk' is 

[Doctorandus_P]

It may have nothing to do with the PFC at all.

It looks like you did a firmware update, then cycled power and it does not work anymore...

[Kaluluka]

It may have nothing to do with the PFC at all.

It looks like you did a firmware update, then cycled power and it does not work anymore...

Failed firmware update and no 5V STBY Power from this PS, relation explanation please?