Y sustain keeps blowing FETs ? (LG)

**angelos** · 10-24-2018, 02:30 AM

Re: Y sustain keeps blowing FETs ? (LG)

And it does not help the situation that there is so much fake spares being dumped. Getting original spares is also a challenge. I feel just like you. Frustrated.

**Dannyx** · 10-24-2018, 02:44 AM

Re: Y sustain keeps blowing FETs ? (LG)

I did find this board as a replacement, but it's not exactly cheap (not cheap at all in fact) and you run the risk of blowing it up immediately and wasting all that money....

**tvtimmy** · 10-24-2018, 04:14 AM

Re: Y sustain keeps blowing FETs ? (LG)

If the x and y mains are connected in series, a bad x can take out your y, and vise versa.

**Dannyx** · 10-24-2018, 05:20 AM

Re: Y sustain keeps blowing FETs ? (LG)

Yes, these are in series indeed but isn't that just a "pass-through" ? How is the X (or Z I suppose) interact with the power devices on the Y sustain ?

**Andrew F. Ali** · 10-24-2018, 05:49 AM

Re: Y sustain keeps blowing FETs ? (LG)

are the Y-sus and Z-sus connected in series or parallel??

**Dannyx** · 10-24-2018, 05:53 AM

Re: Y sustain keeps blowing FETs ? (LG)

Originally posted by Andrew F. Ali

are the Y-sus and Z-sus connected in series or parallel??

Series: cable goes from PSU to Y and then another one from Y to Z if that's what you're referring to.

**Andrew F. Ali** · 10-24-2018, 12:55 PM

Re: Y sustain keeps blowing FETs ? (LG)

Yes, in parallel. The Vs (+) and GND (-) goes to the Y-Sus and then the Vs (+) and GND (-) go to the Z-Sus without passing through the Y-Sus circuitry. If they were connected in series then the Vs (+) will enter the Y-Sus circuit and the Y-Sus GND (-) will then connect to the Z-Sus Vs +. So it is connected in parallel and NOT in series. Also, we could disconnect the Y-Sus and the Z-Sus will still be powered. I am sharing this so as not to confuse the 'Newbie'.

**Dannyx** · 10-24-2018, 01:03 PM

Re: Y sustain keeps blowing FETs ? (LG)

Oh I get it, you're talking about the circuit-level wiring and not the physical wire harness...yes, it's parallel then.

I found it strange that you said "Vs going to GND in a "series" setup" but I mulled over it and that's how "series" circuits works...never thought "series "like they thought us in school with batteries and bulbs can be performed at such high voltages with "power" components...seems a bit counterintuitive and stupid really

**Andrew F. Ali** · 10-24-2018, 01:28 PM

Re: Y sustain keeps blowing FETs ? (LG)

So you got it then? The Vs and GND from the PSU go to the Y-Sus and the the Vs and GND is taken from the Y-Sus then go to the Z-Sus. I am sharing this so as not to confuse the 'Newbie'.

**Dannyx** · 10-24-2018, 01:45 PM

Re: Y sustain keeps blowing FETs ? (LG)

Glad you pointed out the last bit again

Yeah, not too vexed with plasmas I admit, though that causes me to be even more ambitious and curious to learn about them....for instance why there's a small SMPS transformer on the Y-sustain as an example of curiosity...there's little resources about the in-depth operation on these boards, or at least one with "decent" terminology that even average-joes with moderate electronics knowledge like me can comprehend

Newbies as you so kindly put it

**foxyUK** · 10-24-2018, 03:08 PM

Re: Y sustain keeps blowing FETs ? (LG)

I know nothing about LG plasmas, but reinforce Angelos' comment: on Panasonics it's almost always fake components that cause repaired boards to fail. These 'copy' power components work after a fashion but can't reliably handle the high power...sometimes for seconds, sometimes for tens of hours only. You need to track down 100% genuine parts or known good equivalents.

**tvtimmy** · 10-24-2018, 04:15 PM

Re: Y sustain keeps blowing FETs ? (LG)

Originally posted by Dannyx

Yes, these are in series indeed but isn't that just a "pass-through" ? How is the X (or Z I suppose) interact with the power devices on the Y sustain ?

Im not sure, but it has happened to me on a 42". I found some shorted igbts on the zsus. Changed them.Pluged it in and nothing. Checked the ysus and found some more. Replaced and pluged it in again. Nothing.looked at the zsus again and all my igbts shorted again. Replaced them and checked the ysus.pluged it in and I had a good tv.

**lookimback** · 10-25-2018, 01:27 AM

Re: Y sustain keeps blowing FETs ? (LG)

One thing I learned a while back is that some boards will blow FETs if you don't put all the screws back in when remounting the board, and it was an LG I was working on. The thought process was, "I don't know if this is gonna work so I'll just put two screws in for now." And pop. "Yup, still doesn't work, glad I didn't put all those screws back in." Did that about 20 times before finding a thread about it. It's because the screw is the only path to ground, and when it's not there, it finds or makes an alternate path. Another thing is the driver can be shorted. I usually scope the gate before replacing the FET. Someone already mentioned a short on the other SUS, and it's good practice to pull both and check everything under the heatsinks when you have a problem on one.

**steviewonder** · 10-26-2018, 05:50 AM

Re: Y sustain keeps blowing FETs ? (LG)

I had the exact same problem with the same board,think the set's model number was a pj350...Turns out it was a 1NF blue disc cap that was leaky-had a hole blown in it on the side i couldn't see-it was mounted in front of a 2-pin diode bolted to the heatsink.study that one-and all the others for cracks,etc.You never know!

**Dannyx** · 10-26-2018, 10:30 AM

Re: Y sustain keeps blowing FETs ? (LG)

Mine also had a blown cap, except mine was clearly blown apart and I replaced it. I closely studied the other ones but they didn't show any signs of damage sadly

**lookimback** · 10-27-2018, 03:08 AM

Re: Y sustain keeps blowing FETs ? (LG)

You can use a light bulb in series with the VS to limit the current. That will usually save the FETs. I'd say probably 100 watt. I tried a 60w tonight and still blew a FET, but 1 is better than 6. I should start to light then go out. If it stays lit for more than a half second or so, you definitely have a short.

**Dannyx** · 10-31-2018, 08:03 AM

Re: Y sustain keeps blowing FETs ? (LG)

Same scenario on a different board, this time for a Samsung...I rest my case: there's no way you're fixing a Y sustain board.....unless by some freak luck. Replaced a couple of transistors and some diodes and although THEY didn't fail upon startup, there was a slight pop and now something else is busted...won't even bother replacing those too...plasmas need to go back to hell where they came from

**lookimback** · 10-31-2018, 01:22 PM

Re: Y sustain keeps blowing FETs ? (LG)

Yeah, that's the crazy part. You replace some bad FETs, and some other ones somewhere else on the board blow. It's like how lightning never strikes twice in the same place. They certainly are fixable though, just a real pain in the ass. I can't find a short anywhere on my board, but Fets will pop 3 seconds after it powers on. I think I may have a bad gate driver keeping a FET on when it shouldn't be. I'm going to try leaving the VS fuse out and probe all the gates with my scope. Maybe that will reveal something.

**tom66** · 11-01-2018, 06:56 AM

Re: Y sustain keeps blowing FETs ? (LG)

You tried the bulb trick in series with the board's Vs?

Also, if you have a scope, this can be very helpful. But you need to have a good understanding of power electronics.

Y sustain keeps blowing FETs ? (LG)

Y sustain keeps blowing FETs ? (LG)

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