LG 42PC1R start-up problems

[PowerAmpFreak]

Set tries to start up VS and VA supplies. Appears to trig overcurrent protection, mains relays keeps clicking.
Disconnecting BOTH Y-SUS and X-SUS makes powersupply run OK. Disconnecting only one of the SUS boards doesn't help, doesn't matter which. Problem is both Y and X seems to be fine. Measured across the IPM's without finding any short. Perhaps there are failures in the IPM's that doesn't show until power is applied. So obviously both SUS boards are bad, seems to be a longshot though.

[tom66]

Re: LG 42PC1R start-up problems

AC detect good?

[PowerAmpFreak]

Re: LG 42PC1R start-up problems

PSU operates fine without Y and X sus boards connected, so the psu AC detect should be ok,

[tom66]

Re: LG 42PC1R start-up problems

PSU operates fine without Y and X sus boards connected, so the psu AC detect should be ok,

Not always - if the line voltage is low because of a lot of load from the sustains, it might be dropping out. Check it with meter.

[PowerAmpFreak]

Re: LG 42PC1R start-up problems

I've been digging into the AC detect circuitry. The primary AC detection is made pretty close to the mains inlet (of course) and that circuit is driving an optocoupler to interface with the microcontroller located on the secondary side of the powersupply. When measuring at the optocoupler itself, the AC detect signal is stable and not in sync with the powersupply restarting behaviour. BUT, on one of the ribbon connectors, a pin named AC-D, that signal is in sync with the restarting behaviour. For fun, I shorted the optocoupler output trying to simulate a fake AC detect signal, but that did no difference. I even removed the opto, but then set will not even try to start (since it's not sensing any AC of course).
If letting the supply restart for a while, can eventually make the restart frequency lower, in other words, longer time of operation, but the screen wasn't looking good, lots of maldischarge in the bottom of the screen (and the rest of the screen all dark) so there are some problem with this dude perhaps in the Y-sustain.
Still strange it would restart no matter which of the Y and X boards connected.