Hyundai HLT-3270 - 32" LCD TV

[tom66]

Picked this up for a tenner, it has had some work done on it. Rails, 12V and 24V, are completely dead. This TV doesn't use a standby power supply, it keeps the 12V and 24V on all the time, it just uses the ON/OFF on the inverter to turn the TV off... seems a little dodgy, ya think?

Problem is -- won't power up at all. When I plug it in, I can hear a faint ticking sound; I also notice one of the diodes has got very hot as the board is discoloured around it. I'm putting two and two together and thinking shorted diode.

None of the diodes are shorted... damn, I think. I notice both have been replaced previously. So, I take a closer look. The topology is a forward converter. It looks like this:



Now, one diode has been replaced with a VF20100SG -- pretty ancient device from when TI used to make silicon. This device wasn't designed for a forward converter; it's for a flyback. It's got one diode instead of two inside the devicel; the outer legs are tied to the anode. The effective circuit now becomes.



See the problem? One secondary has a dead short across it! Hence, of course, the PSU doesn't work.

I will replace this diode and hope this is all that has been messed with.

[budm]

Re: Hyundai HLT-3270 - 32" LCD TV

What happen if you run the power supply by iself with with just the AC input/ Does it have a ZENER at the DC outputs for crowbar protection? It also has PFC.

[tom66]

Re: Hyundai HLT-3270 - 32" LCD TV

Yeah, it has PFC, and that runs continuously too. A single optocoupler for feedback, so no on/off control. No zeners on the outputs. The feedback comes from 24V and 12V. I was going to run it without 24V (which has the dodgy diode) until I realised that it might cause the 12V to rise too high.

Also, there was very little thermal paste on the replacement diode and the guy used an insulated package... great, until you realise the combination means a rapidly overheating diode if it did work.

[tom66]

Re: Hyundai HLT-3270 - 32" LCD TV

To add some insult to injury, I calculated a best-case least power consumption in standby mode.

It has a 470 ohm resistor on the +24V, and two parallel 390 ohm resistors on the +12V. This is 1.22W on the +24V, 0.74W on the +12V. That's 2W at minimum...

On the primary there's two 18k ohm resistors in series to provide the PFC startup current. These have clearly got very hot; there's considerable discolouring around the PSU area. At 325V bus voltage, the power dissipation in these two is 2.93W total.

So, about 5W in standby. That's 1980's era VCR, not modern power supply. What a joke... And this doesn't even include the power consumption of the main board or efficiency losses in the power supply.

[tom66]

Re: Hyundai HLT-3270 - 32" LCD TV

It's fixed. It was just that diode. I'm going to recap it completely as well, but it's working now. As the guy said, it does have a dead pixel -- more like three in a small vertical line -- but that's not a major problem, it's for my fellow students in the dorm to watch DVDs on.