LG 37LC7D-AB Power supply squeal

[mangozac]

This 37" LG LCD was my parents' but when it died early in the year they decided to buy a new one. I intended to work on it eventually and was recently forced to look at it after my Panasonic plasma started playing up.

The LCD would not turn on. Apparently they had been watching it when it made a loud bang and died. Upon further inspection I found that the 2.2R resistor supplying the mains DC bus voltage to the Standby 5V regulator had burned out. I couldn't find a short on the MOSFET of the switching regulator so figured I'd just try replacing the resistor. I did so and strangely enough it worked! I plugged the board back in and the TV was working. Except for one issue: a loud power supply squeal.

Even when the set was in standby the squeal was audible, but when powered on the squeal was far too loud so the TV definitely isn't usable like that.

I read that squealing can be caused by faulty other boards in the TV so I pulled the power supply and set it up on my workbench. I strung a 47R 5W resistor across the STBY5V output so that it was drawing 100mA and the squealing noise was being made. So it's definitely a problem with the power supply board.

I thought perhaps the 2200uF 10V filter cap on the STBY5V was failing but replacing it made no improvement.

I've got the service manual for the 37LC7D but the LG service manuals seem really poor in comparison to Panasonic ones. It does not include schematics for the power supply board, which is an EAX32268301/9.

The Standby 5V regulator IC is an ICE3B1565J and the PSU design is pretty similar to that presented in the datasheet.

It's really strange that the 2.2R resistor just died like that without any other issues. There's no charring on the board so it wasn't a failure due to gradual overheating (as would be caused by, say, mildly excessive current draw).

Do you component level repair gurus have any suggestions?

[budm]

Re: LG 37LC7D-AB Power supply squeal

How is the voltage on the Main DC filter cap on the primary side? Does it have lots of AC ripple (if you have good Fluke True R.M.S), you can set it to AC to read the AC value riding on top of DC?

[mangozac]

Re: LG 37LC7D-AB Power supply squeal

I've got a Fluke 179 which is true RMS, which reports 10VAC ripple. That sounds pretty acceptable, right?

[budm]

Re: LG 37LC7D-AB Power supply squeal

That reading with just power supply plug into an AC with small load on 5V right?, if that is the case, that seems too much.
And what is the DC reading? It should be about = to Line voltage x 1.414.

[mangozac]

Re: LG 37LC7D-AB Power supply squeal

Sorry I was on auto pilot when I wrote that last reply! You're right that 10V ripple with that small a load is extremely excessive! I decided to re-measure and realised that the true-RMS feature of the meter averages over a long period and the measured voltage ripple gradually decreases to under 1V.

I was still not feeling confident so confirm it further I grabbed an oscilloscope and isolating probe and measured the voltage. This measured a beautiful, flat 320VDC on the DC bus, so everything there is definitely fine.

[mangozac]

Re: LG 37LC7D-AB Power supply squeal

She's fixed!

After taking the effort to get the oscilloscope out yesterday I figured I'd keep probing around and checking out the switching signals. This revealed some very anaemic looking pulses on the cold side of the STBY5V transformer. It just didn't look right and there was a lot of ripple on the 5V.

Everything on the cold side checked out ok so I decided the problem lay on the hot side. Resistors all measured OK so I moved on to testing the passive semiconductors. That's when I discovered the fault: zener diode ZD501 was a short circuit.

ZD501 is reverse biased in parallel with R512, which is a 0.68 Ohm resistor used for sensing the transformer current by the switching regulator. This sensing is used as a part of normal operation and hence with the zener shorting the resistor no sensing was possible. The zener simply protects the CS pin from high current spikes causing a large voltage to appear across the resistor.

I pulled the zener off and found it was marked "36B". I took this to mean it was a 1N5236B 7.5V 0.5W part. Unfortunately I didn't have an equivalent handy. I looked at the switching controller datasheet and read that the current limit threshold is actually 1V and the maximum input voltage for the current sense pin is 6V. So the use of a 7.5V zener has to be questioned anyway! But I did have a 5.6V part handy and decided I could get away with that.

I did the swap, powered it up and she was quiet as a mouse. Even under some load. I'm yet to take it home and plug it back into the TV but I'm feeling pretty confident now!

For reference, here's the board and the parts I replaced:




R512 is directly below ZD501 (and standing upright).

Hopefully this information may help someone else with the same issue. I'll report back to confirm that the TV is running as soon as I've tested it out for a short period.

[budm]

Re: LG 37LC7D-AB Power supply squeal

Great troubleshooting. The resistance reading must be close to 0 Ohms, lower than 0.68 Ohms. Most cheaply made power supply leave out that Zener Diode.

[mangozac]

Re: LG 37LC7D-AB Power supply squeal

Thanks budm! I've said in a previous post that despite being an engineer and designing electronic circuits, tracking down faults in equipment that others have designed is a completely different skill set!

It was the resistor that was 0.68 ohms; the zener diode was very close to 0 ohms. The zener diode isn't even suggested in the ICE3B1565J datasheet so it doesn't surprise me that many mould leave it out. So I guess points go to LG engineers for not cutting that corner?

[midnightg35x]

Re: LG 37LC7D-AB Power supply squeal

Long dead post I know, but I had a LG32LF11 with a similar issue. TV would not turn on at all. The 2.2 ohm resistor (R501) was open like the OP. I replaced it and it squealed like the OP's. I found that ZD501 was shorted after finding this post. My TV uses a 1.2 ohm current sense resistor (R503) versus the OP's 0.68 ohm.

For my contribution to this post, I found that my zener diode was a 3.6V zener. I'm not sure if the marking just was not representative on the OP's zener, but the zener on mine is a 3.6V This also makes sense because the max voltage on the CS pin is 5.5V. I thought it curious they used a protection device that did not actually protect. Either way, if someone else runs into this issue know that a 3.6V zener is standard in at least some of these older LG power supplies.