LED 27" monitor PSU issue - np power to LED array

[dougieT]

Hi all, picked up a faulty Hanns-g HL272HPB free for spares/repair hoping it was just a re-cap...

I found 1 cap with a bulge and an out of spec reading.

100uF 100V see pics.

I've replaced it with a like for like value.

Not so fast... still no power on

More testing results next post.

Dougie

[dougieT]

Re: LED 27" monitor PSU issue - np power to LED array

So time for some readings...

Heres a pic of the underside, there are 2 test points labelled on the PCB, one for 5Vdc and one for 24Vdc.

I also read the mains AC (244Vac) and rectified DC at the 2 paralleled 53uF 450v filter caps (336Vdc).

Took some readings at the other caps too, see pic.

Dougie.

[dougieT]

Re: LED 27" monitor PSU issue - np power to LED array

I checked 2 surface mount TO-262 packages in the 24v test point area of the board, part numbers 050N10 and S20N10, desoldered and checked them in the mini tester. Also tested the gate switch function OK with a DMM. See pic for results.

Dougie.

[diif]

Re: LED 27" monitor PSU issue - np power to LED array

E19 capacitor ok ?

[CapLeaker]

Re: LED 27" monitor PSU issue - np power to LED array

Do you have BLon?

[dougieT]

Re: LED 27" monitor PSU issue - np power to LED array

E19 capacitor ok ?

Looks like a slight bulge on it, took it out and measured in the mini tester. I replaced it with 2 x 47uF 63v in series and got a good spec and ESR.

The replacement E19 caps read 39v across it.

Dougie.

[dougieT]

Re: LED 27" monitor PSU issue - np power to LED array

Do you have BLon?

No lights to indicate it's on after mains power applied, soft power switch doesn't light up.

I took some readings from the AV panel driver board, pic attached.

Dougie

[lotas]

Re: LED 27" monitor PSU issue - np power to LED array

Your problem may be in the motherboard, since 5v power is supplied to the board, but there is no command to turn on "EN".
Check power to DC-DC converters.

[dougieT]

Re: LED 27" monitor PSU issue - np power to LED array

Your problem may be in the motherboard, since 5v power is supplied to the board, but there is no command to turn on "EN".
Check power to DC-DC converters.

Here's some shots of the AV/logic board, top and bottom sides.

I don't see any obvious issues or where the fault could be on the board.

Dougie

[lotas]

Re: LED 27" monitor PSU issue - np power to LED array

What are the voltages relative to earth at these points?

[dougieT]

Re: LED 27" monitor PSU issue - np power to LED array

What are the voltages relative to earth at these points?

I've attached the photo with the 2 readings.

Thanks for your help

Dougie.

[lotas]

Re: LED 27" monitor PSU issue - np power to LED array

You have a low voltage at the input should be 5v, at the output 3.3v.

[dougieT]

Re: LED 27" monitor PSU issue - np power to LED array

You have a low voltage at the input should be 5v, at the output 3.3v.

The 5v to 3.3v regulator on my board is a CYT8117T33 and the IC it feeds its output to pin 3 of this package labelled "8912k 245168", can't find any info on the codes.

I checked he caps around them and all ok.

So 1st I'll order the 5-3.3v regulator and replace it.

Dougie

[lotas]

Re: LED 27" monitor PSU issue - np power to LED array

This regulator is the same (1117 33) with a fixed voltage of 3.3v.

[lotas]

Re: LED 27" monitor PSU issue - np power to LED array

8912k - APE8912

[dougieT]

Re: LED 27" monitor PSU issue - np power to LED array

8912k - APE8912

Thanks, good find!

Both regulator IC output readings are low, but the 1.5v reg IC Vin is fed from the 3.3v reg Vout so we can assume the 1.5v reg is OK? It relies on the 3.3v from the other reg IC and Vin has to be 1.5v higher than Vout, its Vin is currently only getting 1.52v.

Could a fault in the 1.5v reg IC take out the 3.3v reg IC?

So, maybe best to replace both...

Dougie.

[lotas]

Re: LED 27" monitor PSU issue - np power to LED array

It is unlikely that 8912k suffered, 3.3v not only power it, you can check by applying 3.3v from a laboratory power supply to this point and check.

[dougieT]

Re: LED 27" monitor PSU issue - np power to LED array

It is unlikely that 8912k suffered, 3.3v not only power it, you can check by applying 3.3v from a laboratory power supply to this point and check.

Yes, I wouldn't expect these ICs to draw much, datasheet notes 3A typical and 5A max and I wouldn't expect it to need that much.

I applied 5v from the bench supply to pin 3/Vin on the 5-3.3v reg IC and got 3.3v pin2/Vout then tested at pin3/Vout on the 3.3-1.2v reg IC and it read 1.2v so all good there.

The power LED lit up and I was able to power on/off the motherboard via the on/off button. I only connected the front panel button/LED board header and nothing else.

So now to find out what has happened to the 5v supply we have from the PSU board header and the 5v-3.3v reg IC.

Dougie.

[dougieT]

Re: LED 27" monitor PSU issue - np power to LED array

Very strange!
I connected all the headers and the panel flat cable to test the 5v from the PSU header board again, the power led lit up and I cot a check cable message on the LCD panel

I'll put everything back in the casing and test with an HDMI input etc...

Dougie.

[lotas]

Re: LED 27" monitor PSU issue - np power to LED array

Check the connector and the passage of 5v to 1117 and the capacitors in the power supply on the 5v bus.