[SOLVED] PCB Repair PC Monitor (Help request).

Thread Starter

rchappy88

Joined Oct 18, 2022
8
Hello,

I've decided to start trying to fix some electronics for consumer items, just because I'm quite interested and I want to further my knowledge and skills in PCB level repair:

I've currently got a PC Monitor that does not turn on. I have its Power Supply PCB dismounted and on my desk.

The transformer that seperates both sides (H and L Voltage) sides of the PCB has all 4 pins shorted together at the top end and reads approx 85VAC against Earth and Neg. The +19v pins for the red wires (going off to the VGA board) has 80-90VAC there.

With this information what could I consider to try? Or what would anyone here think the issue could be? I've looked for this transformer online to check if these 4 pins should be shorted together but I can not find it for the life of me.

1666111186103.png

1666111728558.png
Thanks for your time.

Ryan
 

Attachments

MrChips

Joined Oct 2, 2009
27,681
Welcome to AAC!

You cannot judge the state of transformer windings if you see a short (unless there is a short between primary and secondary windings).

I am going to guess that the transformer is not at fault. This generally the case.
You need to look some place else for a fault. Make sure that you apply a suitable load when testing any SMPS (switch mode power supply). Some will not turn on if there is not a minimum load.
 

Thread Starter

rchappy88

Joined Oct 18, 2022
8
Welcome to AAC!

You cannot judge the state of transformer windings if you see a short (unless there is a short between primary and secondary windings).

I am going to guess that the transformer is not at fault. This generally the case.
You need to look some place else for a fault. Make sure that you apply a suitable load when testing any SMPS (switch mode power supply). Some will not turn on if there is not a minimum load.
I connected everything together as would be when assembled. The voltages were the same at various points on the board.

A big question I have is; the transformer I was suspicious about drops from approx 115 to 85VAC. This is not reduced further to 19V for the VGA board so I'm reading 85VAC at a pin which read +19v.

There's 87VAC everywhere which makes me think there is a short (which I why i suspected the Transformer [red line across 4 pins in the image below]).

1666116692527.png
 

twohats

Joined Oct 28, 2015
362
How have you got your test meter connected?
Power supplies can lead you a merry dance. (Old Anglo Saxon saying).
Please keep us informed,
Good luck.
 

Thread Starter

rchappy88

Joined Oct 18, 2022
8
TwoHats - I'm taking my readings as black prob on earth at the main PCB connector (kettle connector port) and red on various points on this side of the board. Honestly the neg tracks on the low voltage side of the board is live.

I've disconnected the four legs (of the transformer) to the PCB as where i've drawn the red line on the above image. The short between the + and - tracks has disappeared. This seems like progress to me but I could be mistaken.
 

LesJones

Joined Jan 8, 2017
3,903
The secondary side of the power supply i probably floating so measuring voltage wit respect to earth is meaningless.
As the transformer works at a high frequency (Tens or hundreds of Khz.) the winding will have very few turns so the winding resistance will be VERY low. (Probably too low to be measured using a normal DMM.
I would guess that the device mounted on the heat sink is the switch mode regulator chip. I suggest reading it's part number an obtaining it's datasheet. This will probably give you an application circuit which is likely to be similar to your board. From that yo can start logical fault finding.

Les.
 

MrChips

Joined Oct 2, 2009
27,681
This is a SMPS. You cannot trouble shoot this the way you would a conventional linear power supply.

In fact, these are very difficult and dangerous to work on especially if you don’t know what you are doing.
 

Thread Starter

rchappy88

Joined Oct 18, 2022
8
The secondary side of the power supply i probably floating so measuring voltage wit respect to earth is meaningless.
As the transformer works at a high frequency (Tens or hundreds of Khz.) the winding will have very few turns so the winding resistance will be VERY low. (Probably too low to be measured using a normal DMM.
I would guess that the device mounted on the heat sink is the switch mode regulator chip. I suggest reading it's part number an obtaining it's datasheet. This will probably give you an application circuit which is likely to be similar to your board. From that yo can start logical fault finding.

Les.
Les,

Thanks for pulling me out of the rabbit hole. There must be DC somewhere on this secondary part though? For the VGA Board?
 

LesJones

Joined Jan 8, 2017
3,903
My first step with switch mode power supplies would be to check the DC voltage across the main reservoir capacitor. It should be about 1.4 times your RMS mains voltage. If that is not present check that the fuse has not blown. After that you need the data sheet on the switch mode regulator IC.

Les.
 

Thread Starter

rchappy88

Joined Oct 18, 2022
8
My first step with switch mode power supplies would be to check the DC voltage across the main reservoir capacitor. It should be about 1.4 times your RMS mains voltage. If that is not present check that the fuse has not blown. After that you need the data sheet on the switch mode regulator IC.

Les.
Thanks Les.

Checked the Res Cap first as it needed discharging, it appears good and capacitance is as stated on it. Fuse still good. Just removed the heat sync now and managed to find the regulator IC so Im going to get up the datasheet now and have a gander.
 

Thread Starter

rchappy88

Joined Oct 18, 2022
8
Update.

I decided to prove something. I hooked up my bench power supply to the secondary side of the PCB and give it +19VDC. I disconnected the transformer prior. The monitor turned on.

I have drawn two boxes (red and blue) for where i connected my Bench PSU.

1666129526547.png
 

Attachments

LesJones

Joined Jan 8, 2017
3,903
You have proved that the fault is on the primary side of the power supply so you are back to NEEDING TO FIND THE DATA SHEET for the switch mode regulator chip. Be aware that working on the primary side you can't connect the ground wire from your oscilloscope to the negative of the reservoir capacitor while the circuit is powered directly from the mains. You will need to use an isolating transformer between the mains and the supply input.

Les.
 
Here is the diagram from the datasheet, check the Mains Input voltage to the supply pins.

/IMAGE
Dave. I saw that too but it doesn't tell me which pins I need to check and what I need to be expecting (unless I'm missing something). I'm unsure whether this chip should produce a half bridge, High Frequency/PRF Square Wave or something else. My graphical DMM is showing that the transformer (between primary and secondary PCB sides) is showing a full sine wave at 115VAC which gets stepped down to 85VAC on the secondary side which is not what is needed to start the Monitor.


[QUOTE

LNK6777K is a definite suspect.
Good luck..............
I've ordered it for £5.55 anyway. I hoped I could come to a more solid conclusion though.
 

LesJones

Joined Jan 8, 2017
3,903
Did you measure the DC voltages with respect to the reservoir capacitor negative of the following pins on the LNK6777K ?
BP (Pin 4)
D (Pin 7)
FB (pin 2)
Where the readings about what you would expect ?

Les.
 

Thread Starter

rchappy88

Joined Oct 18, 2022
8
The Chip is the V package, not K. It won't let me edit my previous post.

1666255516150.png
Les, between these pins and the Resevoir Capacitor Neg is:

Resevoir Capacitor = 320.6VDC (+ve 111.3V [earth pin] 111.3V -ve)

BP Bypass (Pin 4) - Nil VDC
D Drain (Pin 6) - 319.8VDC
FB Feedback (pin 2) - Nil VDC
S Source (Pins 7-12) - Nil VDC

Just In Case:

I'm getting VAC readings respective to Earth though. from pins 1-4 which show this:

1666256250059.png

And at Pin 6 (Discharge)

1666256336721.png

I don't know if those wave form readings are pointless for this. Between the Res Cap and the IC is a grey area of understanding for me.
 

Attachments

Top