Prestigio P3221W 22’, power supply problem

[Sushi1808]

Hello guys,

I am a regular visitor of Badcaps forums for the last two years (mostly laptop repair forum), but this time I decided to ask you for help after two full days spent on this monitor and still without a clue.

A few months ago I got this monitor from a customer of mine as a part of a compensation for a laptop repair. He said that monitor has some problems, but not anything specific.

I opened the thing and measured ESR of electrolyts, I replaced those with extreme deviations with equivalents, some of them with a larger voltage specification.

The thing was working like new for the whole month until a few days ago, when I forcefully unplugged it from mains when turned on, as I needed a power cable for something else, an impulsive decision I now regret.
The next time I plugged it in, it didn't turn on, the thing was dead, power LED also.

The diagnostics I tried:

- I established the problem must be on a power supply board, as I connected only the main board to my bench power supply and gave it 5V, the power LED started working, the ON/OFF button was responding, the current consumption was about 50mA.

- I connected the power supply to mains (230V) without anything connected and heard repeated clicking, it seemed the thing tried to start but was unable.

Measurements:

- voltage on connector pins (without anything connected)
5V line: fluctuating from around 4V to 4.9V (it seems like it was climbing up to 5V and then the thing shut down before it could reach 5V and the cycle repeated,…)
12V line: steady 19V

- Filter capacitor (negative leg as ground)
320V
Out of circuit capacitance OK, around 100uF, ESR value in specification with a lot of reserve.

- Two Schottky diodes (middle leg)
Same as on connector pins, 4 – 4.9V fluctuating and steady 19V.
Out of circuit, DVM in diode mode, they are not shorted and seem OK.

- Power MOSFET (STK0765)
Out of circuit, tested with DVM (applied 2.6V to gate), MOSFET opens, DVM shows around 0.600

- Small SMD glass diodes
Some show 0.0030 and 0.0060 even when probes are switched when measured in circuit, but behave normally out of circuit, 0.600 on one side and OL on other.

- Two larger diodes
In circuit 0.464 DVM in diode mode on one side, other OL.

I re-measured all the capacitors on power supply board and are all deeply within specifications, according to the scale on my ESR meter, picture of ESR scale is included.

Then I tried connecting the load (12V 21W car light bulb and PC fan) to the 5V line. When I initially plugged the board to the mains, the light bulb was dimmed blinking and the fan was trying to start but was unable, just as I expected it would, as the 5V line is fluctuating.

I disconnected the mains cable and connected it again and then the unexpected thing happened - the light bulb starts to shine steadily and the fan starts working as it should.
I measured the 5V line, it is steady at 4.6V the first time, but the 12V line is now steady 30V.

And now every time I connect the dummy load the board starts without a problem, even after cooling it with compressor, shaking it, disconnecting it for a period of time...

I am thinking about connecting it to the main board and LCD panel but I am not sure this is a wise choice because of that 30V on 12V line...

I really don't know how to proceed now and would really appreciate your input.

[viziks]

Re: Prestigio P3221W 22', power supply problem

I would suggest you replace the startup capacitor.I can't make out the part no. on the first pix but it is the small capacitor next to heatsink and connected to D201,value maybe 47uf 50V or similar.

[Sushi1808]

Re: Prestigio P3221W 22', power supply problem

Viziks thank you for your reply.

Yes the cap is 22uF 63V as a replacement for the original 22uF 50V, this is one of the caps I replaced a month ago. ESR on this cap in circuit is 0.50ohm.

[viziks]

Re: Prestigio P3221W 22', power supply problem

Check the voltage across this capacitor and should be stable around 15V. If so you should replace capacitors on both 5V and 12V line.
If you have already recapped you should check around the optocoupler and 3 pin reference Ic TL431 IC203. Also the 12V will normally feed only the inverter and does not go to control board or LCD.

[Sushi1808]

Re: Prestigio P3221W 22', power supply problem

Thank you, I will do as you suggested and report back the findings.

[Sushi1808]

Re: Prestigio P3221W 22', power supply problem

I now replaced all the output caps, they all have ESR 0.0, previous caps had around 0.20ohm max, the results are the same, so I made some measurements you requested.

Measurements:

C204 22uF 64V (without dummy load): fluctuating 11 - 14.9V
C204 22uF 64V (with dummy load - 21W car light bulb on 5V line): steady 13.34V

IC203 - TL431 (with dummy load):
Leg 1 - Ref: 2.49V
Leg 2 - Anode: 0V
Leg 3 - Cathode: 2.51V

IC202 - Optocoupler K1010 817 (tested in circuit with DVM in diode mode)
Leg 1 - Anode to Leg 2 - Cathode, 0.482 both ways
Black probe on Leg 4 Collector, red on Leg 3 Emitter, 0.600
Red probe on Leg 4, red on Leg 3, OL


IC202 - Optocoupler K1010 817 (Applied 5V with bench PS to 5V line)
Leg 1 - Anode: 5.13V
Leg 2 - Cathode: 5.10V

IC202 - Optocoupler K1010 817 (Connected mains plug, disconnected bench PS, connected dummy load to 5V line)
Leg 3 - Emitter: 0V
Leg 4 - Collector: 1.60V

IC201 - PWM Controller LD7575 (Connected mains plug, connected dummy load to 5V line)
Leg 1 - RT: 1.37V
Leg 2 - COMP: 1.63V
Leg 3 - CS: 0.11V
Leg 4 - GND: /
Leg 5 - OUT: 0.37V
Leg 6 - VCC: 13.23V
Leg 7 - NC: 0.11V
Leg 8 - HV: 300V

I would like to measure the switching frequency on pin 1 - RT of the PWM Controller LD7575 with the oscilloscope, but as the scope is not transformer isolated I cannot do this when board is connected to mains.
I found a possible workaround:
"Than, there may be something wrong with the feedback circuit: a good trick is to apply an external adjustable DC voltage to the SMPS output (the SMPS being not connected to the mains). When gradually increasing the DC voltage, you should see the feedback circuit working when you cross a threshold near the nominal output voltage. Since, while doing this test there are no dangerous voltages involved, you can easily use an oscilloscope to diagnose the feedback circuit. You may also have to supply the controller IC (on primary side) with the same low voltage source to see what happens on the other side of the optocoupler. "
source - http://www.giangrandi.ch/electronics.../smpsfix.shtml

Would this work if I inject(with bench PS) around 12V to VCC pin of PWM Controller LD7575 and 5V from some other source to 5V line and measure the switching frequency on RT leg with oscilloscope?

[Sushi1808]

Re: Prestigio P3221W 22', power supply problem

I tried changing the dummy load from a 12V 21W car light bulb to a 12V 1.32W PC Fan and measure the 5V line.
It was steady 5.01V (with previous load it was 4.60V) and the 12V line was around 20V.
I took my chances and connected the power supply board to main board. I got healthy 5.10V on 5V line and 19V on 12V line.
The LCD is working again.

I think the definitive moment of this "repair" was when I connected the 12V 21W car light bulb as a load on 5V line and after I re-plugged to mains, the light bulb started working steadily, also the 12V fan on the same line.

Can someone explain what is happening here, was there a short which cleared itself because of the heavy load?

[Porto]

Re: Prestigio P3221W 22', power supply problem

aren't there any cold or broken solder joints on the board?
Nothing that is broken can heal itself so expect it to be solved temporary.

and... 19v on a 12v line?

[Sushi1808]

Re: Prestigio P3221W 22', power supply problem

Yes I know, I certainly can't sell this monitor as the condition will likely reappear. But now I don't know what to do as the monitor is working flawlessly and the 5V voltage is perfect, I reckon all the measurements would be correct as the output voltages are. Is it possible the 19V was there because no load was on 12V line? I resoldered the power plug connector as it was jerky but other than that everything looks ok, I tried to cool down the board with cold air and shake it in attempt to recreate the fault but the thing doesn't want to stop working.

Tomorrow I will open up the thing again and re-solder the whole board as the broken joint explanation is very plausible, since the problem first appeared after I, not so gently, disconnected the needed power cable and it could be possible some broken solder joint on a PS board disconnected because of the mechanic shock.