Aoc e2070s No power

[Sraros]

I have this monitor, it doesnt power up.
The voltage on the connector CN902 is only 4,5V instead of 19V.
With a quick test I see the voltage on pin 6 of transformer T902 is going crazy from 150 to 250V (tested in DC mode).
Diodes and Q901 seem ok. Any suggestions?

[dick_barton]

Whatd dc voltage do you have between the two legs of the large 47uF 450V capacitor

What is the part number on U901

[Sraros]

300V steady

Q901:SMK0465
U901: AS358M

[dick_barton]

That voltage appears low considering it's a 450V capacitor.

[lotas]

Originally Posted by dick_barton

That voltage appears low considering it's a 450V capacitor.

In this circuit 300v. the norm, there is no PFC here.

[lotas]

Originally Posted by Sraros

300V steady

Q901:SMK0465
U901: AS358M

U901 cannot be AS358M (LOW POWER DUAL OPERATIONAL AMPLIFIERS).

[lotas]

Here is a diagram that might help you figure it out.

[dick_barton]

What voltage do you have across C922

With your meter set to diode mode and the set switched off have you checked D901 and D904

[Sraros]

D901 is ok. 339 and infinity. D904 doesnt exist, but whatever, I guess it was just to split the load.
@lotas U901 well, it is AS358M, look at the photo.
C922 is jumping from 12,3V to 15,1V!

[lotas]

@lotas U901 well, it is AS358M, look at the photo.


AS358M is U801 and U901 is PF6003AG (sot23-6).

[Sraros]

Originally Posted by lotas

@lotas U901 well, it is AS358M, look at the photo.


AS358M is U801 and U901 is PF6003AG (sot23-6).

Yes, sorry! You are totally right only that it’s PF6003AH

[dick_barton]

I found a reference on the internet that said the PF6003 can be a replacement for LD5532GS

The data sheet for the LD5532GS says that when capacitor C922 voltage reaches 16V then the IC starts to operate and drive the mosfet. This initial voltage to charge C922 comes from the resistor chain R924/925/939/937/926

When the voltage across C922 reaches 16 volts the mosfet is driven and the feedback winding on the transformer T902 maintains the voltage on C922 via R920 & D903

I would check those components especially C922

It also says on page 12 of the pdf that the IC can be turned off if the COMP pin of the LD5532 is taken below 1.5V. The Comp pin would be the FB pin on the PF6003

[Sraros]

So, I checked with an LCR tester the cap C922 and is ok. ESR 2Ω for 4,7uF 50V.
Also all the diodes seem ok.
So I checked on voltage mode as many components as possible. The voltage between ground and C922 is steady -80V (black lead in ground). Everything looks steady in general except the drain, which ofc is attached also to the leg6 of the transformer, and it makes me wonder maybe Q901 is not working right and causing the fault...

[lotas]

80 volts is a lot.
The voltage on the C922 must be measured on the capacitor itself, this power supply has different gnd (one in the primary, another in the secondary).

[Sraros]

Originally Posted by lotas

80 volts is a lot.
The voltage on the C922 must be measured on the capacitor itself, this power supply has different gnd (one in the primary, another in the secondary).

Yes right between the legs of C922: trembling from 12,5V to 15,1V

I read again also what dick_barton said. Now I understood what you meant and how it works

[Sraros]

I swapped the capacitor C922 with a new one and still the same.
The diode D903 is S1M.
https://www.vishay.com/docs/88711/s1.pdf

Can I replace with a diode 1N4007 for testing?

[dick_barton]

The diode is probably an Schottky diode due to the high frequency of the transformer switching. The 4007 is a general purpose diode not suitable as a replacement.

[dick_barton]

Test the diodes (D902, D903 & D905) using your meter in diode mode. Two readings per diode by reversing the leads for the second reading.

Have you checked Q901. Plenty of Youtube videos showing how to.

Did you check the resistor network I mentioned since if they have increased in value then the initial VCC voltage for U901 will not be correct.

[Sraros]

I checked the diodes they are all fine.
MOSFET Q901:
I followed instructions from youtube
When it’s in the PCB:
OFF state S-D infinity, charging the gate and then S-D still infinity.
But if I take out of the PCB the MOSFET, and charge the gate then S-D: 340

The resistor chain is fine each one of them