Asus switching drop

[jasko_jacker]

I have a asus power supply that with a load , drop from it's nominal voltage to 16 V. I haven"t a schematic so my question is: it makes sense to open it, it's an easy fault to detect ?

[stj]

Re: Asus switching drop

the question cant be answered till you open it.
you may see caps with the tops burst open - easy fix.
or it may look normal - and take some time.

[jasko_jacker]

Re: Asus switching drop

Hi, thanks for replay. I have opened it and I have desolder all six 1000 mF caps in the output stages, no sign of faul externally, I have measured them with a meter and all have the nominal capacity. I read that it's possible that there is a fault even the capacitance is correct and they must be tested with appropriate equipment, it's right ? I have a mega328 component tester and I will try to check them with it to find more data (but this isn't a professional equipment)
I'm checking all resistor and non electrolitic caps but I don't know if this can help to solve this issue

[jasko_jacker]

Re: Asus switching drop

Caps test:
WCO Vent LoweSR 01/19 1000 mF

1 - 997 0.8 Ohm .5%
2 - 985 0.4 .5%
3 - 1005 0.8 .6%
4 - 1007 0.6 .6%
5 - 1007 0.6 .6%
6 - 973 0.5 .8%

[stj]

Re: Asus switching drop

those esr numbers are crap - they should really be more like 0.05 Ohms or better for low esr,

[CapLeaker]

Re: Asus switching drop

Change those 6 caps with new ones. The ESR on these is too high and should be more like what stj said.

[jasko_jacker]

Re: Asus switching drop

I'm very sorry, I repeated the mesurement and about the electrolitic capacitors I made an error to report values,
the measurement are 0.0X .
I'm studying recently how the smps works but
I think this is a very simple circuit, TL431,BJT,Zener,resistor and capacitors to
drive the optocoupler to provide a feedback.
I have tested individually each component of the
cold part of this power supply and I have only a dubt about tl431 that I test only with a meter in diode mode and not with a test circuit.
When I connect a load, on the anode of tl431, the voltage drop
from 2.5 V to 2.0 V it should be insufficient to operate this shunt regulator. Without a load the voltage remain stable to 19V. If I remove the load the voltage not return to 19V
Can I increase the capacity of C6 or replace it (I repair notebooks so I don't have this type of spares), could this component be the problem?
I still have to test the optocoupler by unsoldering it. But with a tester and i place
I think it's ok

[jasko_jacker]

Re: Asus switching drop

I have removed the NPN, and without it seem that the voltage is stable.
After that I substitute it (1AM) with BC846 but the drop returns.
I have no experience with oscilloscope and I have avery entry level usb unit
but I connect ch1 with opto IN and ch2 with opto OUT.
It's a case that the voltage remain stable without transistor ?

[jasko_jacker]

Re: Asus switching drop

error