Perusal, Synthesis, Bliss

October 15, 2015: repairing the power supply of a LCD screen ACER AL2216W


This screen dates back to about 2006-2007. The symptom I had is the longer and longer time to power on the screen: the ACER logo appears when pushing the ON button, but disappears almost immediately to reappear several seconds later, and so on, during up to 10 minutes, until it get powered on for good. But one day the logo never appeared at all. The problem is failing capacitors, as reported at various places: here, here, and .
To disassemble the monitor, I have followed the explanations here; it was not really complicated. Then I replaced myself the three electrochemical capacitors on the low voltage side, of brand “CapXon”, surrounded in red on the following picture:
I first replaced the top one as advised at stage 16 of this: a simple test (charging it through a resistor with a power supply, and looking at the time constant) shows that it is less than 100µF instead of 1000µF: it is far too less for the power supply still to work.
With this new capacitor, this still does not work: the proof is that, (i) keeping the power supply on the monitor (contrary to the picture above) (ii) connecting the 220V power chord (be careful; this means that there is 220*sqrt(2)=310V on the high voltage side: you may die if touching it: touch it with a voltmeter instead) (iii) measuring the voltage between +5V and GND on the right side connector, we measure 4.95V. Now if the “power on” button of the monitor is pressed, it decreases to 4.15V: far too less.
Seeing this I have also desoldered and replaced the 2200µF capacitor surrounded in red (bottom one); a test similar to above shows that its capacitance has not decreased a lot. Then I changed the right one: instead of 1000µF, it was about 200µF. After this last change, we get 5.05V when the power chord is plugged in, and the same value when pressing the power-on button: it works! And indeed after re-assembling the whole monitor, everything works fine again.

Notes for unsoldering

Capacitor that charges itself

I have had a big surprise when short-circuiting the dead capacitors, while measuring their voltage. Indeed the voltage goes back to zero, but when releasing the shortcut, the voltage rises again to some value; and self-discharging currents seem to have a very limited impact, because one of the old capacitor, though having been short-circuited several weeks ago, is charged to more than 1V! This is because it is not an ideal capacitor, there is a FEM inside that charges the capacitor "from the inside": as soon as the short-circuit is released, open circuit voltage rises (unfortunately when short-circuited it is really equal to zero, otherwise this would be a perpetual motion occurrence). According to here, this effect is not limited to electrochemical capacitors (in such capacitors an electrochemical reaction is going on), and already caused electrocution of electricity practitioners.