Acer H213H strange power issue

[jeanguy1980]

Very strange problem: monitor works for some days perfectly, then some days it won't power on at all. ( Completely no power).
I checked the cable, AC 240 input socket. All good. Capacitors look okay. I don't know what could cause this strange problem? thanks for suggestions or places to check...

 

[Nigel Goodwin]

You can't test capacitors visually, you need to test them using an ESR meter - it could be a duff electrolytic, it's by FAR the most common failing in switch-mode PSU's.

 

[jeanguy1980]

Thanks a lot for the quick feedback and suggestions. Unfortunately i don't have ESR meter at hand to test the Cs.
could faulty Capacitor cause this random power-on issue? sometimes it works, sometimes not. thanks again in advance.

 

[Nigel Goodwin]

Thanks a lot for the quick feedback and suggestions. Unfortunately i don't have ESR meter at hand to test the Cs.
could faulty Capacitor cause this random power-on issue? sometimes it works, sometimes not. thanks again in advance.

Yes, quite easily - a high ESR capacitor could be 'just on the edge' - so depending on various random factors might 'just' start up, or might 'just not start up. If there's a start-up resistor, that could have gone high as well (it used to be a pretty common fault), and again it could be 'on the edge'. A common issue used to be all was working fine until you turned it OFF (or there was a power cut), and then it wouldn't fire up again - because the startup circuit had failed.

 

[Nigel Goodwin]

I've just downloaded the service manual, the start-up resistors looks to be three in series (always a common cause of failure), R904, R905 and R906, all 10K. C908 or C939 would be good capacitors to check.

 

[jeanguy1980]

Thanks a lot.
But somehow I can't find those R C numbers you mentioned...could you please help pointing me to the right ones... thanks again

 

[Nigel Goodwin]

Looks like there's more than one version of that model then?.

 

[jeanguy1980]

hmmm..it appears so...what to do now?

 

[Nigel Goodwin]

hmmm..it appears so...what to do now?

Fault finding, for a start what voltages do you have on the rectifiers D701 and D702?.

 

[rjenkinsgb]

could you please help pointing me to the right ones...

Can you post a photo of the back of the PCB?

 

[jeanguy1980]

Can you post a photo of the back of the PCB?

Yes. Sure. Would this help? Thanks again

 

[jeanguy1980]

@rjenkinsgb​

what do you think? any light? thanks.

 

[rjenkinsgb]

It looks like the start-up resistors in that could be R608,609, 610, below the yellow transformer?

C605, the big green one, appears to the the input smoothing.

TR601/2 is a inrush limiter, "soft-start" NTC thermistor.

What voltage do you get on C605? It should be near 1.4x the AC input voltage.

 

[jeanguy1980]

Thanks. So I should measure the voltage on c605 two legs...it is supposed to be 220V ac times 1.4= 308 AC?

 

[sagor1]

Thanks. So I should measure the voltage on c605 two legs...it is supposed to be 220V ac times 1.4= 308 AC?

That would be DC voltage on the capacitor, not AC

 

[rjenkinsgb]

it is supposed to be 220V ac times 1.4= 308 AC?

That cap is fed via a bridge rectifier from the AC input (which rectifies it to DC), so it should charge to approximately the peak voltage of the AC input waveform.

The peak voltage of a sinewave RMS voltage is 1.414 (Square root of 2) x the RMS.

Some reference info:

Input and Output Voltage of a Full bridge rectifier

I know If we provide a sinusoidal AC Voltage across the Inputs of a full bridge rectifier we get a fluctuating DC Voltage (more or less) with the peaks of source AC Voltage. But what would be the p...

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