Benq G2420HD power supply heat issue?

[2phar]

Hi all..

I have a BENQ G2420HD 24" LCD monitor.

Display backlight seems good, etc. But unit shuts down completely after
about 20-30 mins (no backlight, power LED goes out). Holding down power
switch at this point will cause unit to come back up after about 30-40
seconds. Then it will die again within 5-10 mins, but can be restarted
the same way.

I've already replaced all the elect caps on the power supply board.

Is the power board still the most likely problem here? Any suggestions
as to how to pinpoint the cause would be most welcome.

[N_Cook]

IR thermometer, targetted narrow jet freezer spray or hot air through na
silicone tube

[Phil Allison]

"2phar"

** The fault condition is being brought on by rising temperature - right ?

So you need to apply heat ( ie hot air) SELECTIVELY to various parts of the
PSU until you find the area that is most sensitive. This could take quite a
while.

I have an amplifier module ( from an ART 322A) here that reacted to the
application of hot air by going silent. The cause was a number of cracked
vias ( plated through holes) in the PCB going open when the board became
warm.

The particular vias are about 0.25mm dia and not filled with solder.

Unusual fault and a nightmare to find and fix.


... Phil

[2phar]

Yes.. it sure seems like it's heat related. I redid the compound on the
heatsinks and stuff, and nothign seems extraordinarily hot when it shuts
down. I have gone and got an IR thermometer as N_Cook suggested, so I'm
gonna see if I can track it down.

I'm curious as to why its possible to get it to restart within 40
seconds and it keeps going for several minutes again, if it was due to
some thermal expansion or thermal shutdown, that does seem kind too fast
a recovery?

I also wonder if it could be a fault elsewhere like in the inverter
board that's able to cause the whole power supply to shut down?

[Michael Black]

Only if it's causing a load on the actual power supply which cause the PS
to shut down.

Are you sure you got all the capacitors?

I dragged a 19" Acer monitor home, and it worked fine, or so it seemed.
But if I left it on, it would go into standby and then I couldn't get it
activated again, the Acer logo would go on and nothing more. I changed
the obvious capacitors and it's fine. So it was resetting because one of
those bigger electrolytics in the power supply wasn't quite right. I
nitially thought it was something else, but changed the capacitors because
it was easy.

Michael

[Phil Allison]

"2phar"

>
> I'm curious as to why its possible to get it to restart within 40 seconds
> and it keeps going for several minutes again, if it was due to some
> thermal expansion or thermal shutdown, that does seem kind too fast a
> recovery?

** Nothing unusual in that.

Temperature rises according to an exponential curve, fast at first and then
slower and slower until it settles completely.

It also falls the same way.


.... Phil

[N_Cook]

If you use a standard hot air gun then low temperature of course. And
you have to divert off some of the air, via a silicone rubber tube ,off
a silicone rubber cover or something with plenty of other holes in it or
you will block the air flow and damage the air gun probably and also too
much heat while it still works

[2phar]

On 2/8/2014 1:16 AM, Michael Black wrote:
>
> Are you sure you got all the capacitors?
>
> I dragged a 19" Acer monitor home, and it worked fine, or so it seemed.
> But if I left it on, it would go into standby and then I couldn't get it
> activated again, the Acer logo would go on and nothing more. I changed
> the obvious capacitors and it's fine. So it was resetting because one
> of those bigger electrolytics in the power supply wasn't quite right. I
> nitially thought it was something else, but changed the capacitors
> because it was easy.
>
> Michael
>

I replaced them all except the big 450V one.. mainly cos its not even
included in capacitor repair kits and everyone seems to say failure of
it is extremely rare.

[Michael Black]

It wouldn't. That one is just running at 60Hz (50Hz in some countries),
which requires less effort on the part of the capacitor. It can go bad,
but not in the same way as the capacitors on the output of the supply.

A switching supply is a feedback system, so if a capacitor goes bad, what
the problem is may not immediately be obvious. So I was thinking if a
small value electrolytic was overlooked, it might be more problem than
would seem from the small value.

Phil's comment about bad solder joints is good. You have to look
carefully.

Michael