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]

On 07/02/2014 16:53, 2phar wrote:

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.

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

 

[Michael Black]

On Fri, 7 Feb 2014, 2phar wrote:

On 2/7/2014 8:57 PM, Phil Allison wrote:
"2phar"

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.


** 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





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?

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 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.

** 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]

On 2/7/2014 8:57 PM, Phil Allison wrote:

"2phar"

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.


** 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

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?

 

[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]

On 07/02/2014 18:08, N_Cook wrote:

On 07/02/2014 16:53, 2phar wrote:
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.

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

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]

On Sat, 8 Feb 2014, 2phar wrote:

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.

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

 

[2phar]

On 07/02/2014 18:08, N_Cook wrote:
On 07/02/2014 16:53, 2phar wrote:
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.

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

OK..
to follow up, I ran the unit on the bench and monitored temps with IR
thermometer. Nothing on the PSU board really got more than slightly warm
(under 40C), but the main ASIC (Realtek RTD24282RD) on the controller
board really warms up.. it gets to around 70C before monitor shut down
after 10-15 mins. I bent a piece of aluminum and applied it to the
surface of the IC with some heat transfer compound, and the monitor then
seemed to stay in operation much longer.. over an hour. Eventually shut
off, but on restarting it again stays on for a long time, rather than
less than a minute.

None of the other components are getting as warm.. 50-60C max. One of two
22uF elect caps on the board (near the connector from the power supply
board) is getting warm but not hot.

Could the ASIC develop an overheating issue by itself, or would something
else be more likely?

 

[Jeroni Paul]

Could it be it had a heat sink attached and it fell? Or is it expected to be mounted with the IC in contact with some metal frame?

 

[2phar]

On Mon, 21 Jul 2014 08:18:28 -0700, Jeroni Paul wrote:

Could it be it had a heat sink attached and it fell? Or is it expected
to be mounted with the IC in contact with some metal frame?

Nope.. it's a surface mount device within a metal cage with about 2 cm
clearance between it and cage wall, and no fan. Only cooling is some
perforations in the cage and fins in the sides of the back case.

I'm curious if the controller ICs in other LCD monitors typically run
this hot (pretty much too hot to touch) and without heatsinks.

 

I'm curious if the controller ICs in other LCD monitors typically run

this hot (pretty much too hot to touch) and without heatsinks.

In my experience, no. Look at the power supply lines with a scope and look for noise. I'd still suspect the filter caps here.

Dan

 

[2phar]

On Mon, 21 Jul 2014 09:37:10 -0700, dansabrservices wrote:

I'm curious if the controller ICs in other LCD monitors typically run

this hot (pretty much too hot to touch) and without heatsinks.

In my experience, no. Look at the power supply lines with a scope and
look for noise. I'd still suspect the filter caps here.

Dan

I've actually replaced all the caps with Japanese low ESR, with the
exception of the 450 volt cap. I will see if I can get a scope on the
supply lines.

 

[Jeroni Paul]

2phar wrote:

I'm curious if the controller ICs in other LCD monitors typically run

this hot (pretty much too hot to touch) and without heatsinks.

I have seen ICs run hot as you describe and they seem designed this way, but you should check all its supply voltages. If you find its datasheet it will tell you the operating temperature ranges so you know if it is getting too hot.

The on-off thermal stress over time may have broken some really small solder or PCB trace and it is failing under heat. Fixing that may be difficult if not impossible and a practical solution to be tested is to install a small fan on the cage with a small heatsink on the IC, with luck it may allow it to run enough time to make it useable.

 

[2phar]

On Mon, 21 Jul 2014 09:37:10 -0700, dansabrservices wrote:

I'm curious if the controller ICs in other LCD monitors typically run

this hot (pretty much too hot to touch) and without heatsinks.

In my experience, no. Look at the power supply lines with a scope and
look for noise. I'd still suspect the filter caps here.

Dan

Well.. don't have a scope to check for noise right now, but I was
checking voltages with the DVM on the controller board..

I notice that the supply voltages are all present regardless of whether
the monitor is on or in standby. And the controller IC pretty much
reaches the same high temps even in standby.

There are two regulators on the control board:
a G1084 that is taking in a solid 5.0 volts and putting out 3.44 volts
steady.. I gather for the service manual it should be 3.3 volts - (would
3.45 be high enough over to cause a problem?),

and a 912T45 (fixed 1.2 volt regulator) that has the 3.44 volts on the
input.. and curiously when the monitor is in the ON state, the output is
a solid 1.2 volts, but when off, it is about 1.75 volts on the output.
When initial power is applied from cold (with monitor in standby), the
output voltage from the 912T45 starts at about 2.1 volts and slowly over
a few minutes drops to 1.6 - 1.7 volts and stays there until monitor is
switched to ON state, and returns there after monitor shuts itself down.

Can't find a schematic for the interface/controller board unfortunately.