Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Default Benq G2420HD power supply heat issue?

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?
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Default Benq G2420HD power supply heat issue?

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?
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Default Benq G2420HD power supply heat issue?

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.
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Default Benq G2420HD power supply heat issue?

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
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Default Benq G2420HD power supply heat issue?

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.


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Default Benq G2420HD power supply heat issue?

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.
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Default Benq G2420HD power supply heat issue?

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.
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