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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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#1
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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? |
#2
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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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#3
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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. |
#4
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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 |
#5
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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. |
#6
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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. |
#7
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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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