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.
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.
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
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.
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?
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'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.
I have seen ICs run hot as you describe and they seem designed this way, bu t you should check all its supply voltages. If you find its datasheet it wi ll tell you the operating temperature ranges so you know if it is getting t oo hot.
The on-off thermal stress over time may have broken some really small solde r or PCB trace and it is failing under heat. Fixing that may be difficult i f not impossible and a practical solution to be tested is to install a smal l fan on the cage with a small heatsink on the IC, with luck it may allow i t to run enough time to make it useable.
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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