LCD monitors have a low power switching regulator operating from about 360 volts DC (rectified AC power). Typically there are two resistors connected in series that take standby power from the 360 volts, this power is used to switch the LCD ON. These are high value resistors, about 470K each, resistors do not like high voltage across them, it causes them to degrade and go higher in resistance, sometimes a virtual open circuit. Their heat dissipation is well within rating at about 69mW, but they have voltage stress 24 hrs a day.
When a LCD display has problems switching ON or refuses altogether go looking for these two high value resistors connected to the positive side of the high voltage input capacitor and replace them with THREE resistors to reduce stress eg if the original is 2 x 470K replace with 3 x 270K or 300K. Two of these resistors are straight replacements, the third is a little different. On the underside of the PCB locate the copper track that connects the two series resistors and cut it, next solder the third resistor to the two solder points either side of this circuit break.
Fixed for a few cents worth of components.
This failure is the result of design error where it is not realised that there is too much voltage stress on the resistors. The problem occurs in many products that use the same design. I have found this problem in VCRs.
When a LCD display has problems switching ON or refuses altogether go looking for these two high value resistors connected to the positive side of the high voltage input capacitor and replace them with THREE resistors to reduce stress eg if the original is 2 x 470K replace with 3 x 270K or 300K. Two of these resistors are straight replacements, the third is a little different. On the underside of the PCB locate the copper track that connects the two series resistors and cut it, next solder the third resistor to the two solder points either side of this circuit break.
Fixed for a few cents worth of components.
This failure is the result of design error where it is not realised that there is too much voltage stress on the resistors. The problem occurs in many products that use the same design. I have found this problem in VCRs.
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