For example, the B+ line of a monitor suppose to deliver 50 vdc but it only shows 20-25 vdc. All the other output voltages at the secondary side dropped half.
Faulty electrolytic capacitors with high ESR value in the primary power side could cause all the output power to decrease. Do you know that a defective current sense resistor may also cause output power to drop? Yes, the value increased in the current sense resistor could cause output voltages to drop into half.
Normal multimeter can’t detect such low ohms resistor value (0.1-0.68 ohms). You can get the dick smith esr meter to measure accurately the value of the current sense resistor. If you do not have the meter, just directly replace the current sense resistor and retest the equipment.