Connect the 10VAC to buzzers/sig pins. Connect A battery to A pins on Melco. Connect B bat to sig pins on Melco.
Disconnect the 25pr connector on the melco. On the power supply, put your buttset on the A Battery to A ground. You go off hook and should hear no hum. IF you hear hum in this condition, your power supply is bad. This is very likely due to it's age. If you know how, just replace the filter capacitor or get a tech to do it. Good luck
