I have replaced faulty electrolytic capacitors in a large number of items ranging from Monitors, PVRs to set top boxes.
Disconnect the power and take the lid off the unit and check the various capacitors in the power section. Any that have don't have a flat top i.e. they are bulging to some degree or are leaking, are faulty and need replacing. Symptoms can vary widely, from unable to power up to various malfunctions when on.
I would recommend changing all the electro;ytic capacitors in the power section even if only one ot two look bad. Use Low ESR (equivelant series resistance) capacitors and if there is room try to fit the next highest operating voltage e.g. if 10V fitted try 16V.
Try Googling 'Bad Capacitors'. There is a lot of information about this problem on the Net.
This never ceases to amaze me; when talking imported products, white goods or electronics, they talk % defects however in automotive component manufacturing in Australia, we talk parts per million (1 ppm = 0.0001%).