Computer smps not work properlly what problem

More than likely a bad voltage power rail, balanpv38.

Also more than likely caused by bad Electrolytic Capacitors inside the Switched-Mode Power Supply.

Radial Aluminum Electrolytic Capacitors.

Capacitors in the Output Stage, more than likely.

Capacitors are used as Filters, and Voltage Regulators. In the Input Stage of the SMPS they are used as voltage regulators. In the Output Stage they are used as filters.

Filter the rectified AC voltage, which is now DC. They make the DC voltage 'clean'. No ripples, clean DC voltage.

http://en.wikipedia.org/wiki/Switched-mode_power_supply

The two large blue sleeved Radial Aluminum Electrolytic Capacitors, you see marked with the letter B, can store Up To 100 Volts.
They are in the Input Stage.

IF, your fingers touch the terminals (2) on the bottom of the capacitor/s, YOU could receive the stored charge. Result is a Bad shock to FATAL.

IF, your fingers complete a circuit, that one or more capacitors are in, YOU could receive all of their stored charge. Result is a Bad shock to FATAL.

Capacitors need to be PROPERLY discharged before working on the unit.

This is hwy I cannot guide you in this endeavor. I will not guide you with the slightest possibility of you getting hurt.

Suggestion is to replace the Power Supply.

What can make a Power Supply go bad?

1) Low quality electronic components inside.

A) Low quality Radial Aluminum Electrolytic Capacitors
B) Low quality Power MOSFET/s
C) Low quality Rectifier Bridge
D) Low quality wiring due to the gauge of wiring used is too small.

2) Power Supply, (And inside of computer), is dirty inside.

An SMPS (Switched-Mode Power Supply), has two cooling components inside;

Heatsinks, and a small internal fan.

In the Wikipedia link of the open SMPS above, there are two Heatsink's used in this particular example;

One is in-between the letters B and C, and one is in-between the letters C and D.
Rectangular finned shapes connected by a vertical bar.

The Heatsink absorbs heat from whatever object it is placed against.
The rectangular shaped fins absorb the heat, then radiate it away.
The small internal fan's air flow helps carry heat away from the fins.

If in-between the fins, and around the fins, is clogged with 'Gunk', the cooling capacity drops Tremendously.
('Gunk' = Dirt, dust, lint, hair, etc)

Heat = Wasted Energy
The Power Supply cannot keep up with the call for power, and components inside the Power Supply fail.

Regards,
joecoolvette