Hello and welcome to Bleepingcomputer.
I am thinking that you may have a faulty contact in the power switch.
I do not know if you have any electronics back-ground at all, such as experience with using a VOM (multi-tester) or soldering iron.
If it was me who had this computer in my possession, I would remove the circuit board from the computer case.
I would use the VOM meter set to either continuity or the Ohms reading mode.
When a VOM meter is set to this mode, the meter is partially connected to one side of the VOM meter's battery, while the other half passes through the meter's probes.
If there is a connection between the probes either through the touching of the probes together, or by the probes being applied to a complete circuit, the battery power flows through the circuit and applies power to the meter, thus causing it to move across the scale.
With this being explained we now have an idea how to use the VOM meter to check circuits for problems.
This meter allows us to check a switch for problems.
Applying the meters RED and Black probes to the two solder lugs, we can find that there is no meter fluctuation or deviation, thus we have an open circuit. This is the default position of the power switch.
If we press the power switch in, the switch closes the circuit, thus we should now have a result on the meter, showing a current flow. The meter should respond and move across the scale.
If the switch is functioning like it is supposed to, this should be the result. However, if the switch is bad, contacts are dirty or noisy, the meter may respond with fluctuation that is intermittent.
You should also check the condition of the plug you said looks like a IDE cable plug as well.
The wires going into this plug could be loose where they are crimped into the lug.
The connector can also be victim of contact corrosion. This can happen over time, causing a faulty intermittent connection to the pins found on the motherboard.
Those connectors can also be expanded and be larger than the pins they are supposed to be making a connection to. This would mean that the connector is sitting over the pin, but since it is wider in size, it does not make actual contact with the pin it is supposed to make contact with.
Sometimes I will take one of the probes from the VOM meter and gently pry the connector into the center a bit, so it is a bit smaller in circumference. This reduces the size and makes it a tighter fit with the pin it mates to on the motherboard. Be careful not to over do this, because if you make the hole too small, the pin won't be able to enter the hole and this will end up bending the pin on the motherboard or cause it to brake off!
Also, you can use the VOM meter to check continuity between the circuits on the circuit board that holds the switch to the connector at the other end of the multi-wire connector that would connect to the motherboard.
This would help determine if the board or the connector has a faulty connection.
Putting one probe in each of the connectors holes and testing for continuity on the circuit board should help you determine if there is a fault in the various circuits.
If you find the switch to be dirty or defective, you may be able to replace just the switch and save the rest of the circuit board with out total replacement.
Switches are usually universal and can be found at any electronics parts store, or can be salvaged from another piece of scraped electronics or computer.
Edited by MrBruce1959, 20 September 2010 - 11:11 AM.