The first place to look is the charger output while flexing the leads. Now what can happen is you can be "fooled" into thinking things are OK. Suppose only one strand is connected. You will read the right voltage, but the adapter won't work.
you will then need to test the adapter under load. A couple of 12 V lamps in series would work.
One of the other likely post in the power in connector and that's why a lot of PC's use a separate board for this. Since you have it open, you can check for power whill wiggling the plug.
Used parts may be available here :
www.pchub.com which I have used.