It doesn't seem too odd a model so wiring should be similar to 90% of other desktops. Use your other PC experiences here. I can't help with so little detail but did not find a service manual either.
Can you share the HP.COM page this is at for clarity?
I am not very good at computers and wiring so I need some help re wiring the inside of this pc it is a hp pavilion a640 and i cant find a manual online

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