If you have accepted a partial refund you have effectively closed the transaction, and cannot ask for it to be reopened, or ask for more money from the seller.
Shame the item wasn't fully examined and tested, then you could have refused the partial, and gone for a full refund with eBay.
Not sure if contacting your payment provider is of any help as a partial refund has been accepted, but no harm trying.
@rosjoe007