Hello Celo !
I think you should look here: http://www.museatex.com/bitbox.htm#bitboxUpdate
However, if you are not very, very experienced at soldering & esp. desoldering, I would urge you not to try to fix the unit yourself, since the double-sided circuit board, in which every through-hole component is soldered into plated-through holes, is fairly delicate, so that circuit traces can be quite easily damaged by inexperienced desoldering, esp. the top side traces. Also, the "collateral damage" components are most often surface-mount parts, which also require experienced hands. Please do send it to John Wright to do the work. The cost of the upgrade will also cover the repair as well unless something very serious has happened.
Kind Regards !