I really like my Hornby A4 but recently it became unusable, DCC constantly dropping out. It has loads of pickups, but a prod around under the hood showed that there was no circuit between one side (the left?) and the chip. I dismantled it and found that the pickups on the loco transfer current to studs, with no solder or anything - the pickup strip is sandwiched against the stud by the underframe moulding. I soldered some thin wire on and ran it round and up to the chip, that has fixed the problem.
My word, though, this is a fiddly blighter. If it was in a Haynes manual it would have five spanners and a bloody microscope. I am staggered that I seem to have managed it without bending the connecting rods or breaking the underframe mouldings. I guess it's worth it if the running is restored to normal, this loco drive model is worlds apart from the tender drive A3.