My HP 33C developed a bad digit in the display. Not completely out, it displayed the correct digit, but very, very dim. After disassembling the machine, I pulled the LED module (it just plugs in/out) and blew a dust remover air jet over the row of open sockets on the main board. I noticed a tiny bit of white fluff shoot out. Reassembling revealed a perfect display! I guess the fluff/dust/whatever simply caused a poor electrical contact!
Also, I've taken to using alkaline batteries in my HP's for fear of overheating during recharge, bad rechargable batteries causing electrical anomalies, line spikes while connected to the mains, etc. etc. The little double battery spring that comes in Woodstocks is easily reused. I simply saw out the center plastic bar on the battery pack, remove the old nicads and pop in fresh alkalines (remember to mark the + ans - terminals to make it easier in the future to change batteries). But the Spice machines do not use a similar metal spring. I've found the adhesive foil from the top of a juice can works great! Just peel, cut a small rectangle to fit, and stick it to the plastic wall opposite the battery contacts in the battery compartment. Pop in batteries and calculate while you enjoy some OJ.
cheers,
-John