If it's from leaking alkaline batteries, a Q-tip moistened with white vinegar will clean it up. Hold the calc oriented such that if there's any running or dripping, it'll run or drip
out of the calc, not in. If too much of the material is gone, that flex circuit is available brand new to replace the old one. It might be good to buy one to have on hand, but don't open the calc to put it in as long as it doesn't seem necessary. The reason is that there are posts that tend to break when people open it. There are ways to repair them, but why not just avoid the situation for now if the flex circuit is still good so far.
Replacement:
https://www.clonix41.org/Maintenance/IO ... _Block.htmvideo on how to install the replacement:
http://youtu.be/jUTAZSEn-H8forum topic for updates and info on fakes:
https://hpmuseum.org/forum/thread-15742.htmlThese links are from the HP-41 section of the links page of my site, linked in my signature line.