I know this is a relatively old thread, but I am a new user and just read this topic today.
The idea of fixing buttons on a remote has always been interesting to me, and I recently repaired three MX-850 remotes that all had partially or non-working buttons. In all cases, they were the volume up and down and the channel up and down buttons. It seems that on an MX-850, these are the first to go.
Like Ken H, I went with the aluminum foil fix. I used simple household aluminum foil. I cut out tiny squares, and after opening the MX-850s (which was no easy task and requires controlled brute force,) I simply used scotch tape to affix the aluminum squares to the protective pad inside the remote. The plastic pad is a simple covering put in the remote by the manufacturer, and I find it works well for the purpose at hand. It is relatively easy to tape the aluminum squares securely to this pad. I made sure the tape does not come between the plastic buttons and the aluminum. The aluminum is secured on that pad pretty good.
Bottom line: the remotes have worked like new for about five months (I know I am going to jinx myself by writing that ;) . . . the volume and channel buttons work great again.
Do I think this will last forever? Well, probably not. The aluminum could wear out, or the adhesive on the tape could lose its grip. Or who knows, I might get lucky. It seems to me that the adhesive is the key. While Scotch tape might seem like a relatively simple solution, I find the tape affixes very well to the smoothness of the pad inside the remote. I've also thought that perhaps copper or gold foil would be another alternative. There is copper tape with conductive adhesive for sale on Amazon . . . of course, I have no idea how long the adhesive will last.
I can appreciate mdavej's point about the relative futility of tin foil repairs. But as someone who likes to tinker and see if I can fix things (especially a high-end remote like the MX-850), I don't mind trying possible fixes from time to time. So far, they have worked, and I'll be on the lookout for better options if my current fix eventually fails. There's always another way.
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