This is a success story on how I repaired 4 IR wireless handsets I gave up on 10 years ago.
They all work now. I bought all these off Ebay over 10 years ago. I suspect the reason I got them so cheap is because they were not working right in the first place. So, these are the worst of the worst.
Repairs were all done on the back of the board. It looks like they (China) didn't clean off the flux after they mounted the thru-hole parts. Dendrites are fine crystalline hairs that form in humid conditions using residue underneath layers of old flux. They lead to shorts or resistances that mess with microprocessors. In short, it's poor workmanship that leads to infant mortality.
The two pictures below, one is a controller where one pair of batteries goes dead overnight - powered off. Another is the IR receiver signal line that only works for 5 minutes after being turned on - then it disconnects.
After repeated cleaning cycles of scraping off the old flux, rinsing with solvent alcohol, then using compressed air were these cleaned into reliable operation. Keep doing those steps until all the white sticky residue is gone.
The backs of these units were held on by 4 screws (longer ones to the battery case side.)
It's also important to observe ESD precautions. If you are not set up to work on sensitive electronics, do the work on 50%+ humid days, wear cotton, and ground yourself to a metal cabinet or pipe that is also grounded.
The defects found are usually the controller going nuts like a three year old child running the joystick.
A different problem is the unit shutting off when bumped. While you have these apart, clean off the battery contacts with alcohol. If that doesn't work (like if a battery leaked out), use a small file to get some shiny metal. Wipe on some machine oil to keep the filed contact from rusting. Be sure and also check the removable 'Lid' contacts for any corrosion.
Missed one. After reassembling and testing, this one worked but had a strange problem. I had to press hard on the joystick right and it always jumped several locomotives each time.
Opened it back up finding I missed some dendrites shown below. Cleaned it and it works OK.
On a different controller, I had cleaned these battery contacts. It worked two days ago, but not today. I successfully tinned the positive terminal, but the negative spring was too far gone. Filing took off half the spring.
So, I covered it in solder braid, draped over the spring. The solder braid is soldered to the PCB.
After going through all my Dynamis stuff, I thought I was seeing things. I really have TWO dynamis control boxes?? They both work OK. I also have two of the EZ controllers with the dial that can only talk to 9 DCC locomotives (ID numbered 1-9).
I have no clue where I picked up the second Dynamis system. I was probably buying more hand controllers for $30 and they threw this in for free.