Excuse my ignorance, I'm a complete noob. I got a great RC boat for my kids and it worked for a while, but then pins on the receiver went rusty and fell off, so I bought another, identical Airtronics 75MHz 3CH receiver. I've connected all the wires, everything powers up, but my MX-3 controller does not do anything, as if it has no connection with the receiver. I have then connected everything back to the rusty old receiver and it works, so it's not batteries, wires, etc. So, the question is -- is there something I must do on MX-3 in order to "pair" it with the new receiver?
Many thanks.

75MHz? You need to use the correct crystal.

Thanks a lot!
I assumed the crystal would be the metal thing that can be pulled out, so I simply replaced it with the one from the old receiver and it works!

Should I just leave it as is, or is there somehow a way to use the crystal from the new receiver, and would it even matter? it's not like it's a mechanical device that wears out over time...

Chrystals were a method for fine tuning the exact frequency (i.e. channel) and they come in pairs, One marked TX for the transmitter and one RX for the receiver. Both need to be the same channel number for the transmitter to be able to communicate with the receiver.

75mhz been gone for a long time now. Easier to go the 2.4mhz TX &RX System. May upgrade your servo & motor maybe a Lipo battery. You'll be fine. Its your kid. Get a cheap system like a Flysky FS-GTB 2.4mhz, you can find the on EBay approx 40-50 bucks. Good luck!!! 75mhz is ancient!!!

Got it, guys, thanks a lot!