These openers are very cheap to buy, and are not considered serviceable. The only thing you can check are the battery contacts, and see if there are any cold solder connections.
Many of these openers, depending on the country of use, are working in the
512, 310, 410, or in the 427 mHz range. In different countries, there may be different frequencies allocated for these. There are some new systems coming out that will be using Bluetooth, which is in the 2.4 to 2.5 gHz range. The older systems from before the 1970's were working in the 27 mHz range.
They consist basically of a digital code generator, and a simple transmitter circuit. The output is of AM modulation in the lower cost systems. The more expensive remote systems are using spread spectrum technology, so that the codes would be much more difficult to read remotely.
The power output is usually in the area of about 10 to 20 mW. The range is designed for about 50 to 75 feet operation.