The ICOM M502 was discontinued in 2006. The earliest date I find in the operation manual literature is copyrighted 2000. I guest it could be from 1997, but I would not jump to that conclusion. Take a look at the back and you may find a date. Either way, let's start by recognizing it was a $500 radio when it was new. It's probably worth a little investigation.
First step is really two parts. Yes, as mentioned above, you should unscrew the antenna connection and inspect it. Look at the radio side and the coax side. Look for corrosion, exposed wires, or a lose fitting. If the coax side of the connector looks wonky, they can be replaced. There are a couple of different approaches to antenna connectors. Some are multipart assemblies that push on and then screw down and make contact with the coaxial cable. Others are soldered. I understand the screw-on is preferred, but whatever you have check to make sure it is clean and tight. It isn't a bad idea to examine the entire length of the cable for any damage or splices that need attention or replacement. I have a spare antenna that I keep in the house that I can connect to my radio to see if that improves or cures the problem. This lets me know the trouble is on the antenna side.
The second part often goes overlooked and can cost you hundreds of dollars in new equipment that was unnecessary. Check the power line to the radio. Make sure the power is coming in at 13.8 volts + or - 15%. This comes directly from the manual:
- Power supply requirement:
13.8 V DC ±15%
Too often people think the power supply is fine because the unit powers up. They are wrong. Electronic equipment will power up with less than the required voltage, but won't work properly without the full requirement. And yes, oh yes, on a boat power can and is reduced through bad connections, incorrect wiring, and resistance across fuses that have corrosion. Get a multimeter and measure the voltage coming to the radio. If it is not coming in at 12 volts or better, you need to run a new wire. Always check the values at the source of the power if you are taking it from a fusebox or buss bar. You can run a new wire from a poor source and still have a low value because you were not connecting to 12V in the first place.
Try these things first and maybe, for an out of pocket expense of $5.00 to $35.00 (depending on if you need to buy a multimeter and a coax connector) your radio will be as clear as a bell without spending a single dollar on antennas, coax cables, or a new radio. It is well worth the time and effort to take these steps before you buy a lot of new parts and equipment.
Good luck.