A common problem with Motorola Minitor III pagers is that they stop alerting. They will receive just fine, but don’t alert when the correct tones are received. The root cause is that the printed circuit board that the tone decoder integrated circuit (IC) is mounted on flexes and causes the IC pins to break loose from the board.
To determine if this is the problem with your pager, squeeze it very tightly between the speaker and belt clip and see if the pager will alert. If it does, the fix is relatively simple, but does take some soldering skill.
Disclaimer: Performing this fix will most likely void any warranty you may have on your pager. It will also mean that the pager is no longer “intrinsically safe,” so don’t take it into any explosive environments. Perform at your own risk.
After opening up the pager and removing the guts, find the tone decoder chip (see the picture below). It will be underneath a sticky round cushion that you’ll probably need to scrape off in order to get to the chip. You may not be able to tell by looking, but chances are one or more of the pins’ solder joints have broken, disconnecting the pin(s) from the board. To fix this, simply heat up each pin with the tip of a soldering iron to reflow the solder and reestablish the connection. Apply a little bit of downward force with the tip of the iron while heating the pin up to force the pin to heat the solder underneath it. If you want to add a little bit more solder to the joints go ahead, but be careful not to bridge any pins with a solder blob.
After reflowing each pin, carefully reassemble the pager, and hopefully you’ll be back in business!