OK, here we go. You'll need a DMM (Digital Multi-Meter) and a small Phillips Head screwdriver.
If you don't have those handy and know how to use them, then you'll either need to borrow them and learn to use them, or just bin the controller now.
First you'll need to take the thing apart, and there are 6 screws on the bottom of the controller for that.
After that you can see if the Magnet is there or not. If it's not there, odds are you'd hear it rattling around inside the controller, unless it's become stuck to the Right Trigger's Magnet.
Now, if the Magnet is there, then you'll need to pull the board and have a look at the Hall Effect Sensor. There are 2 screws that need to be removed before the board can be lifted out. These are the only 2 that need to be removed, the other 2 screws hold the Stick/Magnet setup in place, do not remove them, only the 2 that are marked.
After those screws are removed you can lift the board out of the top half of the controller shell. You're looking for HED5 over on the left side of the controller, that's the Hall Sensor for the Left Trigger.
First make sure it's there, if it is not there then that's a red flag for sure and that thing has been abused a bit and I'd have to look around a bit for a replacement as I've never had to install one before. If you know it's been knocked around a bit but everything is still where it's supposed to be, check over the board on both sides for any cracks, as that can cause traces to break and that will for sure cause issues.
If the Hall Sensor is there, then make sure that all 4 of it's connections are soldered to the board, just use a magnifying glass and compare it to the one for the Right Trigger.
Now, if the Magnet is there, and the Hall Sensor is there, then we need to see if the thing is getting power. Plug the controller up to the DreaCast and turn the power on, then use the DMM and check the voltages on the Hall Sensor. Be careful too not short things out while you have the power on and are measuring the voltages, and there's nothing in there that will shock you either in case it's a concern.
If any of those voltages are off, like way off, then it's an issue with the Op-Amp circuit or some damaged trace. You may have 5v or so instead of 4.7v, that's fine, but if you have 3v there instead of 5v then it's a problem.
The 4.7v spot is for power, it should be around 5v or so, mine here was 4.77v, which is fine. The 2 other spots are for the Hall Sensor's Poles and should measure around 2.5v each, again mine were 2.4v so they're good. If either of those are wrong, then the Resistors in the Op-Amp circuit need to be checked, specifically R22, R23, R24, R25, R26, R27 and R31 to make sure they are the correct values. If they are all OK, then it's time to check the Op-Amp output on pin 7, which I'll get into in a bit.
The way these things work are the Hall Sensor changes how much Current flows thru it depending on where the Magnet is at, then the Op-Amp converts that Current change to a Voltage change, then the MCU on the controller can measure that Voltage change. Complex? Sort of. More so than it needs to be, but Sony also later did the same thing for the Sticks on some of their PS3 controllers also. Considering a Potentiometer will, and does, the exact same thing it's interesting why they went this route, but it doesn't ever get dirty and scratchy like a POT will either, and in most cases unless it's subjected to abuse or it's time is just up then it will last far longer.
Here's an older schematic I made awhile back of the Left and Right Trigger's circuit.
If the Voltages on the Hall Sensor are correct, then you need to see if the Op-Amp is working. Measure the Voltage on pin 7 of IC4. This is the Output of the Left Trigger circuit before it goes to the MCU. It should measure around 0.9v with the Left Trigger untouched, then it will go up to around 2.7v with the Left Trigger fully pressed. You can compare this to the Right Trigger side by measuring pin 1 of IC4 and it will swing from 0.9v to 2.7v when the Right Trigger is pressed.
If the Voltage changes on pin 7 of IC4, then check it again, but measure on pin 64 of IC1 this time.
If the Voltage does not change when the Trigger is pulled then there is some damaged trace or Via between Pin 7 of IC4 and pin 64 of IC1 that can be fixed with a jumper wire.
If the Voltage does change the same as the measurement made on pin 7 of IC4, then the MCU is most likely bad and it's now a parts controller, except for IC1 which is bad.
