Apply air or nitrogen pressure to the tube s, not exceeding the design working pressure, and swab the joints with a soap solution and watch for bubbles.
Any leakage will be readily apparent from both the pressure drop and the visible leakage (likely a spray), including the likely location of the leak. Finding and fixing will be much easier.
When fixing, just preheat that area with a fan-flame torch to burn off any dye without harming more fins that necessary before you start brazing process.
Reason for APH leakage :
1. Due point corrosion
2. Scaling inside the tube
3. Erosion in tube
4. Fire side corrosion.
