I am a new member and would like to benefit from your collective knowledge of the S&W model 39.
At one time the trigger seemed to be hard to squeeze, a possible obstacle inside the firearm.
Lately I noticed that the trigger has no spring tension when in the cocked phase. I also have a model 52 and it's trigger is perfect.
I suspect that the internal secondary spring is broken, but I have not discovered any broken pieces.
The firearms diagram is not very clear to see exactly the parts that may need replacing.
Living in Canada getting it to S&W for repairs is a puzzle.
Your help will be appreciated.

Can you explain the phrase "the trigger has no spring tension when it's in the cocked phase"? .

If the trigger will return to the forward position after completing a double action trigger stroke while unloaded, the trigger return spring is functioning as designed. The trigger return spring is in the frame forward of the trigger. You should see a plunger & possibly some spring pressing against the top front of the trigger when the slide is off the frame.

if the trigger is "floppy" when in single action (cocked), but will drop the hammer when pressed, I suspect the trigger play spring is broken. This is a small spring attached to the trigger bar that holds the trigger bar in contact with the trigger. It's not all that difficult to fix, but does require detail stripping of the frame.

You might contact S&W customer service to see about service. If you give a location in Canada, some members might know someone local who may be able to help you.

BTW, the model 39 isn't part of the M&P series. This post really should be in either the S&W Semi Auto Pistol or S&W Gunsmithing section.