Marathon K5209 treadmill, motor not strong enough

Hello. thank you for reading my problem.
My treadmill is a Marathon K5209 .

When I want to start the treadmill the belt was not running. The motor also not running.
I found a broken transistor and diode on the electronic board.

After I replaced the diode and transistor I started the treadmill again and again the transistor and diode blowed out.
So replaced the transistor and diode again and disconnected the motor and connected a lamp to the motor output from the board. The lamp burned and burns brighter when I speed up. So that looks good

So I assume the motor has a shortcut. I took an external power supply and connected the motor with it.
The motor was running a litle noisy but with low speed the motor stops when you step on the belt.

I connected the motor back to the electronic board again and I forced the belt to turn so the motor was turning also.
then I started the running program and the motor start running.

So when the motor get some help then it start spinning. If the motor not get any help it will not turn and create a shortcut what blows out transistor and diode.

But now the belt is running it is still not functioning because when I step on the belt it stops running.

So I would think that the motor is broken. The brushes are still good and the machine is only 2 years old.

Would there be any other reason why the motor has no power to turn the belt after you step on it?

The motor is a 3HP and the belt is not to heavy to turn.

Thank you

gr. Alan