I have tested the 25D metal gearmotor with encoder.( item #4840
I gave 5V for the motor voltage, 10V for the Encoder vcc.
After I checked the encoder output channel A and B, I found quadrature signal on channel A,
But I can only see the 10V, constant, all time high, signal on channel B.
I connected the encoder ground- channel A for differential probe A, oscilloscope,
encoder ground - channel B for differential probe B, oscilloscope.
I also have checked with alternate motor(I bought 2 motors), it showed the same result.
I was wondering if you let me guide to get the proper signal on channel B
Have the B channel signals ever worked on either motor?
It is difficult for me to follow all of the connections in your pictures, but the paths generally look okay. However, there are several connections between the wires included with the motor and your probes, so one possibility is that some of the pins on your breadboard, wires, or connectors are somehow damaged and not making good connections. Can you try connecting the probes more directly to the included wires to try and eliminate that possibility?
Can you also remove the end cap from each of the motors and inspect the encoder and the wire connector for any signs of physical damage? Please post some pictures of that here.