It has a 425E network and when depressing the hook switch, it first reduces the volume of the dial tone like it should (though not by as much as it should), and then when fully depressed, there is a prominent popping sound.
I have another WE 500 that also has a 425E network, and when it reduces the volume of the dial tone, the dial tone becomes so quiet that you can barely hear it, and then when the hook switch fully depresses, you can't hear a pop at all; completely silent.
What's causing the difference between those two phones? Is there a component in the network that can go bad and cause it to not be able to mute the receiver as well as it's supposed to? I'm not even sure what component is responsible for muting the receiver when hanging up, but I figure it must be in the network, because where else could it be?
The phone works normally aside from the popping sound when hanging up.
The hookswitch red and black wires are connected in parallel with the receiver wires on terminals GN and R. They short out the receiver before the line is opened, in order to reduce the loud pop in the receiver.
The red wire from the hookswitch is connected to the GN terminal. It's in parallel with the green wire from the handset modular connector, as well as with a white wire from the dial.
The black wire from the hookswitch is connected to the R terminal. It's in parallel with the white wire and the red wire from the handset modular connector, as well as with a different white wire from the dial.
I've attached a recording of the sound of hanging up (which I did slowly so you can hear how much reduction there is in the sound before completely hanging up) along with a recording of the quiet WE 500 for comparison. I've also attached images of their waveforms.
I did an experiment where I directly shunted the receiver with a piece of wire screwed to the two screw terminals (see attached picture) and it made the receiver almost 100% silent. I couldn't hear the dial tone at all, and when depressing the hook switch, it made only the faintest sound imaginable when it reached the hangup point.
Based on that, it seems that the only possible explanation here is that my "quiet" 500 has a lower resistance shunt going on when hanging up than my "popping" 500 does, so I tested it:
The resistance between the GN and R network terminals when the hookswitch is depressed is only 0.6 ohms on the "quiet" 500, whereas it is 6.1 ohms on the "popping" 500, ~10 times the resistance.
The "quiet" 500 is older so it has the older cloth-insulated wires from the hookswitch. I don't know if those are heavier gauge wires than the newer, pathetically thin, PVC-insulated wires or not. They obviously appear heavier from the outside, but I don't know what the gauge of the conductors are.
The shunt circuit consists only of 2 wires and a leaf switch, so the additional resistance can only be caused by the wires and/or the switch and/or their connection to the screw terminals / receiver wires. I've already sprayed the hookswitches with electrical contact cleaner but I just did it again for good measure, along with loosening the GN and R terminal screws and retightening them, and it got down to 1.5 ohms. It's a lot quieter when hanging up, but still not as quiet as the older 500.
I would have liked to test the resistances of the two hookswitch wires by themselves as well as the leaf switch at the points that the wires attach to it, but that area isn't really accessible with meter probes, not while the hookswitch mount is riveted down, that is.