Can I connect directly from the audio output port of the MRX to the music input on my PE-501 intercom with an 1/8" phono stereo cable? I see that there's an audio cable available to connect into the headset cord, but I want to go directly to the "audio in" of my intercom.

Where is the audio output port located on the unit - I didn't see it in the user manual.

Yes, you can certainly plug the audio from the MRX directly into your intercom, but the plug on the MRX is a sub-mini 2.5mm stereo connector, so you may need to adapt. The audio port is on the left side of the unit towards the front, if looking at it from the front (about 1/2" back). It's on the same side as the antenna connector.

What are the signal connections on the jack? If they're standard stereo audio connections, then all I need is a male-male 2.5mm patch cord.

They are indeed standard stereo connections (tip, ring and sleeve). The patch cord you mentioned would work fine if the receiving end was 2.5mm as well.

I tried using a standard stereo patch cord from the MRX audio port (with mini adapter) to the audio input of my PM501 intercom, and I couldn't hear the warnings thru the headset. I tested the audio port with an MP3 player with a stereo cable and the port works fine.

I haven't tried the cable provided (headset cable), and will do that and report back.

But I'd really like to just plug it directly into the intercom with a simple patch cord, so I want to sort out why that's not working.

Is the audio output on the MRX wired out to the tip or ring (or both) on the jack? My intercom audio input is mono.

I can see through the plastic case that the jack is wired out (red wire) to a pin on what looks like an unpopulated test connector on the circuit card. Is it possible that the wrong pin location was wired out? What is the correct location - I can verify visually to rule that out. A white wire appears to pick up a return off another component pad.

Is it possible that the input impedance of the PM501 audio port is not what the MRX audio output wants to see? What impedance does it want to drive into?

I'd like to get this sorted out without having to return the unit, as I have a trip planned in a few weeks. Seeing as it's to Dallas, if I can't get it resolved for the trip down, I'll pop into your shop and see if I can get it looked then for the trip back.

OK, got it to work today plugged directly from the MRX audio port to the audio in on my PM501 intercom.

It seems the problem was (and still is) with the audio jack on the MRX. When I plug the stereo plug in, it tends to back out about 1/16" from the jack, and that opens one of the connections (likely the tip). I tried a bunch of stereo plugs from different manufacturers and they all do the same, so I have to assume it is a problem with the jack.

If I push on the plug, it works fine. If I don't, it backs out and then doesn't work.

I'll be in the Dallas area in a few weeks so will stop in and have it looked at then.