I just picked up an MX-600 with an MRF-100 base.
Right out of the box, the MRF-100's status light remains a solid red as soon as it's plugged in.
I removed the antenna and moved the unit around the room, same thing.
I tried using aluminum foil to shield the antenna and casing, same thing.
The only time I've been able to get the status light to go out is if I put my hand over the top of the MRF-100 near the antenna, or I block the emitter ports with my hand.
Here's the strange thing: It works perfectly with the MX-600.
However, the unit runs very warm. I temped the top of the case with a Raytek gun, and it's a constant 95 degrees F. The bottom side of the case runs at a constant 110 degrees F.
Is this normal, or will it burn itself out if it continues to run in this condition?
As I mentioned, it works flawlessly with the MX-600, but it does concern me.

Are you using a denon receiver? I used mrf 100 one time. As soon as the denon kicked on it the units receiving light turned on. The second I turned the denon off it went off. The cabinet I was using looked like swiss cheese after I drilled holes to move the mrf. It quickly became known as th MFR 100. If you want rf solutions at a good price go to nextgenerationhomeproducts.com They have a bizarre system that works very well.

Thanks for the link jarnold.
No, I'm using a Klimo Merlin tube preamp that has a remote volume control.
The IR eye on the Klimo is extremely sensitive, and the MRF-100 initially overwhelmed it with IR signals, causing it to act erratically.
I have the MRF-100's blaster window partly blocked to tone it down, and it seems to do the trick.
The status light remains on regardless of where I move the MRF-100, whether everything in the room is powered off or not.
It actually works fine even though the status light stays on.
My concern is that the MRF-100 runs warm as mentioned.
I've used RF base stations from One For All, Mastermind, and SIMA, and they all ran extremely cool compared to the MRF-100.
I'm just wondering if the heat is normal, or if it will burn a resistor or internal component out in a short time.
Since the MRF-100 isn't RF channel addressable, I'm wondering if it's internally set to Channel 0, the test mode channel, and it's sensitive to any RF signal that it sees?