Annoying Audio Hum In Signal From XTL5000

[wiredwrx]

I am stumped, and hope someone from the braintrust can help me out. I have an XTL5K UHF High split radio, that I also use for ham, installed in the auto. While I am not very active on the ham band since my interest is kinda cyclical, recently, while using a P25 machine, I was informed that my audio has a heavy hum on it. I went about trying to figure out what is causing the hum, but to no avail. I determined something fairly strange about the behavior. I put the XTL in direct mode, and determined that If I key the mic, and don't modulate the signal with audio, there is no hum, but as soon as I modulate the signal, the hum appears, and does not go away for the remainder of the transmission, even if I stop modulating the signal. But, if I unkey, the nexy key up is the same, no modulation, no hum, modulation, then hum. Turned the engine off, and tried again, the hum is still present.. You should hear two transmissions in this audio/video clip

I then tuned to an analog frequency, and checked. With the car off, the signal is quiet. With the car running, I can hear an extremely faint alternator whine, which does change with engine RPM.

Thinking it was the microphone, I swapped through two other mics, same behavior, it hummed. Thought it might be the 05 head, swapped to a different head and CHIB and flex, still present. Swapped in a different brick, and different TIB and Flex, still present. This surprised me, because I figured it was the brick, but apparently it is not.

Brought the brick onto the bench, and connected to my PS, and wouldn't you know it, same hum. The only thing that remained consistent between the radio bricks is the codeplug (and perhaps the firmware etc., but I am just focused on one for now.) Here is the that info:



The only other thought I had was the AGC settings, (especially because of the analog signal alternator whine being present, thinking that the AGC is amplifying the alterrnator whine so it shows up in the signal, BUT, there is still a hum on P25 when on the bench supply) but I am fairly certain I am using the "recommended" settings I found on this forum. Here are my settings:



I am gonna throw them onto my service monitor and tune them up, just in case, but I don't think that's the issue.

Does anyone have any suggestions, ideas, or a fix?

Thanks,
Michael

 

[wiredwrx]

Well, I think I figured it out. First, I threw the brick on the service monitor, and ran through the tune, and nothing was out of tune at all. After, I check just to be sure, but there was still hum. Following my hunch about AGC, I went about checking that. I started first turned AGC off in both internal and external, and set the values to zero, and turned off noise suppression. Lo and behold, the humming stopped. I then added things back one by one. Once I added noise supression (Advanced) the noise returned. I then turned off internal and external AGC, and there was no hum. So, I started to experiment with what combination causes the hum. Turns out, noise suppression (Advanced) and Internal AGC don't seem to get along. WIth Extenral AGC on, and noise suppression on, there is no hum. I then kinda figured, and proved, that if Internal AGC for Digital is on, along with Noise Suppression, the hum occured on P25. So, for now, I have Internal AGC all off.

Does anyone know what Motorola means by "internal" mic?

 

[Colt45ws]

I would probably start looking at hardware. If you have the capability to use an o-scope and backtrace from the input of the gain circuits I would start there. I have started to see problems with the MOSFET-based switches used in several places on the tx audio circuit. They'll short to ground, or short the outputs together. Essentially there is one tx audio circuit and theres these switches that enables the radio to have the microphone input, the aux_mic and aux_tx inputs all go through that one circuit.
I guess I should ask since this is an O-head if its remote mount. The remote mount O-head converts the audio to digital to pass it over one of the CAN bus lines in the remote cable. Ive never looked at how much audio stuff is done in the CHIB and/or TIB.

 

[jry]

There are potentially two microphone sources
1. Control head
2. rear accessory connector ...may also be on the front TIB but the rear is where you will always have a connection. Usually used for equipment intercom /radio connect.

I usually never mess with those settings ...thought we had gone through optimizing that years ago and most codeplugs I use already have them set correctly.

Good post though .

 

[Astro Spectra]

The internal mic is an option where if you are carjacked and the perp cuts the mic cable or unplugged the mic you still have a live mic in the head that is enabled when you hit the concealed emergency switch.

 

[wiredwrx]

The internal mic is an option where if you are carjacked and the perp cuts the mic cable or unplugged the mic you still have a live mic in the head that is enabled when you hit the concealed emergency switch.

Ummmm, what? Really?