[arm-allstar] High pitch hum on audio

[john gnatowsky]

Hey all...need some hive mind input.
 
 
The layout...
 
Pi 3b+, 2 X ra-35, one cm108 modded.
 
220 repeater (on ra 35) built with pair of tait tm-8100's.
 
Uhf half duplex link. Moto cdm750 (on ra35)
 
Gmrs repeater (on modded cm108) built with pair of moto cdm750's.
 
The problem...
 
There is a high pitch hum on both tx and rx audio on the LINK ONLY. Both repeaters are perfect audio.
 
What I've tried....
 
Grounding! I've run extra grounding to radio, pi, interface. No change at all.
 
Usb ports. I've changed usb ports/towers with the link interface.
 
Changed radios, cables (usb and harness).
 
Rerouted all cables and power wires.
 
Swapped ra35's.
 
Changed power supply to pi.
 

 
None of this has had any affect at all! While testing i noticed that manual key of link radio (grounded ptt wire at ra35 connector without interface connected) and the tx audio is clean. I've injected signal in from service monitor without audio and noise is there. The only thing i can think of is the pi itself? Anything else i can try?
 

 

 
John "Jason" Gnatowsky
 
KG4FJC
 
434-841-9290