> Thanks for all of the suggestions. However I think you
> guys are missing the problem. Both repeaters work fine when
> they are not linked together. As stand alone machines they
> work fine and the audio sounds fine. Once I turn on the
> secondary port on the NHRC-4/m2 is when the noise occurs.
Do you hear the hum on both repeater Tx's, or just the slaved repeater? And
does it matter which Rx the user comes in on when the hum is heard?
I took a quick look at the NHRC-4 schematic. One thing I see is that they
use polarized caps on the Tx audio outputs. The op-amp that drives those
outputs is centered at half the supply voltage (i.e. about 6.9 VDC in a
13.8V world). The polarization of the caps is with the + toward the op amp.
If you're driving the mic hi line in a Mastr II, it's going to have DC on it
to power the stock mic element. You should lift the resistor that supplies
DC bias on the exciter; you don't want the voltage from the radio to be
higher than the 6.9V on the other side of the cap.
Maybe you mentioned it and I didn't catch it, but are you feeding
discriminator (vol-sq hi) audio to the controller, or something else? Are
you using PL? If so, how/where are you filtering it, and how/where are you
encoding it?
Do you have speakers hooked up to the M2's? If so, do you hear hum when
there's receiver activity?
If you physically disconnect the PTT line from one of the transmitters so
that it can't transmit, do you still hear hum?
--- Jeff
