At 10/20/2008 07:21, you wrote: > > 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.
If it's had +12V applied to its audio output for some time, it's possible the capacitor is already damaged. I have an NHRC-micro that's had nearly half of the chip capacitors replaced due to failure. The last one shorted & vaporized a supply trace as well as a small part of the circuit board itself. Oddly enough, the output coupling cap is still OK, but the 12V mic bias was removed from the MVP before the controller was installled. BTW, while we're talking about coupling capacitors, Digikey has some really tiny non-polarized ceramic capacitors up to 47 µF @ 16 V: <http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=445-2905-ND>. I've used the 10 µF value of the same capacitor series for squashing some nasty EMI problems at work. Their ESR is way lower than standard electrolytics, & they were able to totally knock out power supply noise that the electrolytics didn't even touch. >Do you have speakers hooked up to the M2's? If so, do you hear hum when >there's receiver activity? ...but if you DON'T have speakers connected, have you terminated the speaker outputs into 4 or 8 ohm power resistors? I've seen Mastr II audio PAs go bonkers when those outputs aren't terminated. I usually disable the entire audio PA by cutting off it's A+ supply so the termination is no longer necessary (I never use the speaker output). Bob NO6B
