SSB on 3870. How dare you
It was humming last night. I got ran off the band. Its about 5 watts of hum on that carrier. I am suprized nobody has any solutions. I guess I will try a big cap on the B+ to that first tube. Maybe that will help.
Brett what do you mean about the diode and cap?
C
5 watts of hum on a 300 watt carrier, and people are complaining? Man, some people have SUPERB hearing!
I'd say you pretty much narrowed down the stages where the hum is coming from in your other post, where you pulled tubes and could get the hum to go away.
Check for wiring in the audio stages that goes near either the ac leads, or near a run to the filaments, since the radio has is AC powered on the fils.
Since the hum is there when you put shorting BNC's in the radio, I'd say that the first tube is where it's picking up the hum. Can you try putting a shield in place?
Wiring is king!!!! MAKE SURE you have the audio leads going to / from the mod / speech amp away from everything carrying AC or hw rectified DC.
Another thing you could try is balanced feed, but since it's getting in there even with the BNC's shorted, I HIGHLY doubt it would fix it.
Could be simply ground loops. All grounds TIGHT! I know you've fought them in auto installs, we all have. Maybe a GOOD ground run from deck to deck inside the chassis would help out. 1 volt of difference would cause a hum, methinks.
With a shorted BNC on it still humming (to the normal level), I'd expect it to be either wiring issues or something getting inside the first stage.
The other thing you can do is after the 1st stage, build a 60 or 120 hz notch filter, depending on the frequency of the offending hum.
--Shane
