[TV MAN]

The project progresses. I have spent quite a few hours tracking down the source of a hum. Mainly it is a full wave 120 cycle hum with a 60 cycle hum in the background. I'll take a few to go through the steps I did to test, track down and eliminate the hum. I am still working out bugs and fine tuning, so any input will be welcome. I spent quite a bit of time looking inside and for a solution in my construction, the design, different antennas, even tried in a different house on a different power supplier. That hum followed me wherever I went. I experimented with grounding the audio grid of the 6888, still hummed. So, that eliminated the preamp. I fed the signal directly into the grid via a cap, and found there was enough gain that this could be used without the preamp, but it really is a plus to have that extra stage. 

I could still get the hum even when I disconnect the power cord, while the tubes still warm until the caps discharge. So, I didn't feel it was in the power supply or filament supply. But, just to prove my theory, I did some experiments. I read in the ARF Homebrew section that another member had a similar problem. He claimed he switched the B+ take offs between the 6888 plate, G2 grid and the supply for the 6AV6. So I did the same. Didn't help. In fact, then I started having modulation problems and distortion. So I switched it back. Next I tried different types of grounding. The AC supply ground really didn't help much. So I ran a ground to my tower ground rod. That helped for a radio close to the transmitter, but not distant radios. 

I decided to verify if my power supply was bad by adding capacitance. Nooope. I went so far as to build a "breadboard" power supply using half wave rectification to rule out my full wave power supply. So - the result? Full wave hum with a half wave power supply. Nope, didn't help. I also tried powering the 6888 filament with another, separate transformer. Noooope, still hummed. So, my next attempt - I built another output stage using a simple diagram, different layout, different parts. I powered the oscillator from an old 5v phone charger, hopefully to eliminate any possible feedback into the cathode or G1. I used my Isotap unit as the power transformer, another 6v transformer to feed the filament. Did it work? Yes. Did it hum??? Yes.  Oh, for laughs, look at this contraption! Actually, it was kinda fun to build and easy to experiment with. I found the AM radio antenna coil worked better than the 1mh choke. I used a tuning cap from a solid state radio, which worked pretty good. I could peak the antenna. 
This was built using Norm's diagram (ARF). 

     

Okay - what next? Gotta think outside the box on this one. 2 separate transmitters, different layouts and builds doing basically the same thing. My brother (a ham operator) mentioned that some years ago, while he was house sitting, that he brought his setup out here and had nothing but trouble getting it to work properly. When he brought it back to his house, things worked fine. So, I started grounding this and that, ran a cord from our pole direct to the transmitter, then started flipping breakers in the house. Bingo! It appears there is some type of interference from something in this house. I killed a number of breakers and finally hit one and the buzz went almost silent! Then as I continued, I found another that virtually eliminated the hum. I even went back up to check the transmitter to see of it was on or not. It was. And so was the test radio. I found I could turn on all but 2 breakers and keep the hum silent. One would make it hum a little, and tracked that one to a couple phone chargers and our television. The other really cranks the hum, but I haven't found the source yet. Unfortunatly, this breaker goes to my work bench. Killing the switch on the bench doesn't help, so I am going to have to do some hunting. I have tried moving the transmitter to a different room and on a different plug, different directions on the antenna. Still hummed. When I peak the antenna (according to my scope and ears), the hum also comes up. Here is my output waveform. Thanks Tom (35Z5) for your ideas on things to check!

   


So, what if I cant eliminate the source? Any ideas aside from killing that breaker when I want to use this transmitter? It seems to me the problem is external, not inside the unit. I did try a "EMI" filter, along with my .01 mfd line caps. I don't know if somehow RF is getting into the electrical system causing this hum, returning to the transmitter, of if it is mixing somehow in the air. I am open to suggestions.