[EdHolland]

It would be easy to combine or fit the dip meter with one of the frequency counter modules that are available these days...

[TV MAN]

I made a little progress today. I made a PC board with socket to make changing osc crystals easier. Thanks for the input on the coils. Since I dont have a dip meter, it will probably be some trial and error. I am trying a 1 meg crystal to start out with.

   
   

[jcassity]

if 35z5 says his works,, i trust it.

ironic ,,, i was actually wanting to do a project like this.

[TV MAN]

I had a little time to work on things again over the weekend. I decided to tie all the grounds via wire and solder to eliminate the chance of a bad ground through a loose screw or nut. I also got an inductance tester via Ebay to check the values of my coils. My antenna coil reads 78 uh which is very close to the value specified on the schematic. It is not tunable, but I have quite a bit of range on the capacitor.

[TV MAN]

I have had a couple of afternoons to work on things. I started doing the ground system, then the hard wiring, small parts then some of the bigger parts.  I had to think on the larger parts and moved some things. I used shielded audio cable from the input to the level control, and used the same from the antenna tuning cap to the output terminal. Here is my progress:

   
   
   

[Eliot Ness]

Looking pretty good Tim

[TV MAN]

I finished the ac input section, installing the line caps, fuse clips and power cord. Did a few resistance checks from B+ to ground and around, then an initial no-tube power up to about 50v while monitoring current draw. Flat as expected. Then plugged in the tubes and osc. crystal followed by a slow power up while closely monitoring voltages and current. So far so good - no smoke, no firecrackers. I tuned a nearby radio to the 100khz frequency thinking I would hear a carrier. Nothing. huh... so I dug around and found some needed cables and connected a portable CD player and powered up again. Imagine my delight when I turned the dial on my old Philco to the 100khz spot and heard my CD playing! Then I went downstairs to a cranky digital radio and heard it there too. I had only a couple clip leads coming off the antenna out and ran it off my isolation transformer/variac. It sounds fairly strong. That's where I quit for the time. Going to document resistance and voltage measurements before I go much further.

   
   

[KCMike]

Nice job Tim. I've been following along.
Might try to build one in the future. Looks like fun.

[Phlogiston]

Do I have to have 4 VOMs to build one?

[TV MAN]

Russ, you might need 5 lol  I like to monitor EVERYTHING when I am powering up for the 1st time. I really don't like fireworks in the radio room.

[TV MAN]

Ok so I have been experimenting some. I can  adjust the antenna cap for stronger and louder, but during silence I get both a 60 and 120 cycle hum. I also get a rushing sound like water running. That is on a 12 ft wire hung between a shelf and blind. I don’t know just what I’m looking for in tuning. Highs are flat but plenty of bass.

[TV MAN]

Tom, could you please repost the schematic in post 17 of this thread? I am comparing notes... 

Thanks.

[TV MAN]

Sam, in post 18, are you saying you are connecting the antenna to each post in a loop (shorting the terminals together)? How long of antenna are you using?

[35Z5]

Tom, could you please repost the schematic in post 17 of this thread? I am comparing notes... 

Thanks.

[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.