TH main board mystery adjustment

Typically "L" is the designation for a coil or inductor, and of course an inductor is a type of coil.

Yor L300 is most likely a variable inductor, and is probably either part of a filter system, designed to reduce harmonics, or the antenna tuning system, designed to peak the output.

Unless there are instructions for adjusting it, it is probably factory set to the right setting.

Is it in a small metal can about 1/2" square?

On the schematic, L300 is part of the feedback loop for the PLL circuit.  Not positive but the adjustment seems to be used to set the center of the operating range.

I experimented with it when my first Talking House had an FM'ing problem.  We called it a "warble".  It seems to have a lot of latitude in adjustment but if you go to far one way or the other the PLL has problems locking in on the ends of the band.

I ended up putting a trimmer capacitor across the PLL reference crystal to "Net" the frequency and that got rid of the warble.

Yeah, that's what I thought it could be. I dug out an old electronics book and did some reading up... It's probably set at the factory right where it should be, and likely wouldn't do me much good playing with it.

To give you guys an update on my experiments, I have attempted several ATU placements in a few different heights and locations, and still could not get any usable range out of the thing... barely 1/2 a block in any direction, regardless of elevation. In fact, the audio quality seems to be worse with the ATU then with the standard wire antenna you attach to the chassis - the ground is good, the meter is at a solid 7 or 8, but I still get a lot of "picketing" even with three different receivers. I even tried different strands of coax, the result is the same. In comparison, the wire output attached to a simple vertical 4 ft long 3/4" copper pipe sounds just fine, no noise.  

There may be a problem with the ATU (it is brand new, though) but I'm kinda thinking that it may be a problem with the ATU's output on the main box... Typical issue with these things? Any way to test it and know for sure, maybe put a meter across it?

Thanks... MM

There is not to much to the ATU.  There is a torroid input transformer, two switchable loading coils, a meter circuit and some micellaneous capacitors and resistors.

The input will look like an open across the coax as there is a capacitor in series with the input transformer.  If you measure from the antenna rod to ground it should be a fairly low resistance as the selected loading coil is in series with the input transformer secondary to ground.

The resistance between the antenna rod and ground might change a little bit when you change the position of the band switch.

I don't know off hand what that resistance is but I wouldn't think its much.

I have a schematic for the ATU if you'd like.

It appears to me from my experience, you are receiving significant interference from another source or broadcast station. It is very likely skywave coming in from a distant station or a licensed station close by on the band. Above 1600 KHz, most licensed stations are at 10 kilowatts so they can have quite a lot of skywave, even during the day. Licensed stations 20 to 30 KHz from your transmit frequency can cause your receiver to de-sense and make the received signal to appear to be weakened. If one of the local stations down the dial from you has harmonic product on their signal, it could be mixing with your transmitted signal in your receiver to create the picketing effect. It's very likely your transmitter and ATU are working properly.

Did you try different frequencies?  Interference can cause what you're describing, but then, so could a defective ATU.  If you could try several others (frequencies that is), up and down the band, and not near any other strong stations, that would provide more information.

And how did you ground the ATU (if you did)?

I have been using 1700 for my tests, b/c it's a nice, even, easy-to-remember spot that sounds good with produced imaging. Would like to stick with an even freq of that respect, but of course, it won't do me any good if I can't get any decent range. 

It could very well be interference. There isn't much on 1700 to be picked up during the day, but at night it can get noisy. The tests I've done were both at night, way after sundown, and during the day... same results at either time as I described using the ATU. The wire antenna's signal was fairly clear, but only covered the area surrounding my building (as the system was designed for). The ground used with the ATU was a metal roof in all instances, giving strong readings on the tuning meter.

The closest adjacent active stations are 1640 at 5 miles, 1580 at 15 miles, and 1490 at about 13 miles. I used Radio-Locator to find everything within 200 miles, and I have a 1000 at almost 200 miles away, but 990 and 1010 at less than 100 miles away.

Too close? Even for daytime? And should the heritage "clear channel" freqs of the big stations be avoided, or does it really matter? 

Tough to say.  That's why I would test at several other clear frequencies, to eliminate interference as the potential cause.

Grounds can also cause interference, depending on what you're grounding to.  Are you using any sort of ground with the ATU, or leaving it ungrounded.  You could try both to see what happens.

You might also try another coax cable - maybe there's something wrong with either your coax to the ATU, or the connectors.

Unfortunately, sometimes it's just plain old trial and error that gets things resolved.  Trying out different things may lead to the root of the cause, which could end ultimately up being just a defective ATU.  And if you had another ATU, you could definitively say yes or no to that one.

Artisan said "grounds can also cause interference" and this is an idea worth exploring, especially since we are talking about a ground point that is ABOVE the ground, and itself connected to an unknown number of other electrical points.

You have a roof ground, I tried a basement I-beam, a 40-foot long steel beam that holds up the building.

The first time I tried clamping my transmitter ground to the I-beam I got plenty of buzz in the carrier. It turns out the I-beam has metal pipes, furnace ducting, and electrical metal conduits laying in contact with it. The stray voltages in the I-beam were inter-acting with the electric service connected to that transmitter.

Interestingly enough, I now have a different transmitter clamped to the other end of the I-beam and it works perfectly with no interference.

You might also try switching power outlets, if that's possible.  I once had an interference problem that went away when I did that - your transmitter is sharing that power with whatever else is on that circuit, and the 'whatever else's' could be very noisy electrically (computers are notorious noise generators, particularly at AM band frequencies).

Let's not forget ground conductivity. Even at these low power levels the quality of ground conductivity can have either positive or negative results. Maybe isolated ground rods seperate from the mains would make a bigger difference than just one ground rod or the grounding in the home wiring. Also there is a remote chance that the ATU has a problem. I have never had one but i assume it has screws and maybe you can check over all the contacts and solder joints just to be sure they are solid.

mram1500 mentioned he has a schematic which could be very helpful as a reference when checking for loose connections, bad solder joints. Good Luck