Help with Hamilton

I would pull the transmitter down and try it at ground level to make sure it is working correctly. Then go after any gremlins in the system.

It could be that the ambient noise floor is getting into the rcvr..

Are there any AM stations around that can be picked up in your area? If so, See if the interference that your hearing is with this station also. If this is the case, It's going to take a very strong signal to override the noise..

Have you made a test outside of the campus area.. Maybe 1/4 mi. away from the buildings? If it is set up correctly, You should be able to hear it that far away. This takes you away from all of the noise generated within the buildings. Or, take a portable radio with you and test right at the xmtr location. It may sound a little "Fuzzy" from overdriving the receiver but the noise should not be there unless it's getting into your audio chain..

Changing frequency may help. The only way to know is to try..

other AM stations in the building come in fine, with no buzzing
like my broadcast has. do you think that if the lightning supression
system's little 6 inch tall 1 inch wide spike antenna is 5 inches
away from it and running parelell to it that the rf is being absorbed
by it and channeled into the buildings earth ground? im going to
go try the 1620 crystal i have, i hope it helps. I think my ground
is crap also, my ground wire runs along a metal roof, touching
it, then goes into this electrical box where the guy from maintence
says it runs down to the earth inside the builidng along with
some other wire... i dont know what kind...

how could the noise get into my audio chain? there is a ferrite
ring around the transmission (power/audio) line where it comes
out of the dc adaptor and sound board and TWO (cause i had
em) where it goes into the transmitter. the cable is sheilded, i think
i may have to talk to maintence to make sure the cable is unbroken
all the way from the station to the xmitter. also, when i unlplug
the audio source from the sound board the buzzing can still be
heard when the xmitter is on, but when you unplug the xmitter,
the buzzing stops and all you can hear is the chattering from
very faint signals from other stations. it sounds like the xmitter
is broadcasting this buzz, but only when youre on the 3rd
floor (remember, the xmitter is on top of the building, there
are 4 floors in the buiding)

requires an organized, systematic approach.

1. Get the transmitter off the roof
2. Set up an accessible test location where you can isolate possible problems
3. Use a simple antenna for testing
4. Use battery power for the transmitter to eliminate power supply issues
5. Start by using a very simple audio source like a battery powered device placed in a metal box.
6. Your first goal should be to produce a clean RF signal, then a clean audio signal, then work on distance, then fine tune.
7. You will probably get better results with a transmitter and antenna on the ground but away from noise sources than you will get from a system mounted high in the air with serious noise source issues. For example, the metal roof, the lightning system, the ballasts from lights, the electrical system in the building, PC network infrastructure and even dimmer switches are all the enemies of a clean signal.
8. This is a web forum, not an IM screen. Use punctuation, break up your posts into readable paragraphs and provide clear questions to maximize your chances of getting useful answers
9. Keep trying

Experimental broadcasting for a better tomorrow!

Thanks for the ideas, i will do them in the order you suggested.

the only question i have is in step 6 of your instructions;
what exactly do you mean by fine tune?
Do you mean in the case of the rangemaster to peak the voltage w/ the jumper and the variable capacitor?

Also when you said try to increase your range are you
referring to things such as adding a good ground plane?

Thanks

MD

Sounds like the culprit may be a ground loop.

Ground loops are an audio chain problem that can happen when you ground at more than one point. Theoretically, they're both the same "ground", but often they'll actually be at a slightly different potential. The most common cause being that the ground wiring in the building represents a conductor of usually unknown length between each piece of gear and true ground. That makes a big "loop" made up of all that wire which acts as an antenna which can pick up anything, but in practice will mostly pick up hum off the nearby AC line and put it into your audio signal since the "baseline" of a theoretical zero volts is not actually being zero volts but a low level AC wave.

Now, in cases where there is an actual difference of potential (voltage) between two "grounds" it *can* be enough to damage gear or present a shock hazard.

In most cases though, it'll just be a heck of a lot of hum, sometimes also with a wee bit of any local large AM transmitter sneaking into the mix.

There are a couple simple ways to deal with it. Basically you want to avoid long cable runs and make sure your whole chain only grounds once. In a small studio that can usually be managed by running everything off a breaker strip or two off one outlet box and keeping all cords and cable runs short. Not always practical for the real world, though, so you may need to look into getting a "loop lift" or "ground isolator" box. You can usually find those from any supplier for recording gear or there are plenty of circuits out on the net if you look around a bit.

It's a very common problem with small home recording studios, and not all that hard to fix.

I'd say bring your Hamilton in off the roof for a test, and plug *everything* in your chain into one breaker strip or two breaker strips going into the same outlet box. If the hum is gone or much less, then I'd say it's a ground loop issue and a loop-lift or ground isolator on gear other then the transmitter will most likely do the trick.

Since you have to ground once, make it at the transmitter end (for rf reasons). Use the remedies on the other gear in your chain.

Daniel

by MRAM 1500 kHz

I recently assisted with the install of a licensed Advisory AM station for our city. The equipment was provided by ISS.

The antenna is located on a metal deck, flat roof (2 story) building. The transmitter is located just below inside the building. There's plenty of HVAC and electrical equipment close by.

Now even though the equipment and cabling is good quality stuff mounted in an enclosed rack, one misplaced audio wire left the audio feed unbalanced and we got squealy-hollow sounding transmit audio until that was corrected.

Even though the antenna SWR was only 1.3/1, enough RF got into the audio input to cause the feedback.

My point: like the others have advised, check the obvious, it could be a loose wire in the mix.

This Advisory system runs 2.5 watts into an antenna that looks like it could be Part 15 compliant. It is center loaded with an adjustable stub at the top for tuning. The loading coil is only about 2" in diameter and about 2' long. The metal roof deck serves as the ground plane. You can see the antenna at the ISS home page.

With that 2.5 watts it covers a 5 mile radius with no problem. I reduced the power out to less than 100 mw to see how well the antenna works and although the signal got spotty coverage you could still hear it almost as far.

I was just going to say that it sounds like unbalanced audio. If the source (computer) is unbalanced, I would urge you to buy a balanced to unbalanced device like a Henry Matchbox http://www.henryeng.com/matchbox.html or similar device. You at least need to get a 10k to 600 ohm transformer with a center tap to do the same task, you can get those from Sescom http://www.sescom.com/audio/mi_series.html like the MI-96 or MI-77

The Rangemaster uses a 600 ohm balanced input. In my case, my Alesis limiter output is unbalanced, so I used a "Clean Box" to convert the audio into balanced 600 ohms. It's available through Broadcast Supply Worldwide (www.bswusa.com). If you have a ground loop, you might need a ground loop eliminator. BSW has those, as well.

Frank
www.easthillradio.com

what exactly do you mean by fine tune?
Do you mean in the case of the rangemaster to peak the voltage w/ the jumper and the variable capacitor?

In the case of #6 from scwis, fine tune is more of a process step than a specific adjustment. It's an important step when trying to optimize the performance of any complex, interdependent system. First get everything working, then establish a base line, then do what ever is needed to improve performance.

It's called fine tune because the amount of improvement over the baseline is generally small. After a while you reach what is called a point of diminishing returns where more and more effort is required to get less and less improvement.

That's where you know you've done enough, and you can stop 🙂

Peaking voltage is a good example, because you will get a baseline reading once everything is working properly, then you can tweak the audio and note its effect on the voltage, and adjust the transmitter and note that effect on the voltage, and keep working back and forth until it just doesn't get any better.

Also when you said try to increase your range are you referring to things such as adding a good ground plane?

Yes, an enhanced ground is very helpful to improving range and might be one of the more important efforts in that regard.

If you are located on a college campus the Part 15 regulations are slightly different as far as your field strength. You can be a bit more creative as long as your signal is within Part 15 field strength requirements off the campus.

Experimental broadcasting for a better tomorrow!

Daniel,
Do you still feel this might be the problem even if the
hum/buzzing is present even when the audio signal
is not plugged into the transmitter? When I turn the
transmitter on, the test radio just starts buzzing like
crazy. Plugging the audio source in does nothing, I can
get the audio to be heard in the test radio but not
loudly and certainly not clearly.

do you still feel it might be this ground loop?

thanks,
md

Nope. If it's present even when the audio chain (with it's ground) isn't connected at all, then ground loop wouldn't be what is causing it. It was worth checking though, since they're a common source of "mystery" ac hum, and fairly easy to fix.

Next thing I'd wonder about is the power supply. Is there any way to hook the transmitter up to a battery to see if the hum is still present then?

Daniel

well, thats a shame. just to be clear, when i said the audio wasn't
connected, i did not mean the 2 wires at the transmitter, i meant
the 1/4" stereo jack coming from the aux send on my mixing board
wasn't plugged in.

anyway, about the power supply, its supposed to be 12v but when
i tested it with my volt ohm meter at the end of the wires it was
showing something like 21 VDC.

as far as testing it with a battery goes, where would i get a
12v battery? is that a common voltage? the last time i tried
broadcasting w/ my old ramsey am25 transmitter the problem
was strikingly similar, and it turned out to be a crummy power
source. this power source im using came with the transmitter
though, so i thought it'd be ok.

any further thoughts?

md