For all you radio transmitter experts out there.

[Dan C]

This is the 2nd year I've used a Ramsey FM25B transmitter. I've noticed this year that it is having trouble locking onto the channel. I hear the music pretty good, but occasionally it has some static over the radio. I checked the connections and everything is tight. The red led on the front seems to be solid, but then I noticed when I hear the static, the led flickers slightly.

I have the power cord plugged directly into a wall outlet and the transmitter is located in a second floor room of my house, the same room I have my computers in.

The wall adapter power lines are not shielded. Just insulated wire, so I installed a Ferrite core from Radio Shack, but that didn't' help.

Any ideas on what would be causing this or how to fix it?

I was considering modifying the power block and replacing the power cord with shielded cable, not sure if that would help. Or would it be better to get a different regulated/shielded power supply?

I know most of you use EDM, and I'll probably get one next year, but I need to fix this in the next few days.

Thanks!

[Dan C]

anybody????

[JBullard]

If the transmitter is drifting, then it sounds like a fault with the PLL circuits.

Fooling with the power supply probably would not correct a frequency drift problem.

It's just being a Ramsey.

[Max-Paul]

This was a kit you put together? If so, was there a procedure to tune the PLL? Is the frequency that you are transmitting on near the top or bottom of the range you can set it for?

As you might know. A PLL is a frequency generator that compares the out put frequency to a reference crystal freq. If not the same a error voltage is created and fed into a speical diode that causes the output freq to be shifted. But the diode will only operate over a small voltage range. The error voltage is exceeding the range I suspect. If there is a tuning procedure you did when you first put the kit together, you might want to run that procedure again. BTW even the reference crystal will age and shift. Usually there is a warping cap near the crystal to tune it back to the proper frequency.
Sorry if TMI, was a radio tech in USAF.

[Dan C]

Thanks for the suggestions Paul,
I tried to re-tune the pll like you said, It didn't really make any difference.

I have been playing around with it this morning and discovered something. When I shut the show off so its not broadcasting any audio, the red light goes solid. I started to play some songs through windows media player and discovered it would cause more static on certain songs than others. For example, when there was a lower bass note it seemed to have more static.

I hooked the transmitter up to a different computer. The red light still flickers a little, but the audio is much cleaner and doesn't seem to distort/static as much.

So for now, I'm in the process of moving all my sequences and show over to my main computer. I don't like having my main computer on all the time, Its a beast! but I guess it will have to do for now.

Thanks!

[H2_Whoa]

2 Things I would check:

1-Computer "equalizer" or "bass enhancement" settings causing distorted output.

2-Ground loop problem: I have worked around this by using a ground lifting plug on the computer (removes the round ground prong on the ac plug). I know this does not sound safe but see if it fixes the problem then work on a solution from there.

Good luck & happy Thanksgiving!

[Max-Paul]

Ok, now a different kind of problem. Just turn down the audio leaving the computer. Sounds like your over driving the audio input circuits. Audio in a FM transmitter is called deviation. The louder the audio is the more the frequency shifts. And from what you have said, I think the frequency is trying to shift to much and the PLL is unlocking as you noted. So, try it with less audio. BTW does this transmitter have a audio input adjustment, if so might try turning it down also.
Sounds like your problem has gotten easier to solve now.

[Dan C]

It does have two adjustments (left and right).

what would be a good level to output from the computer?
its about 3/4 right now.

Since the show is running right now. I will try to adjust things tomorrow.
Thanks Paul!

[Max-Paul]

Well, even this expert (cough, cough) can screw things up. Last year I discovered near the end of the year that my louder parts of the songs sounded distorted. So, this computer has not been used since last year. I remembered the sound problem and checked the volume slider on the computer. Sure enough it was like 2/3s up. I slid it down to about 1/4 volume. Listened to it on the living room receiver, sounds great now. So, drop that volume back and I am sure you will be happy with the results. Then if you want to hear you music at the computer, add a set of amplified speakers and adjust the volume via the speakers.

[progressivetek]

Just a note to all, according to Hobbytron, the FM25B does not have a PLL circuit... I too had the same problem with the same indications and also found the sweet spot on the computer volume at approximately 25%. Any less and the 60Hz hum would be pronounced and more would distort the input circuits.