Total posts : 45366
Okay let’s talk about these microphones.
To this very day I still have two of the original Mr. Microphones.
They are no longer in that silly plastic case with the orange foam ball.
But let’s go back to the beginning here.
Back in the day, I purchased a Radio Shack FM91 wireless microphone.
It was the beginning of my first radio station back in 1976.
I simply planted the microphone in a sound proof cooler along with two speakers for the left and right stereo channels and then attached an SO-239 coaxial connector to the antenna wire. From there, I attached a 10 foot piece of RG-8 coaxial cable which was hooked up to a .64 wave ground plane CB antenna. Back then, I didn’t know that the antenna had to be a certain wave length, but it did carry a signal for about a half mile on an outdoor FM antenna.
In later years, I realized the frequency spread of 88.1 to 92.0MHz in my area was noisy and filled with broadcast stations. I mostly used 90.1MHz, but suddenly one day when I turned the transmitter off, I noticed a dead carrier was on 90.1MHz. Long story short, WECS http://www.wecsradio.com/ a collage radio station from Willimantic Connecticut was just going on the air was covering us up.
There was no open channels in that frequency range, so I had to experiment with some values of the parts of the FM91 to change the tuning range of the tuning coil. I changed one coupling capacitor and one resistor and the range moved up to the upper FM band.
I moved to 106.5MHz. The only station that was on that channel was a religious radio station out of Boston MA. that wasn’t always receivable in my area even on a large hill.
As time moved on, I bought two of those Mr. Microphones from Ronco. They transmitted something like 10 feet, that was about it.
One day I tried an experiment. I removed the circuit board from the plastic casing. I hooked the audio output of an FM mono radio that was tuned to 106.5MHz to the two poles of the circuit board that the microphone was hooked up to. I attached an SO-239 connector to the antenna circuit of Mr. Microphone. It originally used two AA batteries for power, I hooked up a 9 volt adapter so it had constant power. Finally, using 10 feet of RG-8 coax, I had it transmitting into a Citizens Band 1/4 wave ground plane antenna. This signal reached two miles line of site.
Lastly, Radio Shack sold a Science Fair AM transmitter kit. It had those springs that the parts were placed in so kids would not have to use a soldering iron. I never used the included crystal microphone when I used this kit. I did the same thing as I did with Mr. Microphone, I used an FM radio tuned to 106.5MHz and hard wired the audio output of that radio into the AM transmitter’s audio input. I also powered this transmitter up using a 9 volt adapter. For an AM antenna, the building had a 25 foot television antenna tower up on the roof. I used a SO-239 connector on the AM transmitter and feed a 25 foot piece of RG-8 coax up to that tower and attached the coax to the tower by using an SO-239 connector and a copper plate that was attached to the aluminum tower using bolts so there was an electrical connection between the feed line and the tower. That signal at 1000KHz reached 1 mile line of site. I was using the tower as the actual transmitting antenna. Mind you the tower was not grounded so there was no problems with grounding out the transmitted signal.
I had transferred that AM transmitter kit to a home etched circuit board I had made and moved the original plastic coil frame and tuning capacitor onto the new circuit board. All the parts were soldered in, making for a better connection over the springs the original kit used.
I no longer have that FM91, I’m not sure what happened to it. I still have the owner’s manual that came with it though.
That AM transmitter is also gone, I still have two copies of the owners manual that came with the kit and could build a new one if I wanted to. I still have that tuning capacitor from the kit so that is one part that does not need to be found.
I have not looked at the owner’s manual lately, but I know I drew a circuit board diagram on it, maybe if I get up into my attic, I can look through that box of service manuals I have and make a photo copy of the road map I designed, then you could create your own circuit board.
Most of the parts are easy to get if you wished to duplicate that kit.