General

Why does my FM transmitter sounds fuzzy?
Or
Why do I hear a constant tone in my transmission?

FUZZ or a constant tone is usually caused by the input audio interfering with the 19 kHz sub-carrier of the stereo generator in the transmitter. Radio stations eliminate this by sending the input audio through a low pass filter. The filter must suppress the 19 kHz audio by a minimum of 45 dB. To attain this suppression and maintain good frequency response this filter must contain many poles. The BH1415F IC does contain an audio filter; however, this may not be enough filtering if the audio comes from an inexpensive computer sound card or a CD player. To try and isolate this try a different audio source. We suggest a tape player because they rarely have an audio output component at or above  19 kHz. If your fuzz problem goes away with the tape player we suggest filtering your audio. This can be achieved in many ways the easiest being the addition of our STC-1 kit to your system.

Why do I hear hum from my transmitter?

HUM can come from a couple of areas. The first place is from the power supply. The
easiest way to eliminate the power supply as a problem is to replace the wall transformer
with a good regulated 12 volt DC power supply or a 12 volt battery (car battery with the
engine off works well). If the hum goes away you should upgrade your 12 volt DC
source or wind the power cord through a round ferrite core. If the hum doesn't go away
the problem is probably RF feedback. For this problem you may want to insure your
audio cables are shielded, keep audio cables away from AC power cords, keep audio
cables away from antenna and RF transmission line, decrease your output power, use an
external antenna at a good distance from your transmitter and audio source, and finally
check for good grounds on your audio source.

What is the range of my FM transmitter?

The range of the FM transmitters can vary depending on antenna used, height of antenna,
type of terrain, type of obstacles (walls, buildings, etc.) between transmitter and receiver,
and receiver sensitivity. Line of sight should yield about ¼ mile (1/2 km) range.
 

What is the difference between the FM25B and the MP3FM?

Both transmitters have line in and loop out 1/8" stereo jacks. Both transmitters have whip antennas, and are fully synthesized.

The MP3FM was designed to be used with a computer to broadcast digital audio from a sound card. The MP3FM has filtering built in that cleans up the problems associated with using a computer as an audio source. The MP3FM does not have an external antenna connector, and uses an different stereo encoder chip. The FM25B does have improved filtering over the FM25A, (But not the same type the MP3FM has) and has an F type coax antenna connector for adding an external antenna. If you plan on adding an external antenna or filtering, the FM25B is your best choice. If you want a less expensive "plug and play" solution for broadcasting your MP3s around the house, the MP3FM is the way to go.

What is the difference between the FM25A and the FM25B?

The FM25B uses a BH1415F stereo generator IC. The FM25B also has an F Type antenna connection and loop though audio connections.  The FM25B has been completely redesigned and now uses a few SMT parts as well.

I keep reading about these "mods" or changes that should be done to improve the FM25B, should I change anything on my old FM25A?

No! please don't attempt the changes\errata changes for the FM25B on your FM25A! It won't do anything good for you! 

Do I need an antenna for my FM25B or my FM100?

If you want to "Solidify" your coverage, an external antenna is an excellent idea. You also may want an external antenna if you can't place your transmitter in an optimum location. Especially in situations where your transmitter is in your basement, but you need the antenna on the second floor. With our export FM100, an external antenna is required to take advantage of the extra power, so you'll want an antenna like our TM100. Generally, all customers will be pleased with the better performance an external antenna can provide. Our TM100 antenna is recommended for the hobbyist customer.

Your catalog used to mention that the transmitters are "strapable" - Is this still the case?

The term "strapable" no longer applies with our new transmitters. We used to provide a jumper (for export\test bench operation) that would allow you to increase the power by bypassing some resistors and transistor bias. Basically meaning that you would get higher output power. Many of our transmitters now have a potentiometer to vary the output power, instead of an on-off jumper setting.

AM25

What's the deal with the AM25 antenna? Where is it?

We include instructions for building a simple antenna that will work with this transmitter. The antenna we show you how to build uses an 8ft piece of wire wrapped around a 5 inch long 1.25" PVC pipe.

How can I adjust the coverage of this transmitter?

Unlike some of our FM transmitters, the best way to adjust your coverage is just experiment with the placement of the transmitter. Just build it as instructed. Placement will have the greatest effect on your range.
 

FM10A

Why does my FM10A only transmit a few feet?

You have a problem in the RF output section of the transmitter. Check the RF Amp Q1
or the antenna. Bias voltages for Q1 are: Emitter is 0 VDC, Base 0.6 to 0.8VDC, and
Collector is 4 to 5 VDC. If you can't identify the base, emitter, and collector do not pass
go, do not collect $200.00, do not call tech support, go directly to the warranty page
(inside back cover) of the manual and follow the directions for Factory Repair of
Assembled Kits.
 

FM25A

Why does my FM25A only transmit a few feet?

You have a problem in the RF output section of the transmitter. If you can set your
transmitter to any standard FM band frequency (88.1 MHz to 108 MHz in 100 kHz
steps), then your problem is your antenna, low pass filter (L2, L3, C35, C36, C37), or RF
Amp (Q2) circuitry. If you can't set the frequency check Q3's circuitry. Bias voltages
for Q2: Emitter = 0 VDC, Base = 0.6 to 0.8 VDC low power or -0.4 VDC high power,
Collector 5 to 6 VDC. Bias voltages for Q3: Emitter = 0 VDC, Base = 0.6 to 0.8 VDC,
Collector 5 to 6 VDC. If you can't identify the base, emitter, and collector do not pass
go, do not collect $200.00, do not call tech support, go directly to the warranty page
(inside back cover) of the manual and follow the directions for Factory Repair of
Assembled Kits.

Why can't I set my FM25A's audio level high enough?

The output can vary tremendously from one audio source to another. This made it
difficult for us to setup the input circuitry on the FM25A. If you need more audio try
changing R27 and R29 to 22 k ohm resistors. This will change the input voltage divider
giving you more audio to the FM stereo IC.
 

FM25A

Why does my FM25A only transmit a few feet?

You have a problem in the RF output section of the transmitter. If you can set your
transmitter to any standard FM band frequency (88.1 MHz to 108 MHz in 100 kHz
steps), then your problem is your antenna, low pass filter (L2, L3, C35, C36, C37), or RF
Amp (Q2) circuitry. If you can't set the frequency check Q3's circuitry. Bias voltages
for Q2: Emitter = 0 VDC, Base = 0.6 to 0.8 VDC low power or -0.4 VDC high power,
Collector 5 to 6 VDC. Bias voltages for Q3: Emitter = 0 VDC, Base = 0.6 to 0.8 VDC,
Collector 5 to 6 VDC. If you can't identify the base, emitter, and collector do not pass
go, do not collect $200.00, do not call tech support, go directly to the warranty page
(inside back cover) of the manual and follow the directions for Factory Repair of
Assembled Kits.

Why can't I set my FM25A's audio level high enough?

The output can vary tremendously from one audio source to another. This made it
difficult for us to setup the input circuitry on the FM25A. If you need more audio try
changing R27 and R29 to 22 k ohm resistors. This will change the input voltage divider
giving you more audio to the FM stereo IC.
 

FM25B

Why does my FM25B only transmit a few feet?

You have a problem in the RF output section of the transmitter. Check R25 setting, RF
Amp U4, low pass filter L2, L3, C34, C37, C38, or the antenna. Bias voltages for U4:
Input = 2.7 VDC and output is 4 VDC. If you can't identify U4's input or output do not
pass go, do not collect $200.00, do not call tech support, go directly to the warranty page
(inside back cover) of the manual and follow the directions for Factory Repair of
Assembled Kits.

Why does my FM25B lack bass?

Because of a design change, C19 should be removed, and nothing should be installed in it's place. Although leaving C19 will have little effect on the operation of the kit, you should still remove it. We found this mistake very quickly, so there are very few kits that were sold without the change noted.

Why can't I set my transmitter to 87.9 or certain other frequencies?

Because of the limitations set by the programming that is vital to your transmitters operation, your transmitter will only transmit between 88.0 and 107.9; if you attempt to program a frequency outside of this range, the transmitter will default to 88.1

The manual says something about the LED showing that audio is present. Why doesn't it go off when there is no audio source connected?

If we made the LED go off when there was no audio present, you would have a hard time telling when you were having a frequency lock problem, and when you simply forgot to hook up your audio source. You will notice that the LED flickers along with the music to some degree. This is what it is supposed to do.
 

MP3FM

Why does my MP3FM only transmit a few feet?

You have a problem in the RF output section of the transmitter. If you can set your
transmitter to any standard FM band frequency (88.1 MHz to 108 MHz in 100 kHz
steps), then your problem is your antenna, low pass filter (L2, L3, C35, C36, C37), or RF
Amp (Q2) circuitry. If you can't set the frequency check Q3's circuitry. Bias voltages
for Q2: Emitter = 0 VDC, Base = 0.6 to 0.8 VDC low power, Collector 5 to 6 VDC. Bias
voltages for Q3: Emitter = 0 VDC, Base = 0.6 to 0.8 VDC, Collector 5 to 6 VDC. If you
can't identify the base, emitter, and collector do not pass go, do not collect $200.00, do
not call tech support, go directly to the warranty page (inside back cover) of the manual
and follow the directions for Factory Repair of Assembled Kits.

Why can't I set my MP3FM's audio level high enough?

The output can vary tremendously from one audio source to another. This made it
difficult for us to setup the input circuitry on the FM25A. If you need more audio try
changing R14 and R15to 22 k ohm resistors. This will change the input voltage divider
giving you more audio to the FM stereo IC.