SYNTHESIZED AM BROADCAST TRANSMITTER KIT Ramsey Electronics Model No.
PARTIAL LIST OF AVAILABLE KITS RAMSEY TRANSMITTER KITS • FM25B, FM100B FM Stereo Transmitters • AM1, AM25 AM Transmitters • TV6 Television Transmitter • FM100B Professional FM Stereo Transmitter RAMSEY RECEIVER KITS • FR1 FM Broadcast Receiver • AR1 Aircraft Band Receiver • SR2 Shortwave Receiver • AA7 Active Antenna • SC1 Shortwave Converter RAMSEY HOBBY KITS • SG7 Personal Speed Radar • SS70A Speech Scrambler • MX5, MX10 Mixers • MD3 Microwave Motion Detector • PH10 Peak hold Meter • STC1 Stereo Transmitt
Ramsey Publication No. MAM25 Manual Price Only $5.00 KIT ASSEMBLY AND INSTRUCTION MANUAL FOR SYNTHESIZED AM TRANSMITTER AM25 TABLE OF CONTENTS Introduction to the AM25 ....................4 AM25 Circuit Description ....................4 Schematic Diagram ............................7 Parts Layout Diagram .........................8 Parts List .............................................9 Assembly Instructions .........................10 Frequency chart ..................................
INTRODUCTION Many times we wish to be heard via wireless communications. Whether transmitting music or voice, what is required is a stable transmitter tunable to an unused frequency on the broadcast band. Here at Ramsey, we have produced several models of FM transmitters and an entry level AM transmitter. Recent requests for a synthesized AM type transmitter have been answered with this kit.
Low Freq uenc y Audio Wa ve form High Freq ue ncy Radio Carrier Amplitude Mod ulated Signal L7 and varactor diode D2. The Phase-Locked-Loop portion of the circuit controls the capacitance of the varactor diode to keep the frequency exactly as selected. We’ll examine that in more detail later. A buffer amplifier, Q5 increases the output of the oscillator enough to drive final RF amplifier Q3.
amount of power. It is not recommended that a common 9V battery be used to power this kit. Instead, a battery “pack” consisting of eight 1.5 volt cells, a 12V sealed battery, or other external 12V DC supply may be used. The Phase-Locked-Loop portion of the circuit is responsible for maintaining the frequency of the transmitter. U2 is the brains of the whole circuit. This chip looks at the settings of each switch in the dip switch S1 and from this information, calculates the desired frequency.
AM25 •7
PARTS LAYOUT DIAGRAM AM25 •8
PARTS LIST RESISTORS 2 10 ohm [brown-black-black] (R33,34) 1 51 ohm [green-brown-black] (R15) 2 100 ohm [brown-black-brown] (R5,11) 1 270 ohm [red-violet-brown] (R3) 2 470 ohm [yellow-violet-brown] (R12,32) 5 1K ohm [brown-black-red] (R10,13,20,28,29) 2 2.2K ohm [red-red-red] (R16,17) 2 4.
SEMICONDUCTORS 7 1 1 1 1 1 1 1 NPN transistors [marked 2N3904] (Q1,2,4,5,6,8,10) TO-220 style NPN power transistor [marked TIP31] (Q9) TO-220 style power FET [marked P16NF06] (Q3) 1N4148 style glass bead diode (D1) MVAM-108 varactor diode [looks like transistor with 2 leads] (D2) Red LED (D3) LM-358 8 pin dual opamp IC (U1) MC-145106 PLL synthesizer IC (U2) MISCELLANEOUS PARTS AND HARDWARE 1 2 1 1 1 1 1 1 1 2 80’ 2 2 10.
possible, we install in an order that fits our "Learn-As-You Build" Kit building philosophy. This entails describing the circuit that you are building, instead of just blindly installing components. We hope that this will not only make assembly of our kits easier, but help you to understand the circuit. For each part, our word "Install" always means these steps: 1. Pick the correct part value to start with. 2. Insert the part, oriented correctly, into its holes in the PC board. 3.
We’ll now begin to construct the RF oscillator section of the AM25. Be sure to mount the components as close as possible to the PC board to avoid “radiating” any unwanted signals due to long lead lengths. 4. Install L5, the slug tuned inductor. Solder all 7 tabs. 5. Install L7, a 68 uH inductor, it looks like a resistor with color bands on it, blue-gray-black. 6. Install C35, .01 µF [marked .01 or 103 or 10 nF]. 7. Install H1, the 3 pin header strip.
21. Install R33, 10 ohm [brown-black-black]. 22. Install R34, another 10 ohm [brown-black-black]. 23. Install C26, .1 µF disc capacitor [marked .1 or 104]. 24. Locate a 1/2 inch long piece of scrap component lead. Install it into the black hole near trimmer capacitor C16. Leave about 1/2” standing up from the PC board, this wire point, which we will call TP1, will provide a convenient point for you to connect a meter probe. You have just completed most of the RF carrier oscillator.
41. Install C32, 22 pf disc capacitor [marked 22]. 42. Install C31, the other 22 pf disc capacitor. 43. Install C24, .01 µF disc capacitor [marked .01 or 103 or 10nF]. 44. Install X1, the 10.24 MHz quartz crystal. Inside this metal can is a piece of synthetic quartz that has been precision ground to vibrate precisely at 10.24 million times per second, very similar to a mechanical tuning fork! 45. Install C25, .01 µF disc capacitor [marked .01 or 103 or 10nF]. 46.
Winding L4. Locate one of the binocular ferrite cores and the enameled magnet wire. We will talk about winding complete turns through the core, a complete turn is a full wire course through one hole in the core and back out through the other hole. 63. Cut a length of wire 18 inches long and following the diagram shown, wind 2 complete turns through the core and form a loop 3/4 “ long. 64. Twist this loop fairly Figure 1 Figure 2 tight and continue winding another 8 complete turns through the core.
Just what are these inductors? The inductors you just wound are called broadband matching inductors. They act as impedance matching devices, similar to a transformer. L4, for example has a 4:1 turns ratio and will transform the low impedance output of Q3 up by 16 times. This is because the square of the turns ratio is equal to the ratio of impedances. Now let’s move over to the audio section. 73. Install U1, LM358 opamp.
produce some heat and require a heatsink to carry the heat away from them. When installing these parts, the transistor sits on the metal heatsink and a 440 screw is passed through the transistor, heatsink and PC board, finally to be tightly fastened with a 4-40 nut. 98. Install Q9, TO-220 style NPN power transistor [marked TIP31] as shown in the diagram. Use a 4-40 screw and nut to secure. Solder all three leads. 99. Install Q3, P16NF06 power FET in the same fashion as above. 100.
Maximum Frequency (KHz) C3 and C7 L1 and L3 No. of turns C4 and C5 L2 No. of turns Band Jumper (H1) Jumper pins 790 2200 pF 29 8200 pF 31 2-3 950 1500 pF 27 6800 pF 28 2-3 1150 1000 pF 25 5600 pF 27 1-2 1350 470 pF 23 4700 pF 24 1-2 1710 none 20 3900 pF 21 1-2 Length required = Number of turns x 0.6 nches. For example: 35 turns required. 35 x 0.6” = 21” 104. Wind the toroid cores, L1, 2 and 3 with the proper number of turns. One pass through the core counts as one turn.
PROGRAMMING YOUR AM25 Programming your AM25 is very easy, involving nothing more than dividing (by 10!) and subtraction. The output frequency in KHz is divided by 10 and this ‘Program’ number is then set on DIP switch, S1. These switches are programmed in Binary, you simply add up the closed switch positions: 1, 2, 4, 8 all the way up to 128 to make any ‘Program’ number between 0 and 255. Next to the DIP switch, there are numbers printed on the PC board from 1 to 128.
8. Measure the voltage at TP1, located near the center of the board. 9. Using the plastic tuning tool, adjust coil L5 until you read between 1 and 3 volts DC at TP1. You should also see the red LED light up and be able to hear the AM25’s carrier signal. 10. Connect a 1 volt peak-to-peak audio source to the AUDIO IN jack. Adjust ‘Audio level’ pot R31 for best sounding undistorted audio. The best audio source for testing is the line level output of a tape deck or CD player.
These rules are published in 100 “Parts,” covering anything imaginable concerning the topic of “Telecommunications.” The six books containing the FCC rules are section 47 of the complete Code of Federal Regulations, which you are likely to find in the reference section of your local Public Library. If you have any legal questions concerning your AM25 or any home built device which emits RF energy, it is your responsibility to study the FCC regulations.
An easy way to describe what happens when you “load” the antenna is that we electrically increase the “length” of the antenna without increasing the mechanical length. Our goal is to approximate a functional portion of the full wavelength (i.e. λ/4 or λ/8) Using an 8 foot length of wire, or an old 102” CB radio whip as the antenna, a series inductance of about 350 µH is required to properly load the circuit.
The Ramsey Kit Warranty Please read carefully BEFORE calling or writing in about your kit. Most problems can be solved without contacting the factory. Notice that this is not a "fine print" warranty. We want you to understand your rights and ours too! All Ramsey kits will work if assembled properly. The very fact that your kit includes this new manual is your assurance that a team of knowledgeable people have field-tested several "copies" of this kit straight from the Ramsey Inventory.
AM-25 AM BROADCAST TRANSMITTER Quick Reference Page Guide Introduction to the AM-25........................................... 4 AM-25 Circuit Description .......................................... 4 Schematic Diagram.................................................... 7 Parts Layout Diagram ................................................ 8 Parts List .................................................................... 9 Frequency chart .........................................................
