Text preview for : sstran_amt3000.pdf part of sstran amt3000 AM-Transmitter amt3000
sstran
Back to : sstran_amt3000.pdf | Home
AMT 3000 Assembly and Operating Instructions
AMT 3000
Low Power AM Broadcast Radio Transmitter Kit
Assembly and Operating Instructions
Instruction Manual Revision: R4 PCB 3000-05-1 April, 2009
Copyright (C) 2009 by Sstran. All Rights Reserved. Web: www.sstran.com email: [email protected]
1
AMT 3000 Assembly and Operating Instructions
Table Of Content
1 Getting Organized..................................................................................................................................................................................3 Inventory and Organize Your Parts..................................................................................................................................................3 Tools You Will Need........................................................................................................................................................................3 2 Part List..................................................................................................................................................................................................5 3 Circuit Board Component Locations.....................................................................................................................................................8 4 Tips for Inserting and Soldering Components.......................................................................................................................................9 Inserting Components......................................................................................................................................................................9 Soldering..........................................................................................................................................................................................9 Temporary «Tack Soldering».........................................................................................................................................................10 5 Step-by-Step Assembly Instructions....................................................................................................................................................10 Fixed Resistors...............................................................................................................................................................................10 Surface Mount IC...........................................................................................................................................................................10 Chokes............................................................................................................................................................................................10 Rectifiers and Diodes.....................................................................................................................................................................11 Resistor Network............................................................................................................................................................................11 IC Sockets......................................................................................................................................................................................11 DIP Switches..................................................................................................................................................................................11 Jumper Headers..............................................................................................................................................................................11 Ceramic Capacitors........................................................................................................................................................................12 Ceramic Trimmer Capacitor..........................................................................................................................................................12 Rear Panel Jacks.............................................................................................................................................................................12 Variable Resistors...........................................................................................................................................................................13 +5V Voltage Regulator and Transistors.........................................................................................................................................13 Electrolytic Capacitors...................................................................................................................................................................13 RFI Suppression Coils...................................................................................................................................................................13 Quartz Crystal................................................................................................................................................................................13 +15 Volt Regulator and Heat Sink.................................................................................................................................................13 LED Power Indicator Lamp...........................................................................................................................................................14 Insert 16-Pin ICs into Sockets........................................................................................................................................................14 Jumper Plug Installation.................................................................................................................................................................14 Antenna and Ground Wires............................................................................................................................................................15 6 Final Checks........................................................................................................................................................................................15 7 Power On Check..................................................................................................................................................................................15 8 RF Tuning............................................................................................................................................................................................15 Setting the Frequency.....................................................................................................................................................................15 Setting the RF Output Tuning Range.............................................................................................................................................15 Peaking the RF Output...................................................................................................................................................................15 9 Audio Adjustments..............................................................................................................................................................................16 Front Panel Control Functions.......................................................................................................................................................16 Gain Control.............................................................................................................................................................................16 Modulation Control..................................................................................................................................................................16 Compression Control................................................................................................................................................................16 Audio Adjustment Procedure.........................................................................................................................................................16 10 Board Jumper Options.........................................................................................................................................................................16 A Word About AC Building Wiring Interference Problems..........................................................................................................16 Jumper Plug S1: Audio Source RF Isolation.................................................................................................................................16 Jumper Plugs S2 and S3: AC Adapter RF Isolation......................................................................................................................16 Jumper Plug S6: Treble Boost........................................................................................................................................................17 Jumper Plug S7: Meter Resistor Bypass........................................................................................................................................17 11 Mounting the Board in the Enclosure..................................................................................................................................................17 12 Final Installation..................................................................................................................................................................................17 FCC Antenna Rules.......................................................................................................................................................................17 Antenna Location...........................................................................................................................................................................17 Ground...........................................................................................................................................................................................18 Check Tuning.................................................................................................................................................................................18 Finish Assembling Enclosure.........................................................................................................................................................18 13 Troubleshooting...................................................................................................................................................................................18 14 Theory of Operation............................................................................................................................................................................19 Oscillator and Frequency Synthesizer Section...............................................................................................................................19 RF Power Output Section..............................................................................................................................................................19 Preamplifier and Compressor/Limiter Section..............................................................................................................................20 Modulator Section..........................................................................................................................................................................20 Power Supply Section....................................................................................................................................................................20 2
AMT 3000 Assembly and Operating Instructions 15 Warranty, Return and Service Policy...................................................................................................................................................21 Warranty.........................................................................................................................................................................................21 Returns...........................................................................................................................................................................................21 Service............................................................................................................................................................................................21 How to Contact SSTRAN..............................................................................................................................................................21 16 About FCC Part 15 Regulations..........................................................................................................................................................21 17 Component Identification....................................................................................................................................................................22 18 Color Code Charts...............................................................................................................................................................................24 19 Synthesizer Frequency Selection Chart...............................................................................................................................................25 S4 Switch Setting For 10 kHz Channel-Spacing Models..............................................................................................................25 S4 Switch Settings For 9 kHz Channel-Spacing Models...............................................................................................................28 20 RF Output Tuning Range Selection.....................................................................................................................................................31 S5 Frequency Range Switch Settings for 3 Meter Antenna...........................................................................................................31 Frequency Range Graph for 3 Meter Antenna...............................................................................................................................31 21 Specifications.......................................................................................................................................................................................32 22 Circuit Diagram...................................................................................................................................................................................33
1 Getting Organized
This manual provides step-by-step assembly instructions to help you complete your kit with the least possible chance for error. Keeping your parts, tools and work area organized will further reduce the chance for error and keep you from getting frustrated looking for a part or tool.
Inventory and Organize Your Parts
Unpack your kit carefully and check each part against the parts list in section 2. Refer to section 17, «Component Identification», for help with identifying parts. During your inventory, put a check mark in the box next to each part in the parts list. Some parts, such as resistors and ceramic capacitors, are marked only with color codes or numeric codes. Other parts, such as transistors, voltage regulators and integrated circuits are marked with part numbers. A generic part number shown in the parts list may not correspond exactly with what you see on a component. However, the generic part will appear as part of a sometimes longer component part number. For example, a 74HC40103 (generic part number) integrated circuit may be marked as CD74HC40103E. The generic number is embedded in the actual part number along with varying prefixes and suffixes which may be ignored. If there is a missing or damaged part, e-mail SSTRAN at [email protected]. and we will send you a replacement. Describe the part completely in your e-mail. Include Reference Designator, Value, Part Marking, and Description from the parts list. Also include your name, shipping address and date of original order.
Tools You Will Need
You will need a few inexpensive tools to assemble your kit: · · · · · · · · a low-wattage (15 25 watt) pencil-type soldering iron with a small diameter tip small gauge electronic solder, 0.032'' (0.81 mm) or smaller diameter flux-core for use in electronic assembly small long-nose pliers flush-cutting «nippy» wire cutters wire insulation stripping tool (a sharp knife is an acceptable substitute) solder removal tool small screwdriver with approximately 0,1'' (2,5mm) wide blade No. 1 Philips screwdriver
3
AMT 3000 Assembly and Operating Instructions If you don't have one ore more of these tools you can purchase them at your local Radio Shack store or from a mail-order electronics supplier such as www.mouser.com or www.digikey.com. Here are some examples from the Radio Shak on.line catalog on the web at www.radioshack.com. Radio Shack catalog #: 64-2051: «15-W with Grounded Tip. Good choice for integrated circuit work.» (It is very important that your soldering iron have a small diameter tip like this one.)
Radio Shack catalog #: 64-005: «Standard 60/40 formula rosin core solder. 0,032'', 2,5 oz.»
Radio Shack catalog #: 64-2033: «Long-Nose 4-3/4'' Mini Pliers»
Radio Shack catalog #: 64-1833: «Flush-cutting nippy cutters fit tight places and lets you trim leads close to PC boards. Spring action.»
Radio Shack catalog #: 64-2129: «Adjustable Wire Stripper/Cuter»
Radio Shack catalog #: 64-1963: «6-piece Precision Screwdriver Set»
Radio Shack catalog #: 64-2098: «Vacuum Desoldering Tool»
4
AMT 3000 Assembly and Operating Instructions
2 Part List
Refer to section 17, Component Identification, for help with parts identification. Refer to section 18, Color Code Charts, for instructions on how to read resistor and RF choke color codes. Reference Designator R8 R3 R25, R28, R29 R7 R18 R12, R21, R31 R17 R27 R4 R23, R30 R10 R6 R19, R22, R32 R16 R1, R2, R5 R33 R9 R26 R14 R11, R15, R20 R13 Qty 1 1 3 1 1 3 1 1 1 2 1 1 3 1 3 1 1 1 1 3 1 Value 22 ohms 100 ohms 220 ohms 510 ohms 820 ohms 1K ohms Part Marking red-red-black brown-black-brown red-red-brown green-brown-brown gray-red-brown brown-black-red Description ¼ watt carbon film resistor ¼ watt carbon film resistor ¼ watt carbon film resistor ¼ watt carbon film resistor ¼ watt carbon film resistor ¼ watt carbon film resistor ¼ watt carbon film resistor ¼ watt carbon film resistor ¼ watt carbon film resistor ¼ watt carbon film resistor ¼ watt carbon film resistor ¼ watt carbon film resistor ¼ watt carbon film resistor ¼ watt carbon film resistor ¼ watt carbon film resistor ¼ watt carbon film resistor ¼ watt carbon film resistor ¼ watt carbon film resistor ¼ watt carbon film resistor ¼ watt carbon film resistor 10K ohms x9, 10 Pin Common Bus Resistor Network RCA phono jack r/a pcb white RCA phono jack r/a pcb red Power jack pcb 2,1mm RCA phono jack r/a pcb yellow RCA phono plug w/yellow band 50K ohms 20K ohms 12-100pF 50K 20K Black point dot PC Mount Black Shaft Variable Resistor PC Mount Black Shaft Variable Resistor Black, 12-100pF, 7mm Variable Trimmer Capacitor Ceramic Ceramic Ceramic
Fixed Resistors and Resistor Network
1,5K ohms brown-green-red 1,8K ohms brown-gray-red 2,7K ohms red-violet-red 4,7K ohms yellow-violet-red 6,8K ohms blue-gray-red 8,2K ohms gray-red-red 10K ohms 15K ohms 27K ohms 30K ohms 47K ohms brown-black-orange brown-green-orange red-violet-orange orange-black-orange yellow-violet-orange
100K ohms brown-black-yellow 220K ohms red-red-yellow 1M ohms 10K ohms brown-black-green 10A103GA
Jacks, Plug, Variable Resistors and Variable Trimmer Capacitor J1 J2 J3 J4 P4 R34, R35 R36 C5 Fixes Capacitors C12, C13 C30 C15, C27 C23 2 1 2 1 33pF 180pF 200pF 820pF 33 180 201 1 1 1 1 1 2 1 1
821 Ceramic For use with wire antenna. See step-by-step instructions.
5
AMT 3000 Assembly and Operating Instructions Reference Designator Qty 1 Value 560pF Part Marking 561 For use with external baseloaded vertical antenna. See step-by-step instructions. 102 .01 or 103 .1 or 104 Description Ceramic
C17, C21 C14, C16 C1, C2, C3, C6, C7, C8, C9, C10, C11, C22, C28, C31, C100, C101, C102, C103 C18 C20, C24, C25, C26, C29 C32 C4 Coils and Chokes L1, L2 L3 L4 L5 L6 L7 L8
2 2 16
1000pF 0,01uF 0,1uF
Ceramic Ceramic Multi-Layer Ceramic Multi-Layer
1 5 1 1 3 1 1 1 1 1
4,7uF 10uF 100uF 470uF 1000 uH 56 uH 82 uH 180 uH 470 uH 1000 uH
4,7uF 10uF 100uF 470uF 1000 uH green-blue-black-(silver or gold) gray-red-black-(silver or gold)
Electrolytic Electrolytic Electrolytic Electrolytic RFI Suppression Coil Epoxy Coated Choke Epoxy Coated Choke
Brown-gray-brown-(silver or Epoxy Coated Choke gold) yellow-violet-brown-(silver or gold) brown-black-red-(silver or gold) Epoxy Coated Choke Epoxy Coated Choke
Diodes, Transistors, Voltage Regulators and Quartz Crystal D1, D2, D3, D4 D5, D6 D7 Q1 Q2, Q3, Q4 Q5 U1 U4 X1 4 2 1 1 4 1 1 1 2N3906 PN2222A L7815 L78L05 4.000000 (Used in models AMT3000 and AMT3000-SM) 3.600000 (Used in models AMT30009K and AMT3000-9KSM) 74HC4060 74HC40103 SSM2166SA 1,0A, 100V 1N4002 Red with black end band Silicon rectifier, 1N4002GP DO-204AL 1,0A 100V Silicon Diode, 1N914A DO-35 T-1 LED lamp green 2,2v 10mA 32mcd 2N3906 TO-92 PNP Transistor PN2222A TO-92 NPN Transistor L7815 TO-220 +15V 1A VREG L78L05 TO-92 +5V 0,1A VREG HC-49U Microprocessor Crystal 4,0 MHz 20pF HC-49U Microprocessor Crystal 3,6 MHz 20pF
1
Integrated Circuits, DIP Switches and 16-Pin IC Sockets U2 U3, U5 U6 (Check circuit board for presence of pre-soldered IC) U7 6 1 2 1 74HC4060 14-Stage Ripple Binary Counter; 16-pin DIP 74HC40103 8-bit synchronous binary down counter, 16-pin DIP SSM2166A Microphone Preamplifier with Variable Compression & Noise Gaining, 14pin SOIC surface mount 74HC4046 Phase-Locked Loop, 16-pin DIP
1
74HC4046
AMT 3000 Assembly and Operating Instructions Reference Designator IC Sockets for U2, U3, U5, U7 S4 S5 Miscellaneous S1, S2, S3, S6, S7 (5 Jumper headers plus 1 spare) S1, S2, S3, S6, S7 (jumper plugs) PC Board Control Knobs Enclosure bottom section Enclosure top section Enclosure front panel Enclosure rear panel Enclosure screws Enclosure feet PC Board mounting screws Heat sink Heat sink mounting screw Heat sink mounting lock washer Heat sink mounting nut Antenna wire Ground wire AC Adapter 6 0,230 8PIN STR 0,100 Pin strip headers. Supplied as a 6-pair strip to be cut per assembly instructions 2-pos Jumper open black 3000-02 5'' x 3,9'' printed circuit board Black 0,50''D x 0,61''H plastic knob Enclosure bottom 6,08 x 4,26 x 0,375, bone Enclosure top 6,08 x 4,26 x 1,175, bone Enclosure front 5,75 x 1,311, black Enclosure rear 5,75 x 1,311, black Pan Head Philips screw Type B, #4 x ¾'' Plastic self-adhesive protective feet Pan Head Philips screw Type B, #4 x ¼'' Heat sink, TO-220/126/127 low profile 0,750L x 0,750W x 0,375H Pan Head Philips screw Type B, #6-32 x ¼'' Spring lock washer, #6 Machine screw nut, #6-32 White wire, 22AWG, 7/30 stranded, PVC, 118 in. (299 cm) Black wire, 22AWG, 7/30 stranded, PVC, 6 ft. (183 cm) 18VAC 500mA 18VAC, 500mA 2,1mm F AC adapter 1 1 8 Pos DIP Switch, extended actuator 4 Pos DIP Switch, extended actuator Qty 4 Value Part Marking Description DIP Socket, 16 Pin
5 1 3 1 1 1 1 2 4 4 1 1 1 1 1 1 1
7
AMT 3000 Assembly and Operating Instructions
3 Circuit Board Component Locations
The top of the circuit board is screen printed with component outlines and component reference designators to assist you with finding where to insert parts and which way to orient certain parts that must be inserted in a specific direction. The reference designator numbers begin at the top left corner of the board and increment from left to right across the board roughly along each row of components. The references then continue to increment at the left in the next row of components. The highest numbered references are at the bottom right corner. This is a guideline only, since the components are not always in well defined rows an board layout revision may have led to some components being moved.
8
AMT 3000 Assembly and Operating Instructions Feed the solder by touching the end of solder to the soldering iron tip and pad simultaneously. Stop feeding solder when it looks like enough has melted to form a mound over the pad. Let the solder flow around the lead and into the hole before removing the soldering iron. If the solder doesn't meld and flow within 2 or 3 seconds, you probably need to re-clean and re-tin the tip. Never heat a connection for more than 5 seconds at a time or you could damage the component or the board.
4 Tips for Inserting and Soldering Components
Inserting Components
Except as noted later for transistors, all components should be mounted flush with the surface board. All axial lead components such as resistors, chokes and diodes, require the leads to be bent at right angles. Bend the leads to match the hole spacing for each component. Leads can be bent by hand or with pliers. All other components such as capacitors, switches, IC sockets, controls an jacks, have radial leads or pins that do not require bending other than to straighten or align leads to fit the hole spacing. Never force a component into the board. If a component doesn't go easily, remove it and re-bend the leads until they match the board hole pattern. After inserting each component that has wire leads, bend the leads outward slightly at the bottom surface of the board to hold the component flush with the board while you apply solder.
If a soldered connection doesn't look good, clean and tin the soldering iron tip again and reheat the connection while applying a little more solder if necessary.
Soldering
All soldering is done on the bottom side of the board. Assembly will go faster if you insert several components before you turn the board over for soldering. Before soldering, be sure the iron is hot. This may take up to 5 minutes for some irons. Clean the soldering iron tip by wiping it on a damp cloth or sponge. Tin the tip by melting a small amount of solder over the surface of the tip. The tip should always be shiny before soldering. It will get dull with oxidation after a few minutes. When this happens, wipe and tin the tip again. A component led is soldered correctly when solder flows onto both the lead and the board pad, and forms a small, shiny mound over the pad. This creates a good electrical connection. A poor solder connection will cause failure of your kit to operate properly. All components used in this kit are highly reliable. If the kit doesn't work properly, the cause is very likely to be a bad solder connection. To solder a connection place the soldering iron tip so that it contacts the board pad and the lead at the same time. If you accidentally apply too much solder or bridge two nearby pads on the board, use your solder removal tool to remove the excess solder. Then, re-apply solder if necessary until the connection looks good. After soldering, cut any wire leads just above the solder mound using nipper wire cutters. Components that mount with lugs (controls and jacks) or pins (IC sockets and DIP switches) do not require the leads to be cut after soldering.
9
AMT 3000 Assembly and Operating Instructions R19 10K ohms (brown-black-orange) R22 10K ohms (brown-black-orange) R32 10K ohms (brown-black-orange) R16 15K ohms (brown-green-orange) R1 27K ohms (red-violet-orange) R5 27K ohms (red-violet-orange) R33 30K ohms (orange-black-orange) R9 47K ohms (yellow-violet-orange) R26 100K ohms (brown-black-yellow) R14 220K ohms (red-red-yellow) R11 1M ohms (brown-black-green) R15 1M ohms (brown-black-green) R20 1M ohms (brown-black-green)
Temporary «Tack Soldering»
At several places in the assembly instructions, we refer to the term «tack solder». This is a technique for holding a part in place before you apply solder properly. To tack solder a lead or pin to its board pad, clean and tin your iron and hen apply addition solder to the tip until a small ball forms. Immediately touch the tip to the wire and pad to get some solder flow. This is only temporary. After you solder the other leads or pins on the component, go back an re-solder the tacked pads.
5 Step-by-Step Assembly Instructions
Fixed Resistors
All resistors are ¼ Watt. Only the first three color bands are shown below. You may ignore the fourth tolerance band color. R8 22 ohms (red-red-black) R3 100 ohms (brown-black-brown) R25 220 ohms (red-red-brown) R28 220 ohms (red-red-brown) R29 220 ohms (red-red-brown) R7 510 ohms (green-brown-brown) R 18 820 ohms (gray-red-brown) Install R18 if you will be using he supplied wire antenna for in-home broadcasting and you want to use a frequency below 1000 kHz. If you will be Operating exclusively above 1000 kHz, do not R18. Also, do not install R18 if you will be using the external baseloaded vertical antenna described at http://www.stran.com/pages/sstran_buildant.html.
Surface Mount IC
This IC is already soldered on your circuit board. There is no assembly step required. You should look at it to verify it is present and familiarize yourself with its appearance and location. It is marked «SSM2166A».
Removal of R18 is one step in the transmitter modifications required for the use with the vertical antenna. See the complete modification instructions at:
http://www.sstran.com/pages/sstran_ant3000mods.html
Chokes
The epoxy chokes look similar to resistors and use the same color code as resistors. They have various diameters, but all are larger than the diameter of a resistor. If you are not sure whether a device is a resistor or a choke, measure its resistance with a meter. All chokes will measure less than 15 ohms. The DC-resistance of a choke is a measurement of the resistance of he wire coil an is not related to its inductance value. The measured DC resistance of a choke will not correspond to a resistor color code interpretation. All four color bands are shown below. The fourth «tolerance» band color may be either silver or gold for any of the inductors L4 L7. Be sure you read the color bands correctly. Sometimes gold can be confused with brown and silver can be confused with gray. Look carefully!
R12 1K ohms (brown-black-red) R21 1K ohms (brown-black-red) R31 1K ohms (brown-black-red) R17 1.5K ohms (brown-green-red) R27 1.8K ohms (brown-gray-red) R4 2.7K ohms (red-violet-red) R23 4.7K ohms (yellow-violet-red) R30 4.7K ohms (yellow-violet-red) R10 6.8K ohms (blue-gray-red) R6 8.2K ohms(gray-red-red)
10
AMT 3000 Assembly and Operating Instructions L4 56 uH (green-blue-black-gold) L5 82 uH (gray-red-black-gold) L6 180 uH (brown-gray-brown-gold) L7 470uH (yellow-violet-brown-silver) L8 1000 uH (brown-black-red-silver)
DIP Switches
Rectifiers and Diodes
Important: Orient rectifiers and diodes so that the band on end of the component is over the line on the end of the board outline. Bend leads 90 degrees at the component ends to match the board hole spacing before inserting. D1 1N4002 silicon rectifier D1 1N4002 silicon rectifier D3 1N4002 silicon rectifier D4 1N4002 silicon rectifier D5 1N914A silicon diode D6 1N914A silicon diode Important: Orient each switch package such that the pin-1corner is over the dot on the board outline. Before inserting, visually check that all pins are straight. Don't force the switch into the board holes. If you feel resistance, straighten any pins that aren't aligned properly. Seat the switch against the board. While holding the switch in place with a finger, tack solder 2 diagonally opposite end pins on the bottom side of the board. Solder the remaining pins. Then properly solder the previously tack-soldered end pins. S4 8 position DIP switch S5 4 position DIP switch
Resistor Network
Important: oOrient the resistor network SIP package so the dot marking pin-1 on the package is aligned with the dot on the board outline. R13 10K ohms x 9, 10-Pin SIP Common Bus Resistor Network (marked 10A103GA)
IC Sockets
Important:Orient each socket such that the semicircular notch on one end of the socket is over the notch on one end of the board outline. If you mistakenly orient a socket the wrong way, it will still work. Don't bother trying to remove it. Just be careful later when inserting the IC to orient it according to the board outline and not the reversed socket. Before inserting, visually check that all pins are straight. Don't force the socket into the board holes. If you feel resistance, straighten any pins that aren't aligned properly. Sean the socket against the board. While holding the socket in place with a finger, tack solder 2 diagonally opposite and pins on the bottom side of the board. Solder the remaining pins and properly solder the previously tack-soldered end pins. U2 16-pin DIP socket U3 16-pin DIP socket U5 16-pin DIP socket U27 16-pin DIP socket
Jumper Headers
The jumper headers are supplied as one strip containing six jumper header pairs. Five will be used. One spare is included in case you damage or lose one while separating them. You will need to cut this strip into individual single-pair pieces. Lay the strip on its side on a flat surface. Place the blade of a sharp utility knife in the first groove separating the jumper header pairs. Hold the knife so the blade will cut vertically downward. If the blade is angled to one side, you damage the parts. When cutting the jumper header strip, one of the both parts may fly. Use two fingers to restrain the strip on both sides of the utility knife. Apply downward pressure until the strip snaps at the groove.
11
AMT 3000 Assembly and Operating Instructions Repeat this procedure until you have separated all of the jumper pairs. Insert the short leads into the board. The long leads are the right length to receive the shorting plugs (installed later). S1 Single-pair jumper header S2 Single-pair jumper header S3 Single-pair jumper header S6 Single-pair jumper header S7 Single-pair jumper header C7 0,1 uF (104 or .1) ceramic multi-layer C8 0,1 uF (104 or .1) ceramic multi-layer C9 0,1 uF (104 or .1) ceramic multi-layer C10 0,1 uF (104 or .1) ceramic multi-layer C11 0,1 uF (104 or .1) ceramic multi-layer C22 0,1 uF (104 or .1) ceramic multi-layer C28 0,1 uF (104 or .1) ceramic multi-layer C31 0,1 uF (104 or .1) ceramic multi-layer C100 0,1 uF (104 or .1) ceramic multi-layer C101 0,1 uF (104 or .1) ceramic multi-layer C102 0,1 uF (104 or .1) ceramic multi-layer C103 0,1 uF (104 or .1) ceramic multi-layer
Ceramic Capacitors
Ceramic Trimmer Capacitor
The number shown in parenthesis is the value code marked on the device. C12 33 pF (33) ceramic C13 33 pF (33) ceramic C30 180 pF (181) ceramic C15 200 pF (201) ceramic C27 200 pF (201) ceramic Both capacitors are supplied with your kit. Install the 820pF capacitor if you will be using the supplied wire antenna for in-home broadcasting. Install the 560pF capacitor if you will be using the external base-loaded vertical antenna described at: http://www.sstran.com/pages/sstran_buildant.html. Match the shape of the trimmer capacitor to the board outline for C5. The trimmer will snap into the holes, holding it in place for soldering. C5 12 - 100 pF 7mm, ceramic trimmer capacitor
C23 820 pF (821) or 560 pF (561) ceramic
Rear Panel Jacks
Important: All jacks must be seated squarely in place against the board before soldering to ensure proper alignment with the holes in the enclosure rear panel. The RCA jacks will snap into the board holes, holding them in place adequately for soldering. You may need to end the three outer tabs slightly outward from the center to get proper snap-in action. The power jack (13) does not snap in. You can press it against the board with the finger while tack soldering several pins to hold in place. When soldering all jacks, flow plenty of solder until all holes are completely filled for mechanical stability. J1 RCA phono jack with white insulator J2 RCA phono jack with red insulator J3 Power jack J4 RCA phono jack with yellow insulator
Installation of the 560pF capacitor is one step in the transmitter modifications required for use with the vertical antenna. See the complete modification instructions at:
http://www.sstran.com/pages/sstran_ant3000mods.html.
C17 1000 pF (102) ceramic C21 1000 pF (102) ceramic C14 0,01 uF (103 or .01) ceramic multi-layer C16 0,01 uF (103 or .01) ceramic multi-layer C1 0,1 uF (104 or .1) ceramic multi-layer C2 0,1 uF (104 or .1) ceramic multi-layer
C3 0,1 uF (104 or .1) ceramic multi-layer C6 0,1 uF (104 or .1) ceramic multi-layer
12
AMT 3000 Assembly and Operating Instructions the hole marked with a «+» next to the board outliner. During assembly think: «longer lead goes in the '+' hole». The number shown in parenthesis is the value marked on the device. C18 4,7 uF (4,7uF) electrolytic C20 10 uF (10uF) electrolytic C24 10 uF (10uF) electrolytic C25 10 uF (10uF) electrolytic C26 10 uF (10uF) electrolytic C29 10 uF (10uF) electrolytic C32 100 uF (100uF) electrolytic C4 470 uF (470uF) electrolytic
Variable Resistors
Important:Like the jacks above, the variable resistors (R34, R35 and R36) must be seated squarely in place against the board before soldering so they will align properly with the holes in the enclosure front panel. They will snap into the board holes, holding them in place adequately for soldering. You may need to bend the side tabs slightly inwards to et them to snap in properly. R34 50K ohms (marked 50K) R35 50K ohms (marked 50K) R36 20K ohms (marked 20K)
+5V Voltage Regulator and Transistors
RFI Suppression Coils
These are the three large coils. L1 1000 uH RFI Suppression Coil (marked 1000uH) L2 1000 uH RFI Suppression Coil (marked 1000uH) L3 1000 uH RFI Suppression Coil (marked 1000uH)
Important: All transistors and the +5V voltage regulator have the same TO-92 package style. Because they all look alike, carefully check the part markings before insertion to get them all in the right places. Orient each component to match the board outline. Beginning at the body of the component, bend the two outer leads outward as shown in the illustration. After bending the outer leads, you should be able to insert the component until the bottom of the plastic body is about 1/8 in. or less from the board. Don't force it down. Remove and reform the outer leads if necessary. U4 L78L05 TO-92 +5V 0,1A VREG Q1 2N3906 T-92 PNP transistor Q2 PN2222A TO-92 NPN transistor Q3 PN2222A TO-92 NPN transistor Q4 PN2222A TO-92 NPN transistor Q5 PN2222A TO-92 NPN transistor
Quartz Crystal
X1 HC-49U Microprocessor Crystal (4'000) for models AMT3000 and AMT3000-SM X1 HC-49U Microprocessor Crystal (3'600) for models AMT3000-9K and AMT3000-9KSM
+15 Volt Regulator and Heat Sink
Electrolytic Capacitors
Important: Electrolytic capacitors are polarized and must be inserted in the proper direction. The shorter negative (-) lead is on the side with the vertical stripe. The longer positive (+) lead is opposite the stripe. You must insert the longer «+» lead into
U1 L7815 TO-220 +15V 1A VREG (L7815)
First, insert the +15 volt regulator at location U1, oriented such that the metal back plate on the regulator face the rear edge of the board. Solder the regulator in place. Now, following the illustration, mount the heat sink on the regulator.
13
AMT 3000 Assembly and Operating Instructions The heat sink must be oriented such that the mounting hole in the heat sink is toward the top. In this position, the bottom of the heat sink will be dose to the board. Insert the mounting screw through the regulator and then through the heat sink. Install the lock washer and nut, and tighten until lock washer is compressed. Heat sink Heat sink mounting screw (#6-32 x ¼) Heat sink mounting lock washer (#6) Heat sink mounting nut (#6 -32) The ICs must be mounted in their sockets in one direction only or they may be damaged when power is applied. The ICs are keyed with a notch in the top of one end or a dimple in the top corner of one end. Align the keyed end with the notched end of the socket (which will also be the notched end on the board component outline if the socket was installed properly). Before inserting the ICs into their sockets you must first bend the leads on each side slightly inward to engage the socket pins properly. This can be done neatly by laying the IC on its side on a flat surface and pressing down ward lightly on the IC body until the bottom leads start to bend slightly. This will keep all leads in a straight line. Do this to each side equally, Straighten any misaligned pins with your fingers or small pliers. Trial fit the IC into its socket. Re-bend any pins that are still out of alignment. When you are satisfied that all pins are aligned, press the IC as far as it will go down into the socket using firm, equal pressure on both ends. If it won't go, one or more pins may not be aligned. Remove the IC and re-bend any pins as necessary. You can remove an IC from its socket by alternately prying each end slightly with a small screwdriver until it is free. To allow the LED lamp to properly engage the front panel hole, the leads must be bent as shown. The illustration above is printed actual size so you may use it as a template. Hold the LED horizontally with the shorter (cathode) lead facing you. Bend both leads downward at a right angle at a point 0,21'' (7/32'', 5,3 mm) from the LED body. Insert the leads into the board at location D7 with the longer lead on the «arrow» side and the shorter lead on the «line» side oft he D7 «arrow/line» board outline. Slide the LED up or down in the holes until you measure 0,43'' (7/16'', 10,9 mm) from the top of the board to the bend in the leads. Tack solder one lead on the top side of the board. Re-check the height measurement and then solder the other lead and reflow solder to the tacked lead. D7 Green LED lamp U2 74HC4060 14-Stage Ripple Binary Counter, 16Pin DIP (74HC4060) U3 74HC40103 8-bit synchronous binary down counter, 16-Pin DIP (74HC40103) U5 74HC40103 8-bit synchronous binary down counter, 16-Pin DIP (74HC40103) U7 74HC4046 Phase-Locked Loop, 16-Pin DIP (74HC4046)
LED Power Indicator Lamp
Jumper Plug Installation
Initially install the five jumper header plugs according to the following steps. You can change the options later, but this will get you going. A jumper plug may be installed in either the shorted or open position. A plug is in the shorted position when it is plugged onto both header pins. It is in the open position when only one terminal is plugged onto one pin of the header. This provides a convenient place to store the jumper plug for possible later use in the shorted position. S1 Install in shorted position S2 Install in shorted position S3 Install in shorted position S6 Install in open position S1 Install in open position
Insert 16-Pin ICs into Sockets
Important: ICs are static sensitive. Before handling ICs you should be sure to discharge any static electricity from your body by touching a grounded metal surface. Avoid shuffling your feet or sliding around on a chair while handling the ICs. Touch that grounded surface frequently while working with the ICs.
14
AMT 3000 Assembly and Operating Instructions For initial testing, just string the antenna and ground wires out away from the board. The ground wire doesn't need to be connected to ground for initial testing. Connect an audio source such as a CD or cassette player to the audio input jacks, J1 and J2. Plug the AC adapter into a l20-volt outlet. The green LED power indicator should light up. Follow the procedures in section 8, «RF Tuning» and section 9, «Audio Adjustments» to verify the unit is operational.
Antenna and Ground Wires
Using wire strippers or a sharp knife, remove about 1/8'' (3,2mm) of insulation from one end of the white antenna wire and one end of the black ground wire, being careful not to nick or cut any of the wire strands.
8 RF Tuning
Setting the Frequency
Find a clear frequency on a nearby AM radio receiver between 530 kHz and 1700 kHz. Try to choose a clear frequency at the high end of the band. The antenna operates more efficiently at higher frequencies. Find your desired frequency in the chart in section 19, «Synthesizer Frequency Selection Chart», of this manual. Set the 8-Position frequency selection DIP switch (S4) according to the chart. Use a small screwdriver to slide each of the eight switches to the specified on or off position.
Separate the handle from the plug by unscrewing the handle. Pass both wires through the handle as shown. Solder the wires to the plug terminals as illustrated being careful to connect the white wire to the center pin terminal and the black wire to the outer ring terminal. Screw the handle back on to the plug. P4 Antenna Wire White, 118 in. P4 Ground Wire Black, 72 in.
Setting the RF Output Tuning Range
Set the tuning range that includes your desired frequency on the 4-position DIP switch (S5) according to the chart in section 20, «RF Output Tuning Range Selection», of this manual. Use a small screwdriver to slide each of the four switches to the specified on or off position.
Congratulations! Your board is now fully assembled.
6 Final Checks
Your board is now fully assembled. Take a little time now to visually check your work. Here are some final inspection steps. · · Are all step-by-step assembly instructions checked in this manual? Are all electrolytic capacitors, diodes, transistors, voltage regulators and ICs installed in the proper direction? Visually inspect the board with a magnifying glass. Look at every pad on the bottom of the board systematically from one corner to the opposite diagonal corner. Look for: unsoldered or poorly soldered connections, wires not clipped properly, solder bridges between adjacent pads, and wires that may be bent over and touching a nearby pad.
Peaking the RF Output
Locate the meter test point holes labeled T1 and T2 on the right side of your circuit board. Insert the positive (red) probe from your meter into T1. Insert the negative (black) probe into T2. The meter mode and range switches should be initially set to measure voltages as high as about 15 VDC. The meter must be a 10 megohm/volt or higher analog or digital meter to give a good reading. Turn the GAIN, MODULATION and COMPRESSION controls fully counterclockwise. Use a small screwdriver to adjust the trimmer capacitor, C5, through its entire range (one full turn) until you observe a voltage peak. Usually you will see two voltage peaks as you rotate one full turn. Either peak may be chosen. Set the trimmer at the high point of the peak. Note this voltage reading. Next try setting the output tuning range switch, S5, to the next lower or higher range and re-tune the trimmer capacitor. Repeat until you find the S5 switch setting that give the highest voltage peak. If you are using a digital meter, peak by adjusting the trimmer slightly and waiting for a stable reading, Repeat until you are satisfied with the peak. Don't try to get a precise peak down to the last digit! A reading within one volt of peak is satisfactory.
·
7 Power On Check
Place the assembled circuit board on an insulated surface such as a wooden table top or a sheet of cardboard, plastic or heavy paper. Plug the AC adapter cord into the power jack (13).
15
AMT 3000 Assembly and Operating Instructions
9 Audio Adjustments
Front Panel Control Functions
Gain Control The gain control varies the audio level feeding the SSM2166 preamplifier stage and serves to compensate for a wide range of possible audio input levels. Rotating the control clockwise increases the gain. Modulation Control The modulation control sets the limiting level of the compressor/limiter. Rotating the control clockwise increases the limiting level, which in turn, increases the maximum amplitude of the audio to the modulation stage, increasing the modulation percentage.
10Board Jumper Options
The four jumper headers labeled S1, S2, S3 and S6 on the circuit board provide grounding options and a treble boost option. Jumper header S7 should always remain open.
A Word About AC Building Wiring Interference Problems
RF isolation of the transmitter ground from the AC building wiring may be of benefit if you experience hum induced on the transmitted signal due to interaction of the transmitted RF field and the AC building wiring. The RH suppression coils (L1, L2 and L3) may optionally be inserted in series with the ground paths to an AC powered audio source device (L1) and to the AC wiring through the AC wall adapter (L2 and L3). With all three coils enabled (not shorted by jumper plugs), you must connect the transmitter black ground wire to an earth ground or the transmitter will not function properly. In general it is better to operate without RF ground isolation. Your transmitter range will generally be farther without RF ground isolation. Before you use the isolation options to reduce possible hum, thoroughly check your audio source connection for hum problems. Hum on the audio signal is usually caused by a break in the shield connection in the audio cable or in an audio connector.
Compression Control
The compression control varies the audio compression over a range between 1:1 (no compression) and 5:1 (maximum compression). Rotating the control clockwise increases compression.
Audio Adjustment Procedure
Initially set the 3 front panel controls as follows: Gain control fully clockwise, Modulation and Compression controls fully counterclockwise. Connect an audio source playing music that has a fairly constant volume range. Listen on a nearby radio. Follow these steps: Rotate the Modulation control clockwise until the received signal begins to distort due to over-modulation. Back off from this point until the distortion goes away. This is approximately the 100% modulation point. Rotate the Gain control counterclockwise until you reach a point where the volume just begins to decrease. At this point limiting will only occur on audio peaks. The Compression control may be set in any position according to your preferences. A good way to observe the effect of the Compression control is to play a classical music CD with very quiet and very loud portions. As the Compression control is rotated clockwise, you will notice that the volume of the quiet and loud portions begin to be the same. Don't confuse over-modulation distortion with distortion that may be caused by your receiver being overloaded by the strong RF signal from the transmitter. If necessary, move the receiver farther away or reorient it to reduce the signal being picked up by it's antenna. Your audio source level may not be high enough if you can't reach the distortion point in step 2 with both the Gain and Modulation controls in their maximum clockwise positions and the Compression control in the full counterclockwise position. You can compensate for lower level inputs by increasing the Compression. This will boost the audio signal, but you may get more compression than you desire, depending on your tastes.
Jumper Plug S1: Audio Source RF Isolation
Jumper S1 allows you to optionally connect the transmitter ground directly to the audio input cable ground (S1 jumper in shorted position) or to insert L1 in series with the ground side of the two audio jacks (S1 jumper in open position). If you choose the isolation option (S1 jumper open), you may not experience much difference unless you also choose the open positions for S2 and S3. Otherwise, there is still a ground path to the AC wiring through the AC adapter. In addition, with S1 in the open position you may need to connect the black transmitter ground wire to a good earth ground point. You will need to re-peak the trimmer capacitor, C5, each time to change the S1 option.
Jumper Plugs S2 and S3: AC Adapter RF Isolation
Jumpers S2 and S3 allow you to optionally isolate the transmitter from the RF ground path through the AC adapter transformer to the building wiring. With jumpers S2 and S3 removed, the RH suppression coils, L2 and L3 are placed in series with the two wires from the AC adapter. You will need to re-peak the trimmer capacitor, C5, each time to change the S2 and S3 options.
16
AMT 3000 Assembly and Operating Instructions 4. Lift the rear of the board slightly and hold the black plastic rear panel over the jacks while sliding the rear panel down into the slot at the rear of the bottom enclosure section. Reposition the board so the four mounting screw holes in the board align with the four plastic screw bosses on the bottom enclosure section. Check that the power indicator LED is engaged into the hole in the front panel. You may need to bend the LED leads slightly to properly position the LED in the front panel hole. Secure the board to the bottom enclosure section with four #4 x ¼'' thread forming screws. Tighten the screws only to the point where there is no longer any movement of the board. Mount the three control knobs on the front-panel control shafts. To mount a knob, loosen the set screw with a small-blade screwdriver, slide the knob onto the shaft as far as it will go without contacting the panel and then tighten the set screw lightly against the control shaft to hold the knob in place.
Jumper Plug S6: Treble Boost
Installing jumper plug S6 in the shorted position will add about 8 dB of treble boost to the audio with the midpoint of the boost curve at 2 kHz. Installing S6 in the open position will result in a flat audio response. Since treble boost is introduced after the SSM2166 compressor/limiter, you will probably need to readjust the modulation control counterclockwise somewhat to prevent over-modulation distortion at the higher audio frequencies. 5.
6.
Jumper Plug S7: Meter Resistor Bypass
This jumper header must always remain open for normal operation with the supplied wire antenna. Jumper S7 allows shorting across resistor R14 in the meter driving circuit when you use the extern al base-loaded vertical antenna described at: http://www.sstran.com/pages/sstran_buildant.html.
7.
Shorting across resistor R14 is one step in the transmitter modifications required for use with the vertical antenna. See the complete modification instructions at:
http://www.sstran.com/pages/sstran_amt3000mods.html
Leave the top enclosure section off for now to allow access for re-tuning after you move the transmitter to its final location.
12Final Installation
The location of your transmitter will affect its range. Generally the higher the better, However, its range is adequate to cover a whole house no matter where it is located, even in a basement. Because it draws so little AC power, you may elect to place it out of the way on a shelf or in a closet and leave it powered on continuously. Once you set the tuning and audio controls properly, your transmitter will not require further adjustment. You may use extra long audio cables (up to 25 ft) to feed the transmitter if it is located remotely from the audio source. Cables longer than 25 ft. may attenuate higher audio frequencies due to the additional capacitance of the cables. If you use longer cables, you may be able to compensate for audio high frequency loss by enabling the transmitter treble boost option or by boosting the treble at the audio source.
11 Mounting the Board in the Enclosure
First locate all enclosure parts: · · · · · · · Plastic enclosure bottom section (0,375'' high) Plastic enclosure top section (1,1'' high) Black plastic front and rear panels Four #4 x ¼'' thread forming screws Two #4 x ¾'' thread forming screws Four self-adhesive enclosure feet Three control knobs
FCC Antenna Rules
When installing your transmitter, keep in mind the FCC rule that governs antenna and ground lengths. Rule 15.219 states: «The total length of the transmission line, antenna and ground lead (if used) shall not exceed 3 meters.» Your transmitter does not have a transmission line, so you only need to be concerned with the total length of the white antenna wire and the black ground wire (if used). This total length is not to exceed 3 meters. You may need to cut the wires that are supplied in order to comply with this requirement.
Mount the printed circuit board and assemble the enclosure using the following steps: 1. Separate the four self-adhesive enclosure feet with scissors or a sharp knife. Peal off the backing paper and attach the feet to the bottom of the enclosure near the four corners. Slide the front panel down into the slot on one of the wide sides of the bottom enclosure section (the enclosure is symmetrical with respect to front and rear, it doesn't matter which wide side you pick). Lay the board down on the bottom enclosure section and slide it forward so that the control shafts protrude through the front panel holes.
2.
3.
Antenna Location
For best range the white antenna wire should be strung as vertically as possible and should be positioned away from grounded metal objects and electrical wiring. Secure the antenna wire as needed to keep it from moving around to prevent tuning variations.
17
AMT 3000 Assembly and Operating Instructions 3. Secure the top enclosure section to the bottom enclosure sections using two #4 x ¾'' thread forming screws inserted through the two holes on the bottom side of the enclosure. Tighten only to the point where there is no longer any play between the enclosure sections. These two screws may be removed at any time to access the circuit board for frequency changes or re-tuning if the location of the transmitter or antenna is changed.
Ground
A good, low impedance ground is required for best range. Just as a transmitter will not work without an antenna, it likewise will not work without a ground. A «ground» is· simply any path from the transmitter circuit ground to earth ground that provides a low RF impedance. The AMT3000 has been designed to provide a low RF impedance path to ground through the AC adapter and your building wiring. Additionally, when you connect the transmitter via audio cables to an AC powered audio source, there is likely to be a low RF impedance through the audio cable outer shields and then through the building wiring connected to the audio source device. These «built in» ground paths will provide good performance in many cases, but variations in building wiring and transmitter location may require you to ground the transmitter directly to a separate earth ground. You should experiment by connecting the AMT3000 black ground wire to whatever ground sources are available at your transmitter location until you find the optimum arrangement. You can try connecting the ground wire to the center screw on a wall outlet, a copper water pipe, a copper heating pipe, or possibly a forced air heating register if your ductwork provides a continuous conductive path back to the furnace ground. Also one or more interconnected grounding rods driven into the earth will work well. Grounding rods are available at electrical supply stores. Also a very good ground can be established by burying bare copper wires so they fan out in a radial pattern from a center common point connected to the transmitter ground wire. Refer back to section 10, «Board Jumper Options» for the optional ground isolation jumper settings. You can experiment with different jumper options and ground wire connections if you are not satisfied with the «default» shorted position of jumpers S1, S2 and S3.
13Troubleshooting
If your transmitter does not work, the cause is most likely one of the following: · · · · · Unsoldered or poorly soldered connections Solder shorts across adjacent pads Components mounted in the wrong locations Polarized components mounted in the wrong orientation Misinterpretation or mistakes made when setting DIP switches.
Start troubleshooting by carefully inspecting all the solder connections. Look for unsoldered connections and solder bridges across adjacent pads. While inspecting, keep your soldering iron plugged in and ready. If you see any connection that looks the least bit suspect, re-solder it just to be sure. Check the installation of all parts for proper location and orientation. Check the 16-pin ICs for any pins that may have been bent while plugging them into their sockets. In the unlikely event of a component failure, you will need some knowledge of electronic troubleshooting methods and an understanding of how the transmitter operates. Refer to section 14, «Theory of Operation» and to section 22, «Circuit Diagram» to learn how the transmitter works. Start by checking the voltages at the power pin of all IC's with a meter. Also check the voltages marked on the schematic around the transistors in the modulator and RF output stages. You can home in on a problem by first determining whether it is in the RF or the Audio section of your transmitter. If you can receive the carrier at the frequency you have selected, but there is no audio, then the problem is in the audio sections. If you can't receive the carrier at all, then the problem is in the synthesizer of RF output stage. Use of an oscilloscope to trace back to the failing area will make troubleshooting much easier. If you are really stuck, or don't want to spend the time fixing it, refer to section 15, «Warranty, Return and Service Policy» for information on our repair service.
Check Tuning
Whenever you change the antenna position or ground connection you will need to re-peak the RF output. Generally, all that is required is to readjust the trimmer capacitor, C5, but in some cases, you may need to set the 4-position DIP switch S5 to the next higher or lower tuning range to get the highest voltage peak on your meter.
Finish Assembling Enclosure
1. Orient the top enclosure section with respect to the bottom section so that the tabs and slots on the sides will engage properly (the top mates with the bottom only one way). Slip the top enclosure section down over the front and rear panels so that the panels engage the slots in the front and rear of the top enclosure section.
2.
18
AMT 3000 Assembly and Operating Instructions
14Theory of Operation
This section presents a description of the operation of the AMT3000. Referring to the circuit diagram in section 22 will aid in understanding the description. The AMT3000 has five major functional sections: (1) Oscillator and Frequency Synthesizer, (2) RF Power Output, (3) Preamplifier and Compressor/Limiter (4), Modulator and (5) Power Supply.
RF Power Output Section
The design of the RF power output stage of the AMT3000 is an important contributor to its overall modulation quality. The output stage operates in class C mode, which simply means that the output transistor switches fully on and fully off at the RF carrier frequency rate. This results in higher efficiency than output stages that operate in the linear region. Modulation is applied to the output stage by varying the on-state emitter current to the output transistor at the input audio rate. Output transistors Q4 and Q5 are connected to form a differential pair switching circuit. The VCO OUT signal from U7 pin 4 is applied to the base of Q4 through C31 and R27. Q4 is driven into saturation (on-state) when VCO OUT is high and to cutoff (off-state) when VCO OUT is low. Due to the biasing and common emitter connection of Q4 and Q5, when Q4 switches on, Q5 switches off, and vice versa. The current, determined by Q2, flows through either Q4 when VCO OUT is high or through Q5 when VCO OUT is low. The transmitter RF output is taken from the collector of Q5, through DC blocking capacitor C22, to the pi-network tuning and antenna matching circuit. Because Q5 switches fully on and fully off at the RF rate, the Q5 collector current switches between zero and the instantaneous audio-rate current determined by modulation transistor Q2. The instantaneous modulation current determines the amplitude of the RF output signal, thus achieving amplitude modulation. For a 100% modulated carrier, Q5 collector current will vary at the audio rate from zero at full negative modulation to twice the unmodulated current at full positive modulation. RF choke L8 allows DC current to flow to Q5 but blocks the RF output from being shorted to the +15 volt supply. Resistor R18 across L8 limits the voltage swing across L8 to maintain a good modulated RF envelope for poorly tuned or very short antennas. It has little effect when a normal 2-3 meter long antenna is used and the output network is tuned properly. The pi-network antenna matching circuit, consisting of C23, L4-L7 and tuning capacitor C5, is designed to transform the low impedance at the collector of Q