manualshive.com logo in svg

Elenco Electronics AM-550K, Assembly And Instruction Manual, Page: 14 / 32

Share
Download
Elenco Electronics AM-550K Assembly And Instruction Manual Download Page 14

-13-

DYNAMIC MEASUREMENTS

Figure 10

Connect the oscilloscope and audio generator to your
circuit as shown in Figure 10.
Set the audio generator for a frequency of 1kHz and
minimum voltage output.  Set the oscilloscope to read
.5 volts per division.  Turn the power ON and slowly
increase the generator output until the oscilloscope
displays 2 volts peak to peak (Vpp) at TP7.  Move the
oscilloscope probe to TP6 and record the input voltage
here:  Vin=___________ Vpp, (at this point you may
want to verify the AC Gain).  Move the oscilloscope
probe back to TP7 and slowly increase the frequency
from the audio generator until the waveform on the
oscilloscope drops to .7 of its original reading, 1.4 Vpp

or 2.8 divisions.  Use the oscilloscope probe to check
TP6 to make sure the input voltage did not change.  The
frequency of the generator when the output drops to .7
of its original value is called the high frequency 3
decibel (dB) corner.
Repeat this procedure by lowering the frequency from
the generator to obtain the low frequency 3dB corner.
Leave the oscilloscope connected to TP7 and turn the
power OFF.  By subtracting the frequency of the low
corner from  the frequency of the high corner, you
calculate the bandwidth of the Audio Amplifier.  Your
bandwidth should be greater than 100kHz.

AC BANDWIDTH

10

μ

F

Oscilloscope

Generator

Output Adjust

TP8

TP8

Probe

Summary of Contents for AM-550K

Page 1: ...DE Assembly and Instruction Manual Copyright 2007 1999 Elenco Electronics Inc Revised 2007 REV S 752550 No part of this book shall be reproduced by any means electronic photocopying or otherwise without written permission from the publisher Elenco Electronics Inc ...

Page 2: ...n Part 1 C1 Variable Tuning 211677 1 C15 001μF Discap 102 231036 2 C3 C10 01μF Discap 103 241031 5 C2 C5 C7 C8 C9 02μF or 022μF Discap 203 or 223 242010 1 C16 047μF Discap 473 244780 3 C4 C11 C12 10μF Electrolytic Radial Lytic Capacitor 271045 1 C6 100μF Electrolytic Radial Lytic Capacitor 281044 2 C13 C14 470μF Electrolytic Radial Lytic Capacitor 284743 SEMICONDUCTORS Qty Symbol Description Part ...

Page 3: ...l Tuning Diode Transistor LM 386 IC Antenna Assembly Coil Coil Plastic Holders Ferrite Core Color Dot Knob pot Earphone Jack with Nut Knob dial Radio Stand COILS MISCELLANEOUS Label Dial Knob Battery Holder Earphone OR Speaker Pad Speaker IC Socket Test Point Pin Screw 2 56 x 1 4 Screw M2 5 x 3 8mm Nut 2 56 Screw 2 56 x 5 16 ...

Page 4: ...t times to signify a decimal point as in 3R3 3 3 103K 100V The letter M indicates a tolerance of 20 The letter K indicates a tolerance of 10 The letter J indicates a tolerance of 5 Maximum Working Voltage The value is 10 x 1 000 10 000pF or 01μF 100V Electrolytic capacitors have a positive and a negative electrode The negative lead is indicated on the packaging by a stripe with minus signs and pos...

Page 5: ...the stages have been built and tested a final alignment procedure is provided to peak the performance of the receiver and maximize the Superhet 550 s reception capabilities The Superhet 550 can best be understood by analysis of the block diagram shown in Figure 1 The purpose of section 1 the Audio Amplifier Stage is to increase the power of the audio signal received from the detector to a power le...

Page 6: ... is completed Be sure to save the extra leads for use as jumper wires if needed Soldering The most important factor in assembling your AM radio kit is good soldering techniques Using the proper soldering iron is of prime importance A small pencil type soldering iron of 25 40 watts is recommended The tip of the iron must be kept clean at all times and well tinned Many areas on the PC board are clos...

Page 7: ...the solder will not flow onto the lead as shown 2 Insufficient solder let the solder flow over the connection until it is covered Use just enough solder to cover the connection 3 Excessive solder could make connections that you did not intend to between adjacent foil areas or terminals 4 Solder bridges occur when solder runs between circuit paths and creates a short circuit This is usually caused ...

Page 8: ...he Emitter to the Collector Your meter should again be reading a low resistance Using an Ohmmeter connect the transistor as shown in Test B Your meter should be reading a high resistance Switch the lead from the Emitter to the Collector Your meter should again be reading a high resistance Typical results read approximately 1MΩ to infinity DIODE TEST Refer to the parts list and find a diode This is...

Page 9: ... type of distortion is shown in Figure 3 In order to eliminate crossover distortion and maximize efficiency the transistors of the audio amplifier circuit are biased on for slightly more than 1 2 of the cycle Class AB In other words the transistors are working as Class A amplifiers for very small levels of power to the speaker but they slide toward Class B operation at larger power levels To make ...

Page 10: ...f the PC board Solder terminal 1 to the pad of the PC board 1 Wire 1 Part 622131 Part 622130 1 GND 2 Tip 3 N C Tip 1 Wires 1 Wire 1 3 2 Jack 1 GND 2 Tip 3 N C Tip Your kit may contain a different type of earphone jack Before installing the jack determine which one you have Solder the GND terminal to the PC board pad GND Pad Nut Jack 2 3 Nut GND Pad From Terminal 3 Figure B If the speaker pad has c...

Page 11: ...own black black gold Figure I Notch TP6 Test Point Pin see Figure E C12 10μF Lytic Capacitor see Figure F C11 10μF Lytic Capacitor see Figure F R13 47Ω 5 1 4W Resistor yellow violet black gold U1 IC Socket 8 Pin U1 Integrated Circuit LM 386 see Figure I Pot with Switch Nut Washer Knob Solder 5 lugs to PC board Top Side Test Point Pin Foil Side of PC Board Figure E Nut Washer Electrolytics have a p...

Page 12: ...ncorrectly Check C14 to see if it s leaky or inserted backwards If you get a reading higher than 10MΩ check for open copper or bad solder connections on all components Figure 6 Figure 7 Set your VOM to read the highest possible current Connect the meter to the circuit as shown in Figure 7 Make sure that the On Off switch SW1 is in the OFF position While watching your VOM flip switch SW1 to the ON ...

Page 13: ...nd go directly to Section 2 DYNAMIC MEASUREMENTS Connect the VOM and audio generator to TP6 as shown in Figure 9 Turn the power ON Normally the AC gain is measured at a frequency of 1 kilohertz kHz Your VOM however may not be able to accurately read AC voltages at this frequency It is recommended therefore that this test be performed at 400Hz Set the audio generator at 400Hz and minimum voltage ou...

Page 14: ... frequency from the audio generator until the waveform on the oscilloscope drops to 7 of its original reading 1 4 Vpp or 2 8 divisions Use the oscilloscope probe to check TP6 to make sure the input voltage did not change The frequency of the generator when the output drops to 7 of its original value is called the high frequency 3 decibel dB corner Repeat this procedure by lowering the frequency fr...

Page 15: ...t the generator at a frequency of 1kHz turn the power ON and adjust the generator output until the peaks of the sinewave at TP7 are clipped as shown in Figure 11 DISTORTION Clipped Crossover Distortion Figure 11 Figure 12 By measuring the DC power taken from the battery at the maximum power output level the efficiency to the Audio Amplifier can be calculated Power from the battery is equal to the ...

Page 16: ... audio at the volume control as shown in Figure 13 The purpose of the automatic gain control AGC circuit is to maintain a constant audio level at the detector regardless of the strength of the incoming signal Without AGC the volume control would have to be adjusted for each station and even moderately strong stations would clip in the final IF amplifier causing audio distortion AGC is accomplished...

Page 17: ... VOM to test point three TP3 as shown in Figure 14 Check that the VOM is adjusted to read 9 volts DC and turn the power ON The voltmeter should read approximately 1 5 volts DC If your reading varies more than 5 volts from this value turn the power OFF and check the polarity of D1 and resistors R11 and R5 Also check that transformer T1 is properly installed AGC ZERO SIGNAL BIAS With the power turne...

Page 18: ...ontrol at full clockwise position Slowly adjust the amplitude of the RF generator output until you hear the 1kHz on the speaker If this test fails turn the power OFF and check C11 R12 volume control D1 and TP3 SYSTEM CHECK Figure 16 Connect equipment as shown in Figure 16 Set the RF generator at 455kHz with 80 modulation at a modulation frequency of 1kHz Set the oscilloscope to read 1 volts per di...

Page 19: ... is shown in Figure 17 Both IF amplifiers are tuned to a frequency of 455kHz and only need to be aligned once when the radio is assembled These amplifiers provide the majority of the gain and selectivity needed to separate the radio stations The gain at 455kHz in the second IF amplifier is fixed by the AC impedance of the primary side of transformer T3 and the DC current in Q3 The current in Q3 is...

Page 20: ...f 455kHz no modulation and minimum amplitude output Set the oscilloscope vertical sensitivity at 1 volt division The scope probe must have an input capacitance of less than 50pF or it will detune transformer T3 Turn the power ON and slowly increase the amplitude of the RF signal until you have 4 volts peak to peak on the oscilloscope Tune transformer T3 for a maximum output while readjusting the R...

Page 21: ...ues shown in Figure 17 BANDWIDTH TEST FIRST IF AMPLIFIER The operation of the first IF amplifier is the same as for the second IF amplifier with one important difference The gain of the first IF amplifier decreases after the AGC threshold is passed to keep the audio output constant at the detector and prevent overload of the second IF amplifier This is accomplished by making the voltage on the bas...

Page 22: ...to R8 as shown in Figure 21 This short prevents the AGC from lowering the gain of the first IF amplifier Set the RF generator to 455kHz no modulation and minimum amplitude output Set the oscilloscope for a vertical sensitivity of 1 volt division and turn the power ON Increase the amplitude output from the RF generator until approximately 4Vpp registers on the oscilloscope Tune the IF transformer T...

Page 23: ... tuned only to the difference frequency This transformer rejects all frequencies except those near 455kHz T1 also couples the 455kHz signal to the base of Q2 to be processed by the IF amplifiers The antenna and the oscillator coils are the only two resonant circuits that change when the radio is tuned for different stations Since a radio station may exist 455kHz above the oscillator frequency it i...

Page 24: ...h the arrow on the PC board Tuning Capacitor Figure J1 Your kit may contain a 3 lead or a 4 lead capacitor Bend the leads as shown Fasten C1 into place on the top side of the PC board with two M2 5 x 3 8mm screws C1 Screw Holes Knob Post Solder leads to pads 3 Leads 4 Leads R1 56kΩ Resistor green blue orange gold L2 Oscillator Coil red dot Q1 2N3904 Transistor NPN see Figure G C3 01μF Discap 103 C...

Page 25: ...ce and stroke the wire with your iron The insulation should come off very easily CAUTION The soldering iron will burn the hard surface that you are working on 3 Wire Type Antenna Solder the 3 colored wires to the PC board Wire A red to the hole marked RED Wire B black to the hole marked BLK Wire C white to the hole marked WHT 4 Wire Type Antenna Solder the 4 colored wires to the PC board Wire A gr...

Page 26: ...F and check components R1 R2 R3 and Q1 Q1 BIAS If you do not have an oscilloscope go to the Final Alignments With No Test Equipment Section DYNAMIC MEASUREMENTS With the power turned OFF connect the oscilloscope to the circuit as shown in Figure 23 Set the oscilloscope for a vertical sensitivity of 1 volt division and turn the power ON The oscilloscope should display a low voltage sine wave The fr...

Page 27: ...ower 3dB corner here Fl ____________kHz Increase the RF generator frequency past the peak to the upper 3dB corner and record that frequency here Fh ____________kHz The bandwidth of the IF amplifiers is BW Fh Fl IF bandwidth should be between 1 to 2kHz This bandwidth will widen as the AGC is approached IF BANDWIDTH With the power turned OFF connect the equipment to the circuit as shown in Figure 24...

Page 28: ... frequency by adjusting the antenna trimmer on the back of C1 see Figure 25 This process should be repeated until both settings of the antenna track the oscillator tuning Once the antenna is properly aligned carefully apply candle wax or glue to the antenna coil and ferrite rod as shown in Figure 27 ANTENNA ALIGNMENT After peaking the oscillator trimmer capacitor return the RF generator to 540kHz ...

Page 29: ...ble station This procedure peaked the IF amplifiers to their maximum gain Tune the radio until a known station around 600kHz is found It may be necessary to listen to the station until their broadcast frequency is announced If no stations are present at the low side of the AM band adjust L2 until a station is heard Once a station is found and its broadcast frequency is known rotate the dial until ...

Page 30: ...esult is a speaker less efficient and distorted To eliminate the low frequency cancellation a speaker is placed inside an enclosure Now the front sound waves are prevented from traveling to the back The speaker will now compress and decompress air inside increasing its resonant frequency and Q relative to the free air values This type of effectively air tight box is called an Acoustic Suspension 3...

Page 31: ... 30 SCHEMATIC DIAGRAM L1 REV A ...

Page 32: ...Elenco Electronics Inc 150 Carpenter Avenue Wheeling IL 60090 847 541 3800 Web site www elenco com e mail elenco elenco com ...

Reviews:

No comments