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Elenco Electronics AM/FM-108CK, Assembly And Instruction Manual, Page: 27 / 56

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Elenco Electronics AM/FM-108CK Assembly And Instruction Manual Download Page 27

STATIC MEASUREMENTS

Q7 BIAS

Connect your VOM to the circuit as shown in Figure 20.

Figure 20

Connect a clip lead from TP6 to the collector of Q7.
This short prevents Q7 from oscillating.  Set the VOM to
read 2 volts DC and turn the power ON.  The DC
voltage at TP7 should be about 1.6 volts.  If the voltage

in your circuit differs by more than .5 volts, leave the
power ON and check the battery voltage.  If the battery
voltage is greater than 8.5 volts, check components
R31, R32, R33 and Q7.  Turn the power OFF.

Set the scope to read 1 volt per division and turn the
power ON.  The scope should display a low voltage
sinewave.  The frequency of the sinewave should

change when the tuning gang is turned.  If your circuit
fails this test, check components Q7, gang capacitor,
C28, C29, C30, C31, L4 and L5.  Turn the power OFF.

DYNAMIC MEASUREMENTS

AM OSCILLATOR CIRCUIT

Connect your test equipment to the circuit as shown in Figure 21.

If you do not have an oscilloscope, skip to the AM Final Alignments.

-26-

Short TP6 to the collector of Q7 as shown below.

V

COM

V

TP15

TP15

Oscilloscope

Figure 21

Summary of Contents for AM/FM-108CK

Page 1: ...Instruction Manual Copyright 2010 2000 by Elenco Electronics Inc All rights reserved Revised 2010 REV L 753508T No part of this book shall be reproduced by any means electronic photocopying or otherwi...

Page 2: ...SW Pot SW with nut and washer 192522 CAPACITORS Qty Symbol Value Description Part r 1 C30 150pF Discap 151 221510 r 1 C44 001 F Discap 102 231036 r 2 C31 38 01 F Discap 103 241031 r 5 C29 33 35 36 37...

Page 3: ...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 Volta...

Page 4: ...s the FM mixer FM oscillator FM RF and the AFC circuits The incoming radio waves are amplified by the FM RF amplifier which is tuned to a desired radio station in the FM frequency bandwidth of 88MHz t...

Page 5: ...dustry but it is now being replaced by lead free solder for health reasons This kit contains lead free solder which contains 99 3 tin 0 7 copper and has a rosin flux core Lead free solder is different...

Page 6: ...ctor Using an Ohmmeter connect the transistor as shown in Test C Your meter should be reading a low resistance Switch the lead from the Emitter to the Collector Your meter should again be reading a lo...

Page 7: ...ximize efficiency the transistors Q11 and Q12 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 amplif...

Page 8: ...top legend Leave 1 4 between the part and PC board Diode Be sure that the band is in the correct direction Figure E 7 EBC E B C EBC E B C Flat Side Flat Side Band Cathode Anode R43 100 Resistor brown...

Page 9: ...NS Figure G If the speaker pad has center and outside pieces then remove them Peel the backing off of the speaker pad and stick the pad onto the speaker Then stick the speaker onto the solder side of...

Page 10: ...you VOM turn the volume to the ON position rotate clockwise until a click is heard The VOM should indicate a very low current Adjust your meter for a more accurate reading if necessary If the current...

Page 11: ...through R44 can now be calculated as 3 8 R44 or 3 8 3 3k which equals 1 15 milliamps Since D5 and R42 are used for biasing transistors Q11 and Q12 the current through Q10 can be assumed to be 1 15 mi...

Page 12: ...300 22 6 which equals 146 When the input signal is positive there will be a current flowing in Q11 which we will call I Q11 This current will then be multiplied by the Beta of transistor Q13 or x I Q1...

Page 13: ...Set the oscilloscope to read 5 volts per division Turn on the power and slowly increase the volume control to a comfortable level Increase the amplitude of the audio generator until the oscilloscope d...

Page 14: ...ping can be calculated using the voltage Vclp obtained in step 4 as follows Vpeak Vp Vclp 2 Vroot mean squared Vrms Vp x 7 Max power out Vrms 2 8 ohms Vclp x 35 2 8 Maximum power output should be grea...

Page 15: ...ower Imax x V supply Battery power ______ Since the efficiency N is equal to the Max power out divided by the Battery power we can now calculate the efficiency of the audio amplifier N Max power out B...

Page 16: ...ase of transistor Q8 to the full gain position when no signal is present Resistors R35 and R36 also forward bias diode D4 just enough to minimize On Condition threshold voltage The purpose of the dete...

Page 17: ...itive lead of the VOM to TP3 and the negative lead to ground pin TP15 Make sure that the VOM is set to read 9 volts DC and turn the power ON The voltage on the VOM should be the same as your battery v...

Page 18: ...e voltage at TP5 just starts to drop This point is called the AGC threshold with no IF gain Make a note of the amplitude setting on the RF generator here ____________ Set the RF generator at 455kHz 1k...

Page 19: ...ne radio station while rejecting all others The second IF transformer T8 acts as a bandpass filter with a 3dB bandwidth of approximately 6kHz The amplitude versus frequency response of the second IF a...

Page 20: ...volts turn the power OFF and check components R39 R40 R41 and Q9 If you do not have an RF generator and oscilloscope skip to Section 4 ASSEMBLY INSTRUCTIONS TP4 Test Point Pin see Figure A T7 AM IF Co...

Page 21: ...ld be greater than 100 If your value is less than 50 check components R39 R40 R41 C36 and C37 Also make sure that Q9 is properly installed Turn the power OFF BANDWIDTH Reconnect your test equipment as...

Page 22: ...mplifier 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 seco...

Page 23: ...han 100 DO NOT TURN THE POWER OFF GO TO THE NEXT TEST AGC ACTION Move the scope probe back to TP4 and adjust the generator for 4Vpp if necessary Remove the clip lead shorting TP3 to R38 The AGC should...

Page 24: ...uning of the desired radio station The degree of accuracy in keeping the oscillator frequency exactly 455kHz above the tuning of the antenna is called tracking accuracy White Black Red White Black Red...

Page 25: ...he second holder into the left hole and twist the tab 90O Slide the ferrite core through the holders Slide the antenna coil through the ferrite core Note If the end of a wire from the antenna should b...

Page 26: ...a screw Rotate the knob fully clockwise Peel off the protective backing on the label Line up the long white lines on the label with the arrows on the PC board ASSEMBLY INSTRUCTIONS PC Board Stand Inse...

Page 27: ...8 5 volts check components R31 R32 R33 and Q7 Turn the power OFF Set the scope to read 1 volt per division and turn the power ON The scope should display a low voltage sinewave The frequency of the s...

Page 28: ...ons are present slide the antenna back and forth on its ferrite core and retune the dial if necessary Adjust T6 until the station is at its loudest Reduce the volume if necessary Adjust T7 until the s...

Page 29: ...enna coil needs less capacitance Adjust the antenna trimmer on the back of the gang until the signal is at its loudest Repeat these steps until the signal heard decreases for both ends of the magic wa...

Page 30: ...width ___________kHz OSCILLATOR ALIGNMENT Set the RF generator at 540kHz 400kHz 80 AM modulation and a low level of output Turn the power ON and set the volume control to a comfortable level Turn the...

Page 31: ...adio placed near the Superhet 108T If the AM section is still not receiving go back and check each stage for incorrect values and for poor soldering Proceed to the FM assembly section 1 The number of...

Page 32: ...nsmitter to the receiver determine the A frequency of the modulation B wavelength of the audio waves C amount of signal picked up by the antenna D type of filter that is used 6 When the two metal plat...

Page 33: ...213010 r 1 C6 33pF Discap 33 213317 r 1 C11 220pF Discap 221 222210 r 2 C4 5 470pF Discap 471 224717 r 3 C3 7 27 001 F Discap 102 231036 r 3 C2 8 12 005 F Discap 502 235018 r 10 C13 22 01 F Discap 10...

Page 34: ...gh one diode will be greater than the other This causes a current to flow in C23 which will produce an audio voltage across C23 If the modulation is of opposite direction than before more current will...

Page 35: ...k red gold D3 1N34A Diode see Figure E T4 FM Detector Coil Pink Dot see Figure 28 FM Detector Coil T4 Test Point Pin Foil Side of PC Board Figure A NPN Transistor Figure C Mount so E lead is in the ar...

Page 36: ...through resistor R22 is equal to the current through transistor Q6 calculate the current through Q6 as follows Current I V Q6 R22 Your calculated answer should be between 0005 amps 5 milliamps and 00...

Page 37: ...and minimum voltage output Set the scope to read 50mV division Turn the power ON and slowly increase the amplitude of the generator until 3 divisions or 150mVpp are seen on the scope With an alignmen...

Page 38: ...Turn the power ON and set the volume control to a minimum Increase the amplitude of the sweep generator until an S curve is seen refer to Figure 33 Using an alignment tool or screwdriver adjust the bl...

Page 39: ...Q5 and T2 If you don t have an RF generator and oscilloscope skip to Section 8 38 ASSEMBLY INSTRUCTIONS TP10 Test Point Pin see Figure B T2 FM IF Coil Green Dot C17 01 F Discap 103 R16 10k Resistor br...

Page 40: ...an be calculated as follows AC Gain 150mV Vb Your calculated answer should be about 20 Record your calculation AC Gain __________ BANDWIDTH With the power turned OFF connect your test equipment as sho...

Page 41: ...Q4 The current in Q4 is set by the resistors R12 R13 and R15 Capacitors C14 and C15 bypass the AC signal to ground C13 and C16 are bypass capacitors from V to ground to prevent feedback on the V line...

Page 42: ...ecord your calculation AC Gain __________ BANDWIDTH Connect your test equipment as shown in Figure 38 Set your generator at 10 7MHz no modulation and minimum voltage output Set the scope to read 20mV...

Page 43: ...odyned with the local oscillator During the mixing process a difference frequency of 10 7MHz is produced This difference frequency is used as the IF in FM radios The collector of transistor Q3 contain...

Page 44: ...itance of 12pF or less otherwise the probe will detune T1 resulting in an incorrect measurement Set your scope to read 10mV per division Set your RF generator at 10 7MHz no modulation minimum voltage...

Page 45: ...iginal value 2 8 divisions or 28mVpp Record the frequency of the low 3dB drop off point here Fl _________MHz Turn the power OFF The bandwidth can be calculated as follows Bandwidth Fh Fl Your calculat...

Page 46: ...or diodes are positioned in such a way that when the 10 7MHz center frequency increases the DC correction voltage will decrease Likewise when the 10 7MHz center frequency decreases the DC correction v...

Page 47: ...he oscillator from drifting If the voltage at D1 still does not change at D1 check D1 R29 R30 C26 and C27 If these parts are inserted correctly and the voltage at D1 still doesn t change then increase...

Page 48: ...hen the inductance of a resonant circuit is changed the resonant frequency is changed also When aligning the oscillator changing the resonant frequency changes the frequency of oscillation Likewise wh...

Page 49: ...ass end near coil L3 Refer to Figure 45 If the station is heard this means that L3 needs less inductance Carefully pull apart L3 until the station is heard Place the iron end near L3 If the station is...

Page 50: ...nd check each stage for incorrect values and for poor soldering Approx 1 16 gap Approx 1 16 gap Figure 46 Figure 47 Turn the power ON Slowly increase the amplitude of the generator until a 1kHz signal...

Page 51: ...out Tune your radio until a 1kHz tone is heard Place the brass end of your magic wand near RF coil L1 If the signal on the scope increases it means that coil L1 needs less inductance Carefully spread...

Page 52: ...n at 1000Hz 150 times Typical distortion at 100 milliwatts output 2 AM Radio Specifications Tuning range 520kHz to 1620kHz IF frequency 455kHz Tracking 3dB from 700kHz to 1400kHz 10dB signal to noise...

Page 53: ...ontrol the audio frequencies in the receiver 5 The ratio detector transformer is tuned to A 10 7MHz B 88MHz C 455kHz D 10 9MHz 6 The ratio detector is used because A it is sensitive to noise B it is i...

Page 54: ...sound wave The end result 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 tra...

Page 55: ...54 SCHEMATIC DIAGRAM AM FM 108T RADIO...

Page 56: ...Elenco Electronics Inc 150 Carpenter Avenue Wheeling IL 60090 847 541 3800 Website www elenco com e mail elenco elenco com...

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