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Elenco Electronics SUPERHET AM-550TK, Сборка и инструкция по эксплуатации

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Elenco Electronics SUPERHET AM-550TK Скачать руководство пользователя страница 20

-19-

SECTION 3

Figure 15

.707

452kHz

458kHz

455kHz

TP4 - Test Point Pin

(see Figure A)

T2 - IF Coil

(White)

Q3 - 2N3904 Transistor NPN

(see Figure C)

R10 - 470

W

Resistor

(yellow-violet-brown-gold)

R7 - 39k

W

Resistor

(orange-white-orange-gold)

R9 - 10k

W

Resistor

(brown-black-orange-gold)

C7 - .02

m

F or .022

m

F Discap

(marked 203 or 223)

C8 - .02

m

F or .022

m

F Discap

(marked 203 or 223)

The purpose of the SECOND IF AMPLIFIER is to
increase the amplitude of the intermediate frequency (IF)
and at the same time provide SELECTIVITY. Selectivity
is the ability to “pick out” one radio station while rejecting
all others. The second IF transformer (T3) acts as a
bandpass filter with a 3dB bandwidth of approximately
6kHz. The amplitude versus frequency response of the
second IF amplifier is shown in Figure 15.

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 set
by resistors R7, R9 and R10. Both C7 and C8 bypass
the 455kHz signal to ground, making Q3 a common
emitter amplifier. The signal is coupled from the first IF
amplifier to the second IF amplifier through transformer
T2. The IF transformers not only supply coupling and
selectivity, they also provide an impedance match
between the collector of one stage and the base of the
next stage.

This match allows maximum power to

transfer from one stage to the next.

SECOND IF AMPLIFIER

THEORY OF OPERATION

ASSEMBLY INSTRUCTIONS - SECOND IF AMPLIFIER

Содержание SUPERHET AM-550TK

Страница 1: ...AM RADIO KIT MODEL SUPERHET AM 550TK 7 TRANSISTORS Elenco TM Electronics Inc Assembly and Instruction Manual Copyright 2003 1999 ElencoTM Electronics Inc Revised 2003 REV D 752550T ...

Страница 2: ...1000 1 Pot SW1 50kW SW Pot SW with Nut and Washer 192522 CAPACITORS Qty Symbol Value Description Part 1 C1 Variable Tuning 211677 1 C15 001mF Discap 102 231036 2 C3 C10 01mF Discap 103 241031 5 C2 C5 C7 C8 C9 02mF or 022mF Discap 203 or 223 242010 2 C4 C11 10mF Electrolytic 271045 1 C12 47mF Electrolytic 274742 1 C6 100mF Electrolytic 281044 2 C13 C14 470mF Electrolytic 284743 SEMICONDUCTORS Qty S...

Страница 3: ...g Diode Transistor Antenna Assembly Coil Color Dot Test Pin Earphone Jack with Nut Speaker Knob dial Radio Stand COILS MISCELLANEOUS Label Battery Holder Screw 2 56 x 1 4 Screw M2 5 Nut 2 56 Earphone OR Speaker Pad 50kW Potentiometer Switch with Nut and Washer Coil Ferrite Core Plastic Holders Knob pot ...

Страница 4: ...erance of 10 The letter J indicates a tolerance of 5 For the No 0 1 2 3 4 5 8 9 Multiply By 1 10 100 1k 10k 100k 01 0 1 Multiplier Note The letter R may be used at times to signify a decimal point as in 3R3 3 3 BANDS 1 2 Multiplier Tolerance 103K 100 Maximum Working Voltage The value is 10 x 1 000 10 000pF or 01mF 100V BAND 1 1st Digit Color Digit Black 0 Brown 1 Red 2 Orange 3 Yellow 4 Green 5 Bl...

Страница 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 550T s reception capabilities The Superhet 550T 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 ...

Страница 6: ...ering Iron Drag Foil 1 Solder all components from the copper foil side only Push the soldering iron tip against both the lead and the circuit board foil 2 Apply a small amount of solder to the iron tip This allows the heat to leave the iron and onto the foil Immediately apply solder to the opposite side of the connection away from the iron Allow the heated component and the circuit foil to melt th...

Страница 7: ...r Your meter should again be reading a high resistance Typical results read approximately 1MW to infinity Refer to parts list and find a PNP transistor refer to Figure D page 8 for locating the Emitter Base and Collector 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...

Страница 8: ...ansistors 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 Transistor Q4 is a Class A amplifier that drives the base of transistor Q5 directly Q5 is a current amplifier that multiplies the collector current of Q4 by the beta current gain B of Q5 The current from Q5 drives the output transistors Q6 and Q7 ...

Страница 9: ...side is in the same direction as shown on the top legend Leave 1 4 between the part and PC board Diode Be sure that the band is in the correct direction Figure E Polarity Mark EBC E B C EBC E B C Flat Side Flat Side Band Cathode Anode R14 47kW Resistor yellow violet orange gold Q4 2N3904 Transistor NPN see Figure C TP6 Test Point Pin see Figure A C11 10mF Lytic see Figure B C14 470mF Lytic see Fig...

Страница 10: ...one 1 wire and strip 1 4 of insulation off of both ends Solder the wires in the locations shown Figure H Figure G Pad Backing Speaker Backing Step 1 If the speaker pad has center and outside pieces then remove them Peel the backing off of one side of the speaker pad and stick the pad onto the speaker Step 2 Remove the other backing from the speaker pad Step 3 Stick the speaker onto the solder side...

Страница 11: ...orts or parts inserted incorrectly Check C14 to see if it s leaky or inserted backwards If you get a reading higher than 150kW check for open copper or bad solder connections on resistors R13 and R14 Figure 3 Figure 4 Set your VOM to read the highest possible current Connect the meter to the circuit as shown in Figure 4 Make sure that the On Off switch SW1 is in the OFF position While watching you...

Страница 12: ...that the power is ON The voltage should be between 5 and 8V higher than the voltage at TP8 All silicon transistors biased for conduction will have approximately 7V from the base to the emitter If your circuit fails this test turn off the power and check that Q6 is properly inserted into the circuit board TRANSISTOR BIAS TEST Move the positive lead of the VOM to test point 6 TP6 Make sure that the ...

Страница 13: ...of the VOM to TP8 Record the voltage at TP8 here V3 __________ volts Once again parallel resistor R13 with resistor R4 as shown in Figure 6 The voltage at TP8 should also drop to a lower voltage Record the new reading at TP8 here V4 __________ volts Remove R4 from the circuit but leave your VOM connected to TP8 for the next test Turn the power OFF Since the DC GAIN equals the DC change at the outp...

Страница 14: ...formed at 400Hz Set the generator at 400Hz and minimum voltage output Set your VOM to read an AC voltage of 1 volt at the output of your Audio Amplifier Slowly increase the output of the generator until the VOM reads 1 volt AC Leave the audio at this setting and move the positive lead of your VOM to TP6 Record the AC voltage input to the amplifier here Vin ___________ volts You may have to change ...

Страница 15: ...cope probe back to TP8 and slowly increase the frequency from the 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 thi...

Страница 16: ...ximum power output should be greater than 200 milliwatts MAXIMUM POWER OUTPUT Connect the generator and oscilloscope as shown in Figure 8 Set the generator at a frequency of 1kHz turn the power ON and adjust the generator output until the peaks of the sinewave at TP8 are clipped as shown in Figure 9A DISTORTION 15 Clipped Crossover Distortion Figure 9 A B By measuring the DC power taken from the b...

Страница 17: ...io at the volume control as shown in Figure 11 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 by ...

Страница 18: ...M to test point three TP3 as shown in Figure 12 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 turned O...

Страница 19: ...d minimum output Turn the power ON and put the volume control 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 14 02mF Oscilloscope Generator GND R15 GND R15 Connect equipment as shown in Figure 14 Set the RF generator at 455k...

Страница 20: ...r is shown in Figure 15 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 i...

Страница 21: ...a frequency of 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 read...

Страница 22: ... second IF amplifier Your results should be similar to the values shown in Figure 15 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 a...

Страница 23: ... your circuit as shown in Figure 18 Using a clip lead short TP5 to R8 as shown in Figure 18 This short prevents the AGC from lowering the gain of the first IF ampifier 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 approximate...

Страница 24: ... 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...

Страница 25: ... green to the hole marked RED Wire B red and black twisted together to the hole marked BLK and Wire C white to the hole marked WHT Figure J Tabs Tabs 1 2 1 2 C white B black A red C white B A green Twisted Together Red Black A green Red Black B Twisted Together C white 1 2 OR White Black Red White Black Red Green R 9 11W R 1 1 5W R 9 11W R 1 1 5W 4 Wire 3 Wire Figure I Determine if you have a thre...

Страница 26: ...apacitor Bend the leads as shown Fasten C1 into place on the top side of the PC board with two M2 5 screws C1 Screw Holes Knob Post Solder leads to pads 3 Leads 4 Leads 25 Cut Holder Coil PC Board Stand Insert the PC board into the stand as shown Turn the dial fully clockwise Remove the protective backing from the label and align the 1600 with the arrow on the PC board Screw Knob Fasten the knob t...

Страница 27: ...eater than 8 5 volts turn the power OFF 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 20 Set the oscilloscope for a vertical sensitivity of 1 volt division and turn the power ON The oscilloscope should ...

Страница 28: ...VOM drops to 0 707 of its peaked value Record the frequency of this lower 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 ...

Страница 29: ...ading on the oscilloscope by adjusting the frequency of the RF generator Now tune the antenna coil to this frequency by adjusting the antenna trimmer on the back of C1 see Figure 22 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 Fig...

Страница 30: ...ation and repeat this procedure until there is no more improvement noticed on the weakest possible 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 stati...

Страница 31: ... 30 SCHEMATIC DIAGRAM ...

Страница 32: ...ElencoTM Electronics Inc 150 W Carpenter Avenue Wheeling IL 60090 847 541 3800 http www elenco com e mail elenco elenco com ...

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