AM/FM RADIO KIT.

Note: parts in the print diagram are designated with numbers such as C1, Q1, and R1. The values are given in a separate parts list. I don't like this way of giving parts values. After I complete writing the text description I will go back and edit in the value of each part. I will post two versions of the description file, one with only part numbers for initial understanding of the circuit, and one with parts values for deeper understanding.

The schematic is drawn on the component side of the printed circuit board and each part is placed over the symbol for that part. The print schematic that brought you to this description page is exactly identical to the one on the circuit board. The FM circuit runs along the top half of the board and runs the full length. The A M section is below it and covers about half the length of the board. The lower right section is where the audio amplifier, speaker, A M / FM switch, power switch, and battery are located.

Many of the connections will go to the positive supply voltage. For reference in this description these positive supply points will be given names that do not appear in the print version of the schematic. The first one to be encountered is FM B plus 3.

FM Section.

At the left edge of the schematic, and circuit board, FM B plus 3 goes to ground through C2.

RF Amplifier.

The RF amplifier begins at the antenna which is a telescoping whip. The base of the antenna connects to a test point, TP14. The antenna also connects through C4 to the emitter of the RF Amplifier transistor, Q1. The emitter of Q1 also goes to ground through R3. The base of Q1 goes through R2 to ground. It also goes through R1 to FM B plus 3. The base also goes through C3 to FM B plus 3. The collector goes through coil L1 then L2 to FM B plus 3. The two coils appear to be magnetically coupled. The junction of L1 and L2 goes through C5 to the base of the mixer, Q3. The rest of the mixer will be described after the description of the oscillator. The collector of Q1 also goes through C6 then through part of the tuning capacitor, C1 to a point labeled on the schematic as V plus. V plus also goes to ground through C8. The symbol V plus does not appear anywhere else in the diagram. It must be a special test point.

The oscillator.

The base of the oscillator, Q2goes through R5 to ground. It also goes through the parallel combination of R4 and C7 to FM B plus 3. The emitter of Q2 goes through R8 to ground. The emitter also goes to the anode of a varicap diode, D1. The cathode of D1 goes to a point labeled on the schematic as AFC. This loose end will be tied up after the detector is described. The emitter of Q2 goes through C9 to its collector. The emitter of Q2 goes through C11 to the emitter of Q3, mixer. The collector of Q2 goes through L3 to FM B plus 3. The collector of Q2 also goes through C10 then another part of C1 to FM B plus 3.

The Mixer.

The emitter of Q3, mixer, goes through R11 to ground. As described earlier, signal from the oscillator is coupled to the emitter of Q3 from the oscillator through C11. The base of Q3 is labeled on the schematic as TP13. It goes to ground through R7. It also goes to FM B plus 3 through R8. At the point where R8 connects to FM B plus 3 there is a capacitor, C12, going to ground. As described earlier signal from the collector of the RF amplifier is coupled to the base of the mixer through C5. The collector of Q3 is labeled on the schematic as TP12. It goes through the primary of transformer T1 to FM B plus 3. There is a resistor, R10, connected in parallel with the primary of T1. T1 is tuned to the intermediate frequency of the radio. A resistor, R9, connects between FM B plus 3 and FM B plus 2. A capacitor, C13, connects from FM B plus 2 to ground.

First IF amplifier.

One end of the secondary of T1 goes to the base of Q4, first IF amplifier. The other end of the T1 secondary goes through the parallel combination of C14 and R13 to FM B plus 2. It also goes through R12 to ground. The emitter of Q4 goes through the parallel combination of R15 and C16 to ground. The collector connects to a tap on the primary of T2. The bottom end of the primary of T2 goes through C16 to ground. The bottom end also goes to FM B plus 2. The top end of the primary goes through R14 to the bottom end.

Second IF Amplifier.

One end of the secondary of T2 goes to the base of transistor Q5. This point is labeled TP10. The other end of the T2 secondary goes through a resistor R17 to FM B plus 2. This point also goes through R16 to ground. This point also goes through C17 to the bottom end of the primary of T3. Note: This is a neutralizing capacitor. The emitter of Q5 goes through the parallel combination of R18 and C18 to ground. This ground point is labeled TP16. The collector of Q5 goes to a tap on the primary of T3. The bottom end of the T3 primary connects to FM B plus 2. The bottom end also goes through C20 to ground. A resistor, R 19 is connected between the top and bottom ends of the T3 primary.

Third IF Amplifier / Limiter.

One end of the t3 secondary connects to the base of Q6. The other end of the T3 secondary goes through the parallel combination of R21 and C19 to FM B plus 2. This end of the secondary also goes through R20 to ground. The emitter of Q6 goes to ground through the parallel combination of R22 and C22. The collector of Q6 is designated as TP8. The collector also goes to a tap on the primary of the detector transformer, T4. The cold side of the primary of T4 goes to ground through C21. The cold side also goes through R23 to FM B plus 2. The high side of the primary of T4 goes to one side of the primary of detector transformer T5. (Note: This is not a mistake.) The other side of the primary of T5 goes through a capacitor back to the cold side of the primary of T4. (Note: This capacitor has no designator. It is clear from the circuit board layout that it is located inside the T4 can.) One side of the T4 secondary goes through C23 to ground. The other side of the T4 secondary goes to the center tap on the T5 secondary. The not grounded end of C23 and the ends of the T5 secondary will be picked up in the detector section to follow immediately.

Detector.

One end of the T5 secondary goes to the cathode of D2. The other end of the secondary goes to the anode of D3. The anode of D2 goes through R26, then through the parallel combination of C25 and R28, then through R27 to the cathode of D3. C25 is an electrolytic capacitor. The positive end connects to R27. The junction of R27 and the combination of C25 and R28 goes to FM B plus 2. This point also goes through c24 to ground. C24 is electrolytic and its positive end goes to FM B plus 2. The not grounded end of C23 is the audio output from the detector. This point also goes to one end of R29. The other end of R29 goes through C26 to ground. The junction of R29 and C26 goes through R30 to the point labeled AFC which was left hanging in the Oscillator description. The AFC point goes through C27 to ground.

The A M / FM Switch, S1.

This is a double pole double throw switch. One pole switches plus 9 volts to either the FM or A M sections of the radio. The other pole connects the audio output of either the FM detector or the A M detector to the top of the volume control and from there to the audio amplifier. The swinger of the B plus section goes to the positive end of C39. The negative end is grounded. The swinger also goes to one end of the on / off switch, S2. The other end of the on / off switch goes to the positive of the 9 volt battery. The FM side of S1 goes to the point FM B plus 1. This point goes through R25 to FM B plus 2. The A M side of S1 goes to A M B plus 1. The swinger of the audio side of S1 is designated TP2. It goes to one end of R43. The other end will be picked up much later. The FM side of the audio pole of S1 goes to the not grounded end of C23. The A M side of the audio pole of S1 goes to ground through C38.

The AM section.

The A M section begins at the left edge of the schematic, and circuit board, with the ferrite antenna. It consists of many turns of litz wire wound on a rod of ferrite material. Ferrite is made of finely powdered iron held in a ceramic material such that the grains of iron are close together but not in electrical contact. The large winding we will call the primary. It is tuned by a section of the main tuning capacitor C1. The secondary is a few turns at the cold end of the primary. The other end of the secondary goes to one end of C29. The other end of C29 is designated TP7 and it goes to the base of Q7, converter.

The Converter.

The base of Q7 goes to ground through R32 and to A M B plus 2 through R31. The emitter of Q7 goes to ground through R33. It also goes Through C31 to a tap on the oscillator coil L5. One end of L5 is grounded. The other end goes through C30 to another section of the tuning capacitor, C1. One end of the secondary on L5 goes through J1, a short length of wire soldered between two PC board pads to the collector of Q7. The other end of the L5 secondary goes to one end of the primary of T6, A M I F transformer. This connection point is labeled TP6. A tap on the primary of T6 goes to A M B plus 2. The other end of the primary goes through R 34 to the collector of Q7.

First A M I F Amplifier.

One end of the secondary of T6 goes to the base of Q8, first A M I F amplifier. The other end of the T6 secondary goes to a point which is labeled AGC and also TP5. This point goes through r35 to A M B plus 2. It also goes to ground through C32. The plus side of C32 goes to the TP5 point. TP5 also goes through R36 then C33 to ground. The junction of R36 and C33 goes to the anode of the detector diode, D4. The emitter of Q8 goes to ground through the parallel combination of R37 and C35. The collector of Q8 is designated as TP4 and it goes to one side of the primary of T7, A M I F transformer. A tap on the primary of T7 goes to A M B plus 2. At this point capacitor C34 has its positive end connected to A M B plus 2. The negative end is grounded. The remaining end of the T7 primary is not connected to anything.

Second A M I F Amplifier.

A M B plus 1 goes to A M B plus 2 through R38. One end of the secondary of T7 goes to the base of Q9, second A M I F amplifier. The other end of the secondary goes through R39 to A M B plus 1. This point also goes to ground through the parallel combination of R40 and C36. The emitter of Q9 goes to ground through the parallel combination of R41 and C37. The collector of Q9 goes to one end of the primary of T8 A M I F transformer. This point is designated TP3. A tap on the primary goes to A M B plus 1. The other end does not connect to anything.

Audio Detector and AGC (Automatic Gain Control).

One side of the t8 secondary is grounded. The other side connects to the cathode of D4 detector diode. The D4 anode supplies AGC to the base of Q8 as described earlier. Reminder there is a capacitor, C33 connected from the diode anode to ground. The diode anode also goes through R42, then C38 to ground. The junction of R42 and C38 is where the audio signal comes out. It is fed to the A M side of the audio pole of the A M / FM switch.

The Audio Amplifier.

The swinger of the audio pole of the A M / FM switch comes out to TP2 then goes through R43 to the positive end of C41. The negative end of C41 connects to the top of the volume control which has no other designation. The bottom of the volume control is grounded. The wiper of the volume control goes to the negative end of C40. The positive end goes through jumper J3 to pin 3 of the audio amplifier I C. Pins 4 and 2 are grounded. Pin 1 connects through R44 to the positive end of C42. The negative end goes to pin 8. Pin 6 goes to the swinger of the B plus pole of the A M / FM switch. Pin 5 goes through the series combination of C44 and R54 to ground. Pin 5 is designated V out and also TP1. Pin 5 also goes to the positive end of C43. The negative end goes to the tip contact of the earphone jack. The sleeve contact is grounded. The through circuit contact goes to one side of the speaker. The other side of the speaker is grounded. (Note: The through contact is connected to the tip contact when there is no plug inserted into the jack. When a plug is inserted the through contact is disconnected.)

Transistor and Resistor Replacement for the Audio IC.

The 8 pin integrated circuit may be unplugged from its socket and replaced with a discrete component circuit that functions the same but is not the identical circuit. The following description is of the transistor and resistor circuit that can replace the IC. The input to the audio IC at pin 3 is TP16 and goes to the base of Q10, NPN. The emitter goes to ground through the parallel combination of R48 and C47. The collector goes through R49 to the cathode of diode D5. The anode goes through R46 to power, otherwise known as B plus. The junction of D5 anode and R46 goes to the base of Q11, NPN. There is a capacitor, C48 connected from the base of Q11 to B plus. The emitter of Q11 connects to the base of Q13, PNP. The emitter of Q11 connects through R50 to the output pin 5. The collector of Q13 connects to the output. The emitter of q13 connects to B plus. The collector of Q10 connects to the base of Q12, PNP. The collector of Q12 connects to the base of q14, NPN. The emitter of Q12 connects through R51 to the output. The collector of Q14 connects to the output. The emitter of Q14 is grounded. There is a resistor, R47 connected from the output to the input. The schematic does not show where pins 1 and 8 are connected to the circuit. End verbal description.
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