Musical AF/IF checker

There are several signal generator circuit proposed by many engineers but few are reliable over a range. Most of the circuits are designed for a fixed or constant frequency range. This circuit produces music instead of 10 KHz oscillation. Music notes are modulated to 455 KHz. The modulated signal is used for checking and alignment of IFTs at the time of servicing of audio equipment.
The main parts of the circuit are audio tone generator, RF oscillator and modulator. For audio tone generator the musical IC UM66 (IC1) used. This IC has 64-note ROM memory.
Circuit Diagram of music af-if checker

The oscillator section consists of low-noise crystal oscillator. The crystal of 455 KHz used for frequency control. There is no frequency drift in the circuit as no tuned circuit is being used. Therefore, the IFTs can be aligned correctly.
The output section of the circuit is modulator which modulates the AF and RF signals. The modulated signal is taped from the output jack. The whole setup is enclosed in a small metal box. The output jack and switch SW1 are fitted on the front panel of the box.
When switch SW1 is in position A, we can get the modulated IF signal from the jack. When the switch is in position B, we get AF signal from the jack. For alignment of 2-band radio we can replace the crystal with another crystal producing frequencies of 550 KHz, 1600 KHz, 600 KHz, 5MHz or 16 MHz and replace the IFT with a small ferrite core transformer (or an IFT without the tuning capacitor can also be used).
Resistors (all ¼-watt, ± 5% Carbon unless stated otherwise)
R1 = 270 Ω
R2 = 220 Ω
R3, R7 = 10 KΩ
R4 = 470 KΩ
R5 = 4.7 KΩ
R6 = 1.2 KΩ
R8 = 220 KΩ
R9 = 1 KΩ
C1 = 1 µF/16V
C2, C4 = 0.1 µF
C4 = 390 pF
C5 = 0.04 µF
C6 = 0.01 µF
C7 = 100 µF/16v
IC1 = UM66
T1, T2 = BF494B
D1 = 1N4148
ZD1 = 3V 400mW
XTAL1 = 455 KHz
SW1 = 1-pole two way switch

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