Kenwood TS-140S

The Kenwood TS-140S is a transceiver which covers:

• all amateur radio bands from 160 to 10 meters, in TX;
• the entire HF range, in RX.

In both operating modes, the TS-140S employs a PLL circuit based on a single microprocessor-controlled crystal oscillator, which allows for the stability and precision (10 PPM) needed to operate on the above frequencies in 10 Hz steps.

The Control Unit circuit governing the TS-140S is headed by a CPU (IC18, BU18400A), which receives:

• the clock signal from IC16 (TC4069UBP);
• the reset signal from IC15 (PST520D);
• the interrupt signal from IC17 (NE555C).

A 16-bit address bus and an 8-bit data bus connect the CPU to other peripheral components, such as:

• a 16K ROM, IC21 (MBM27C128-25JAJ2), which contains the firmware executed by the CPU;
• a 2 kB static RAM, IC20 (TC5518CPL);
• three I/O interfaces (IC22 to IC24, TMP8255AP-5);
• an interface (the optional IF-10C kit, based on a uPD8251AFC USART) that allows you to connect the TS-140S to a PC.

Each of these components is enabled by IC19 (SN74LS138N), which decodes the signals present in the CPU address bus and activates the corresponding chip. Most of these components are located in the Control Unit.

The TS-140S uses a PLL circuit based on a single crystal oscillator and composed of 3 loops covering the frequency range between 500 kHz and 30 MHz, with a 10 Hz interval between a frequency and the next/previous one. On the Control Unit, starting from Q9 (2SC2787L) and X1, a 36 MHz reference signal (Fstd) is generated, passes through buffers Q10 and Q11 (2SC2668Y), and reaches IC8 (M74LS93P), which divides the value of Fstd by 8 and generates the signal Fr at 4.5 MHz, used by each of the loops of the PLL circuit.

• i. Loop 3 (PLL 3): PLL 3 is composed of IC1 (M54927P) and Q1 (VCO3, 2SC2688Y), it produces a signal whose frequency is around 91 MHz, which varies according to the selected modulation. The output of this loop arrives at IC2 (M54459L), is divided by 20 and continues in two directions:
  1. the first destination is IC3 (SN74LS90N), which starts a series of processing that generates the carrier signal Fcar, sent to the Signal Unit;
  2. 2. the second path leads to IC6 (SN16913P), whose output is a signal used to obtain some characteristics, such as the IF shift.
• Loop 2 (PLL 2): PLL 2 consists of IC4 (M54927P) and Q5 (VCO2, 2SC2688Y). Specifically, there are two signals arriving at IC4:
  1. the Fr signal (at 4.5 MHz), which is divided by 4500 within IC4 to generate a 1 kHz signal, used as a reference frequency;
  2. the signal generated by VCO2, which passes through the buffer Q6 (2SC2688Y) and is divided inside IC4 by an M factor, whose entity is established by the radio CPU.
  
  IC4 compares the result of the division by M with the reference frequency at 1 kHz: the result is a signal between 54.5 and 59.999 MHz, depending on the modulation selected and the RIT, if enabled. This signal arrives at IC5 (M54460L), where it is divided by 10, and continues towards IC6 (SN16913P) through a low pass filter made by L15 and L16. At MIX5, the signal produced by PLL 2 meets the signal coming from PLL 3, and is mixed with it.
• iii. Loop 1 (PLL 1): PLL 1 is essentially represented by IC13 (MB87006). The 4.5 MHz Fr signal is applied to pin 1 of this integrated circuit, and is divided by 90 to generate a reference signal at 50 KHz. This signal is compared with the signal from IC12 (SN76514N), also applied to IC13 and divided by a factor N determined by the CPU of the TS-140S. The resulting signal passes through the active low pass filter composed by Q16, Q17 and Q18 (2SC2459BL), and is sent to VCO1 on the Signal Unit.
  
  VCO1 actually consists of 3 VCOs, from VCO1A to VCO1C, which are activated by the CPU according to the selected operating frequency. The output of VCO1 passes through the Q13 buffer (2SC2688Y) and follows two directions:
  1. remains in the Signal Unit as the VCO signal;
  2. returns to the Control Unit as the Fvco signal, passes through buffer Q14 (2SC2688Y) and arrives at IC12 (SN76514N), where it is mixed with the signal generated by PLL 2 and PLL 3.

The transmission circuit provides a double conversion to SSB, AM and CW modes, and a single conversion to FM mode:

• in SSB and AM modes, the AF signal coming from the microphone, after being amplified by IC1 (uPC1158H2) and Q74 (2SC2712Y), enters the balanced modulator IC3 (AN612) to produce the DSB signal centered at 455 kHz, which - after crossing the CF1 filter and being amplified by Q86 (3SK73GR) - arrives at the transmission mixer IC5 (SN16913P) and meets the 39.6 MHz signal, generated by the VCO4 oscillator, with which it produces the first IF signal at 40.055 MHz that continues towards Q79 and Q80 (a pair of 3SK122L) passing through the XF1 filter;
• in CW mode, the microphone signal amplification by Q74 is inhibited, so that the 455 kHz signal produced by IC3 is not modulated;
• in FM mode, the microphone signal modulates the VCO4 oscillator, which generates the signal at 39.6 MHz, and reaches Q79/80.

Therefore, the Q79/Q80 pair constitutes the first TX mixer in FM and the second in SSB and AM. Additionally, the 39.6 MHz signal represents:

• the 1st IF, in FM mode;
• the heterodyne signal in SSB and AM modes.

These two transistors produce the signal that arrives at the RF power stage (the Final Unit), is amplified up to 100W, and continues towards the antenna.

The RF signal applied to the ANT connector passes through the Filter Unit (including the currently selected low pass filter, the TX/RX relay K16, and the 20dB attenuator marked Q3, 2SC2459BL) and arrives at the Signal Unit (connector CN4, pin 1). Here it passes through a resonant trap at 40.055 MHz, which eliminates any signal at that frequency (the same as the first IF), passes through another filtering stage (band pass from 1.6 to 2.5 MHz, high pass for the other frequencies) and a second trap at 40.055 MHz, before reaching the mixer stage consisting of the two transistors Q18 and Q19 (2SK125-5), to which a VCO (transistors from Q9 to Q11, 2SC266BY) is connected to provide frequencies from 40.555 to 70.055 MHz. This produces the first IF (as mentioned before, at 40.055 MHz).

It passes through the XF1 crystal filter, is amplified by Q21 (3SK122L), and reaches the second mixer (transistors Q22 and Q23, 2SK125-5) together with the 39.6 MHz signal coming from the Control Unit. The result is the second IF at 455 kHz, amplified by Q27 (3SK73GR), which passes inside one of the filters from CF1 to CF3 (depending on the currently selected mode).

After that:

• in AM, SSB and CW modes, the second IF is amplified by Q30 and Q31 (3SK73GR), to be demodulated
  • by IC3 (AN612), in SSB and CW modes,
  • by D75 (1N60PSPA), in AM mode;
• in FM mode, the 39.6 MHs signal is demodulated by IC6 (MC3357P).

Finally, the AF signal is taken from IC7 (TC4066BP) in accordance with the selected mode, amplified by Q59 (2SC2712Y), and arrives at the final audio amplifier IC4 (uPC2002V) via the AF volume potentiometer.