Cobra 148GTL-B

Block Diagram

Microphone Connection

3Speaker (Connect to Ground)
4TX Key (Connect to Ground)


VCO Block
Schematic Diagram
2SC1969 Finale Transistor
2SC2166 Driver Transistor
2SC1675 RF Transistor
AN7140 Audio Power Amplifier
Dual OP-Amp
AN612 Balanced Modulator
TA7310P Oscillator, Mixer and Amplifier
PLL02A PLL Frequency Synthesizer

Improved receive gain

Quieting of AM reception and improving gain of incoming signals is a common request from radio operators. In the first stages of the HF input 2SC1675 transistor can be found. This transistor is responsible for the amplification of a small detected signals. A problems exist if the transistor itself is noisy as is such the case of the 2SC1675 when compared to other low noise packages. Along with the amplification of the incoming signals is transistor noise. Replacement of this transistor with a higher gain, lower noise transistor greatly improves the signal to noise ratio of your receiver.
We will use an 2SC2999 transistor that has higher gain lower noise characteristic. Replace the 2SC1675 (Q11) with an 2SC2999 (or similar low noise and high gain transistor) to achieve this improved signal to noise ratio.
Re-Adjust T6 and T7 / T8
The gain will improved with more than 6dB with the same signal to noise ratio.