Cobra 148 GTL

Cobra 148GTL AM/SSB CB-Radio

Block Diagram

Adjustment

Reading Point	Adjustment	Description	Value
OSCILLATOR
TP9	L19	VCO Coil
TP1		AM
TP1		USB
TP1		LSB
TP1	L20	AM	Maximum
TR20 Emitter	L21	AM	Maximum
TP3	CT2	Carrier frequency	7.800MHz
	L48	Carrier frequency	Maximum
	L47	TX Mixer frequency	Maximum
TP10
RECEIVER
	L4 L5 L6	RF
	L7 L8 L9 L10	1st. IF
	L12 L13 L14 L15	SSB IF
	L3	AM IF
	L1 L2	NB/ANL IF

Internal S-Meter	VR1	AM
Internal S-meter	VR2	SSB
TRANSMITTER
TP8	VR9	Driver Bias	10mA
TP7	VR8	Finale Bias	100mA
	L46 L45 L44 L38
	VR4	Modulator Balance


	VR10	AM Power	15 Watt
	VR11	SSB Power	20 watt
Internal Power Meter	VR6

COMPONENTS

MB8719 PLL Frequency Synthesizer	TA7222AP AF Power Amplifier
AN612 SSB Modulator	SO42P TX Mixer
2SC1306 Driver Transistor	2SC1969 Finale Transistor

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 2SC1674 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 2SC1674 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 2SC1674 (TR14) with an 2SC2999 (or similar low noise and high gain transistor) to achieve this improved signal to noise ratio.

Re-Adjust L4 and L5

The gain will improved with more than 6dB with the same signal to noise ratio.

[Volver]