Inorder for the mod to work, the 2SC2999N should:
1. provide the same (or preferably higher) stage gain at the receiver front end, and
2. The device itself should generate less noise.
There are two different receiver rf amp circuits: the common emitter and the common base. The common base circuit is found in the Cobra 2000, 148GTL, and most 10 meter exports. All of these units use the 2SC1674L transistor.
The common emitter circuit is used on the old 858SSB chassis, and most AM sets. In the 858 chassis, the 2SC1730L is used.
We will look at two items from each transistor’s spec sheet and calculate a third item.
Gain Bandwidth Product (Ft) ... is defined as the frequency at which the AC current gain, hfe, equals 1 (0dB). This is the maximum frequency at which the device produces gain as an amplifier or oscillator.
AC Current Gain (hfe) ... is defined as the ratio of the input current to the output current in a device, and depends on the operating frequency. Ft and hfe work together to define the overall AC gain of the transistor at a specific frequency (fo). hfe is calculated using the formula: hfe=Ft/fo.
Example: For the 2SC1730L at an fo of 27mhz, hfe=1100/27=40.8
Noise Figure (NF) ... is defined as the ratio of the input noise to the
output noise. The transistor will add noise (N), which will be be amplified by the hfe of the device just as the signal (S) is, forming signal plus noise output (S+N). The excess, (S+N) minus (S), equals the noise generated by the device.
Looking at the spec sheets, and doing the hfe calculations, we get:
DEVICE...............Ft...........NF..........hfe@27mhz
2SC2999N..........750mhz.....2.2db............27.7
2SC1674L..........800mhz.....3.0db............29.6
2SC1730L.........1100mhz....3.0db............40.8
In the common base circuit, there is no current gain, but because of the low input impedance and high output impedance found in this circuit there is a large voltage gain (because E=IR). In the common base configuration hfe isn’t a factor,so the 2999 is a suitable replacement for the 1674, as the stage gain will be essentially the same, and the noise figure is 0.8 db better. But a 0.8 db "improvement" is inaudible.
However, in the common emitter circuit, hfe (current gain) plays a big part. As you can see from the chart, the 2999 has a staggering 32% less AC current gain at 27 mhz compared to the 1730. Anyone who believes that the receive in their 858 rig is better after this mod needs to have their hearing checked.
CONCLUSION:
I would call the 2SC2999 mod worthless.
Of the 3 transistors compared above, the 2SC2999’s noise figure is only 0.8 db better. A sound level of 1 db is considered to be just barely noticable, so a 0.8 db change would require bionic ears to hear. Besides, the atmospheric and antenna noise at 27 mhz totally overshadows any devce noise.The 2999 mod does not provide any noticable improvement in receiver performance and only benefits your tech’s wallet.
-399
1. provide the same (or preferably higher) stage gain at the receiver front end, and
2. The device itself should generate less noise.
There are two different receiver rf amp circuits: the common emitter and the common base. The common base circuit is found in the Cobra 2000, 148GTL, and most 10 meter exports. All of these units use the 2SC1674L transistor.
The common emitter circuit is used on the old 858SSB chassis, and most AM sets. In the 858 chassis, the 2SC1730L is used.
We will look at two items from each transistor’s spec sheet and calculate a third item.
Gain Bandwidth Product (Ft) ... is defined as the frequency at which the AC current gain, hfe, equals 1 (0dB). This is the maximum frequency at which the device produces gain as an amplifier or oscillator.
AC Current Gain (hfe) ... is defined as the ratio of the input current to the output current in a device, and depends on the operating frequency. Ft and hfe work together to define the overall AC gain of the transistor at a specific frequency (fo). hfe is calculated using the formula: hfe=Ft/fo.
Example: For the 2SC1730L at an fo of 27mhz, hfe=1100/27=40.8
Noise Figure (NF) ... is defined as the ratio of the input noise to the
output noise. The transistor will add noise (N), which will be be amplified by the hfe of the device just as the signal (S) is, forming signal plus noise output (S+N). The excess, (S+N) minus (S), equals the noise generated by the device.
Looking at the spec sheets, and doing the hfe calculations, we get:
DEVICE...............Ft...........NF..........hfe@27mhz
2SC2999N..........750mhz.....2.2db............27.7
2SC1674L..........800mhz.....3.0db............29.6
2SC1730L.........1100mhz....3.0db............40.8
In the common base circuit, there is no current gain, but because of the low input impedance and high output impedance found in this circuit there is a large voltage gain (because E=IR). In the common base configuration hfe isn’t a factor,so the 2999 is a suitable replacement for the 1674, as the stage gain will be essentially the same, and the noise figure is 0.8 db better. But a 0.8 db "improvement" is inaudible.
However, in the common emitter circuit, hfe (current gain) plays a big part. As you can see from the chart, the 2999 has a staggering 32% less AC current gain at 27 mhz compared to the 1730. Anyone who believes that the receive in their 858 rig is better after this mod needs to have their hearing checked.
CONCLUSION:
I would call the 2SC2999 mod worthless.
Of the 3 transistors compared above, the 2SC2999’s noise figure is only 0.8 db better. A sound level of 1 db is considered to be just barely noticable, so a 0.8 db change would require bionic ears to hear. Besides, the atmospheric and antenna noise at 27 mhz totally overshadows any devce noise.The 2999 mod does not provide any noticable improvement in receiver performance and only benefits your tech’s wallet.
-399
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