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2SC2999 MOD ANALYSIS

Discussion in 'CB Radio Modifications' started by unit_399, Nov 10, 2013.

  1. unit_399

    unit_399 EL CAPO

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    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 unit_399, Nov 10, 2013
    Last edited: Nov 10, 2013

  2. Sarcasm

    Sarcasm New Member

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    It's amazing how people interpret mods differently. I hope this thread gets some traction and more input. I think some believe it works because they have to believe they couldn't of wasted their time.
     
    2 people like this.
  3. sonoma

    sonoma Sr. Member

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    I agree with 399 about the trans mods. I do not really see a gain in the trans. the diodes in most cases is all I do and leave the trans alone.the diodes work in some radios and some it makes no difference to me. at least it does not show a increase in receive on the scope or my meter. some it does.
     
  4. ExitThirteen

    ExitThirteen Grumpy and Cranky

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    One thing I'd like to mention here if I could...


    I took a C1730L from a junked D858 chassis and replaced the C1674 in my President Madison (MB8719 chassis) with the C1730L. It actually made a noticeable improvement. Using a signal generator, I was able to still able to pick up an audible 1khz tone on SSB at .032 MICROVOLTS.

    Yes, you read that correctly.

    While the 2SC2999 mod isn't worth doing (and I agree with Unit 399 on it 100%), there is something to be said about possibly doing a C1730L swap in the MB8719 chassis with a proper RX alignment. Something to think about, perhaps?


    ~Cheers~
     
    rabbiporkchop likes this.
  5. Robb

    Robb Yup

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    Be interested in hearing about that RF tune-up and how much you had to turn down the gain stage so that it doesn't distort receive audio.

    If one does change out to the 2999; then they will likely have to have the receive re-tuned. While it has more gain, it will need to be checked.

    Had a local's Magnum S-9 that had an awful receive distortion. Turns out the 1st RX transistor had been switched out, so re-tuned the RX and all was well once again.

    Not worth the mod if it cost you to have the RX re-aligned - IMO.
     
  6. ExitThirteen

    ExitThirteen Grumpy and Cranky

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    Robb:

    I never had an issue with distortion when I swapped out the 1674 with the 1730. If you go back to Unit 399's notes, the MB8719 uses the common base circuit, therefore the current gain (hfe) is a non-factor. What DOES change, however, is the gain bandwidth. That's where I believe the difference is noticed. The 1730's gain bandwidth is wider than the 1674. I didn't have to do any major adjusting when I did the RX alignment, just a slight touch-up of the 1st stage was really all that was needed. (y)

    I did notice a (very) slight improvement in audio quality while retaining excellent RX sensitivity.


    ~Cheers~
     
  7. ExitThirteen

    ExitThirteen Grumpy and Cranky

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    Also, Robb brings up a very good point. If a person does the 2999 mod, and then has to pay to get a proper RX alignment done, it's just not cost effective. Only way the 2999 would even be remotely worth the trouble would be if a person had all the equipment necessary to do a good RX alignment after the mod was completed.

    Now, for the price of the 1N6263 Schottky diodes, this wouldn't be a bad mod to do. No RX alignment needed for those.


    ~Cheers~
     
    1 person likes this.
  8. unit_399

    unit_399 EL CAPO

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    Exit 13 is right. The 1730 would be a much better choice than the 2999 in radios with common base rf amps. Especially since the 1730L is available again (from INCHANGE SEMICONDUCTOR out of China). Check Ebay.

    View attachment 1730 data.pdf
     
  9. Robb

    Robb Yup

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    Is there another place that carries it? I'd like to get 3 or 4 and give 'em shot at the title. Or an equivalent perhaps?
     
  10. sonoma

    sonoma Sr. Member

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    I do not have a junk 858 chassis either so I will have to try to find some just to give it a try. thanks 399 and exitthirteen for your info. the data looks promising if the new replacements are what they should be.
     
  11. 2FB327

    2FB327 Active Member

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    Has anyone ever seen a before and after test as far as:
    "Does it receive beter?"
    Heck I would be happy just to see a video/videogate of it or how the alignment was done.
    I guess it would take two base stations fairly close, one with a contant ouput signal to try it.
    Intersting post.
    As a newby I tried one in a 29 without a proper receive alignment in a truck I drive everyday and all it did was make it pick up more noise from electrial wires, etc which it never did before. Perhaps it will work better when I learn to do a proper alignment.
    I did another for a guy at work for free as a test and he loves it, go figure.
    Both mobile units.
    Looking forward to more on this subject.
     
  12. unit_399

    unit_399 EL CAPO

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    -
    Dunno who is the USA distributor for Inchange, but I emailed them to get the name of their Colombian rep, who hooked me up. You should email them to find their SoCal rep/distributor.
    73s.

    - 399
     
  13. kc8adu

    kc8adu Active Member

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    been there done that debunking supposed "improvements" in the past myself.
    best is the 2scxxxx has twice the dissapation and should therefore double the output.even though i showed it had 25% of the gain of the original.which of course you have to drive it harder to realise any improvement.
    the trolls werent having their precious mod debunked nor their self appointed "expert" made a fool of and the post vanished.i dropped out too.last time i looked years ago that forum was a wasteland of trolls and spammers.
     
  14. unit_399

    unit_399 EL CAPO

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    Hey ET,
    I was wondering how much of an improvement you got from the 1674/1730 swap ? Also, how did you measure it ? O'scope or VOM ? Thanks,
    73s

    - 399
     
  15. ExitThirteen

    ExitThirteen Grumpy and Cranky

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    Hey 399! Sorry I didn't get back to you earlier, been superbusy here as of late.

    I didn't note any huge changes, but I used an oscilloscope to measure, as that is going to be more accurate/sensitive than what any VoM will be. I didn't note the actual figures, other than there was an improvement.

    Keep in mind that the noise floor didn't change any, but the sensitivity and the audio came up a slight amount. It also changed the audio tone/quality a touch too.

    The radio I did this on is a President Madison with the PC-411 (MB8719 dual conversion) chassis in it. The radio also has fresh caps installed too.

    When I get time I'll have to take some measurements and log the differences.

    Overall, there wasn't a huge difference, but since I do not run a receive "preamp" of any type, any little bit helps.

    ~Cheers~
     
    rabbiporkchop likes this.

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