1. In celebration of the launch of the new redesigned site, we just gave away a FREE antenna! Click Here to See Who Won!

Increase modulation on Cobra 25

Discussion in 'CB and Export Equipment and Accessories' started by Cheech, Nov 28, 2013.

  1. Cheech

    Cheech Active Member

    Joined:
    Apr 6, 2005
    Messages:
    703
    Likes Received:
    0
    Anyway to further increase the modulation on Cobra 25 ltd classic without cutting the limiter or using a power mic? Ive got the VR maxed out.


    #1
  2. 9C1Driver

    9C1Driver Well-Known Member

    Joined:
    Aug 13, 2008
    Messages:
    2,339
    Likes Received:
    419
    With out knowing what tune work has been done at some point the law of diminishing returns kicks in. If your at 100% things don't get better with 125 or 150% unless your talking about channel 6 audio quality. (or lack there of)

    Maybe you could have a shop with a scope bench test it. Then it could be set at 100% and they could tell if say the VR5 pot was worn out or something else was keeping the radio from reaching the 100% mark. Beyond that if the radio is properly tuned and aligned and your at 100% modulation then the radio is being all it can be.


    Edit: Cheech, I just saw your General Lee thread, it sounds like your skilled at some radio tech work. Maybe something has failed and is keeping VR5 from working as it should. There are some sharp members here, maybe they can chime in.
    #2
  3. GnG8d

    GnG8d Well-Known Member

    Joined:
    Jun 28, 2010
    Messages:
    2,029
    Likes Received:
    314
    ^^There is nothing true about that statement. Albeit there is nothing you can do with a Cobra 25 except remove/replace the modulation transformer, that's the bottleneck. The TIP120 mod just pinches the carrier off and keep alive mods still produce harmonics. That radio should sound fine at 100% modulation.
    #3
  4. 9C1Driver

    9C1Driver Well-Known Member

    Joined:
    Aug 13, 2008
    Messages:
    2,339
    Likes Received:
    419



    Hmmmm...ok. I think you agree with me that 100% is 100% and it should sound great there right? I have had many 25/29's that when you cranked VR5 wide open the audio quality went to he!! hence the term overmodulated. So then it would be true that 125 or 150% modulation does not sound good.
    #4
  5. GnG8d

    GnG8d Well-Known Member

    Joined:
    Jun 28, 2010
    Messages:
    2,029
    Likes Received:
    314
    No, it sounds great when done with NPC or PPE. And honestly it sounds good on a Cobra which is running the negative to cutoff. Distortion in audio is desirable, such as compression, it's just a matter of controlling the harmonics as a byproduct.
    #5
  6. Cheech

    Cheech Active Member

    Joined:
    Apr 6, 2005
    Messages:
    703
    Likes Received:
    0
    I'm only getting about 80% modulation with a whistle. The radio is stock otherwise.

    I figured maybe something could be changed out to increase the modulation. I know 25s are capable of more than what this one is doing.
    #6
  7. loosecannon

    loosecannon break on through

    Joined:
    Mar 9, 2006
    Messages:
    1,966
    Likes Received:
    174
    look on the solder side of the board for a resistor that is wired in parallel with VR5.
    if there is one there, remove it.

    you can also try changing C91 to a 33uF electrolytic cap.
    this affects the AMC attack time.
    LC
    #7
  8. 9C1Driver

    9C1Driver Well-Known Member

    Joined:
    Aug 13, 2008
    Messages:
    2,339
    Likes Received:
    419

    I had a 2950 & Delta Force years ago that had the PPE mod. Always got great on air reports. His radio is stock I guess so there must be something going on.
    #8
    1 person likes this.
  9. Shockwave

    Shockwave Sr. Member

    Joined:
    Sep 19, 2009
    Messages:
    2,319
    Likes Received:
    680
    If the 25 LTD can't get past 80% modulation with the AMC turned all the way up, your mic is probably not driving the speech amp at a high enough level. Some of the newer stock mics are absolute junk in terms of sensitivity and fidelity. Most modulation meters are no better and a scope is the most accurate way of checking this. Try to check the modulation with a whistle or other loud sound in the mic to determine if the radio is clamping the modulation at a specific level or if the mic is having a hard time driving the speech amp.

    Those that think 125% modulation (positive peaks) can't sound clean, need to listen to some of the Canadian AM broadcast station when the nighttime DX rolls in on that band. Many are operating at that level and stand out from the US stations only hitting 100% positive peaks. If the transmitter has the headroom to reach 125% positive peaks, the negative peak is limited and you have filtering after the limiter, not one ear out here will detect the sound as containing distortion.
    #9
  10. loosecannon

    loosecannon break on through

    Joined:
    Mar 9, 2006
    Messages:
    1,966
    Likes Received:
    174
    if the mic is indeed the problem, and you dont want to use a power mic, then you can change the value of R76 to 1k.

    this will increase the gain of the mic amp, and may allow your lower output mic to fully modulate the transmitter

    normally i would not recommend this, because if people do run a power mic after doing that mod, the radio will sound like complete crap.
    you usually also have to watch the mic gain if you do that mod.

    the only other thing i can think of is that the radio is starving for current.
    LC
    #10


Share This Page