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Increase modulation on Cobra 25

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Old 11-28-2013, 03:38 PM
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Default Increase modulation on Cobra 25


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.
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Old 11-28-2013, 04:50 PM
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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.

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Old 11-28-2013, 05:05 PM
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Quote:
Originally Posted by 9C1Driver View Post
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)
^^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.
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Old 11-28-2013, 07:06 PM
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Quote:
Originally Posted by 359 View Post
^^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.



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.

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Old 11-28-2013, 07:20 PM
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Originally Posted by 9C1Driver View Post
So then it would be true that 125 or 150% modulation does not sound good.
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.
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Old 11-28-2013, 09:12 PM
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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.
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Old 11-28-2013, 11:12 PM
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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.
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Old 11-29-2013, 08:12 AM
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Quote:
Originally Posted by 359 View Post
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.

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.
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