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Variable power circuit for direct injected cobra 29

mattsowders1989

mattsowders1989

Member
Jan 18, 2017
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Central KY
I'm looking to find a variable power circuit design that is compatible with direct injection on cobra 29 radios. The standard TIP120 produces a tone in transmission. I've also tried a tip29 as well. Is there a better design for variable carrier power? Any help would be awesome.
 

Are you certain the tone isn't coming from noise getting into the D.I. setup, the battle to remove/eliminate hum is an on-going one, I was playing with D.I. yesterday and could not get a noise out of the system, until I unplugged my laptop power supply then it suddenly stopped.

I have a TRC-427 here, I recently added a relay to switch the audio signal path after the T/R switch thread.
It has a TIP120 variable modification with a 33uF capacitor, no odd tone detected just a dead carrier; unless of-course hum is getting into the system some how.
 
LeapFrog said:
Are you certain the tone isn't coming from noise getting into the D.I. setup, the battle to remove/eliminate hum is an on-going one, I was playing with D.I. yesterday and could not get a noise out of the system, until I unplugged my laptop power supply then it suddenly stopped.

I have a TRC-427 here, I recently added a relay to switch the audio signal path after the T/R switch thread.
It has a TIP120 variable modification with a 33uF capacitor, no odd tone detected just a dead carrier; unless of-course hum is getting into the system some how.
Click to expand...

The higher the carrier, the lower in volume the tone is. I'll unplug some stuff to see if it is one of those things. Tried that once but I may have missed something. Thanks LF
 
No problem Matt, you might have some RF getting into your audio line this can end up causing oscillation of the audio amplifier that can in turn produce an audible tone, or something along those lines like a squeal, try and use shielded cables plus a ferrite bead. If your audio amp isn't oscillating it could still be amplifying some kind of interference coming from a transmission source (like the radio:))

Here is a good little read on the subject, best of luck.
All the Best!
-LeapFrog
 
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