Help on a "Cobra mod"
k9s
Hi all
Hope someone can help me out here. I just did the following modification to
and old Cobra 29 LTD. Well every thing seems to be working alright but when
I check the the wattage my meter (Colt SWR-1) it's telling me that I'm only
getting 2 Watts out. Waiting Patiently ya right.
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Variable Power for Cobra 25 & 29LTD
Cobra 25 LTD
Find and remove C70 & JP6(near Final).
Disconnect Orange and Brown wires from the RF Gain Control,then connect
these wires together. **** these's wire were Blue & Yellow on my radio******
Install Q1(see diagram) where JP6 was.The right leg(emitter) of Q1 in the
hole near the final.The Center leg(collector) in the other hole.
Solder a wire from the left leg(base) of Q1 to the center tag of the RF Gain
Control.
Remove VR5(near transformer)
Connect the last tag of the RF Gain Control to a 100ohm resistor and solder
to L6 pot.
Solder a wire from the upper tag of the RF Gain Control to the top
connection of VR5(already removed)
Cut D9 out of the circuit (this is the Clipping Diode)
Put plenty of heat sink compound onto the tab of Q1, then using the proper
mounting hardware mount to heat sink (make sure that the tab of Q1 is
insulated from the heat sink)
Cobra 29LTD
Follow instructions for Cobra 25LTD, except remove C73 & JP36 instead of C70
& JP6,remove VR4 instead of VR5, Also cut D11 instead of D9.
Q1 = TIP120 (Radio Shack), capacitor = 22 uF 25 V
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Any Comments on Lou Franklin's Book "the Screwdriver Expert"
73's from Freeeeeeebirrrrrd
Bill Eitner
Lou's book is excellent.
The mod you did comes from the CB Tricks series. It does
work--you just have to make sure you did it right. The
resting Q1 base voltage determines the dead-key output.
Raise the resting Q1 base voltage to raise the dead-key
output.
The thing that you may not understand was that that mod
is basically a CCAM (controlled carrier amplitude modulation)
mod. In other words, it's designed to suppress the carrier.
To get maximum output out of that radio you didn't need to
go to that much trouble. By simply clipping D-11 and removing
the slug from L-14 you basically max-out the transmitter.
I hope this helps.
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PBrown1271
I also have a 29LTD I believe about everthing has been done to it but one thing
i want to do is drop the dead key but keep the full swing now it keying 6 watts
swinging 18, i would like to drop it to 3 watt key
PERRY
Sean
Bill, either my understanding of CCAM is wrong or this is just a
dial-a-watt installation using a transistor. i just CCAM'ed my 2950
yesterday to check results. Sean
Bill Eitner <kd6...@earthlink.net> wrote in article
<353704...@earthlink.net>...
ng9220700-Ostrowski
Sean wrote:
>
> Bill, either my understanding of CCAM is wrong or this is just a
> dial-a-watt installation using a transistor. i just CCAM'ed my 2950
> yesterday to check results. Sean
>
> Bill Eitner <kd6...@earthlink.net> wrote in article
Sean,
It's a dial-a-watt. And a pretty bad one at that, because with the
collector to emitter drop always in series with the final, you will
never again be able to achieve the same dead key output that you had
before the 'conversion' unless you do some other mods to the radio.
A 250 ohm 2 watt linear taper pot bypassed by a 100ufd 16 volt
electrolytic cap and placed in series with the 'driver' transistor works
a heck of a lot better!
Dennis
#12
Professor
Dennis is 100% right. With this method you don't have the "silicon drop" to
contend with...
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Professor - Telstar Electronics
ng9220700-Ostrowski wrote in message <353787...@lucent.com>...
Bill Eitner
Sean wrote:
>
> Bill, either my understanding of CCAM is wrong or this is just a
> dial-a-watt installation using a transistor. i just CCAM'ed my 2950
> yesterday to check results. Sean
Oops. I was wrong. This isn't the same mod that's outlined
in the CB Tricks series. This is just a simple variable
power mod. If the base drive to the pass transistor varied
in step with the modulating audio this would be a CCAM mod.
Sorry about that.
=============================================================
> Bill Eitner <kd6...@earthlink.net> wrote in article
Sean
thanks for the clarification. while i have never seen that specific mod
book (only have volume 1), i would be interested to see how he recommends
doing it. i had to play with several values to obtain my desired results.
while i was able to achieve a 22 watt rms output, this was with only appr.
30% modulation. as i increased the audio percentage, rms levels fell
proportionatly. i played with a few values and was finally able to
configure it so that as more audio drive was applied, more rms output was
available. i ended up with appr. 20 watts rms coming out of my 2950 at
13.8v. i only maintain appr. 100% - 110% modulation now, which resulted in
better clarity according to the locals due to the "audio reduction". i
still have to play with the values somewhat, so i'm still experimenting
with it. due to the fact that the 2950 has a built in dial-a-watt, the
simple "circuit" that i constructed doesn't function linearly, but does so
only at one position. since i only run my radio at one carrier level, it
works fine for me. otherwise, this mod would require a much more
complicated circuit to perform linearly with varying carrier levels. Sean
Bill Eitner <kd6...@earthlink.net> wrote in article