Modified Cobra 29

[RedBeard]

Hi,
I just picked up an older Cobra 29LTD Classic from a driver who didn't know if it worked or not. I plugged it in, it works fine. I thought about trying some mods on it, so I looked up some info, cracked the case, and found it had already been modified. It's a unit from 1994 with side mounted mic jack. Here's what I found so far:

Final transistor has been replaced. The transistor is marked "C1969" with the Mitsubishi triangle logo.
D11 has been cut.
I don't know what was done to achieve this, but the radio has talkback (I can hear myself through the speaker when I key up)
SWR meter doesn't work (though RF meter function does).

What else should I look for? Is this radio something I should sell or trade for a "virgin" radio? I basically have it for a spare, and my main radio is a later-model 29LTD NW that's just been "peaked and tuned" and talkback set up on it by a local CB shop.

 

[Steve H]

What does the swr needle say when you transmit? Zero, or off the scale? Just wondering...

ok...sorry I see you say the swr is not working. My mistake.

 

[RedBeard]

What does the swr needle say when you transmit? Zero, or off the scale? Just wondering...

ok...sorry I see you say the swr is not working. My mistake.

Right, the SWR function and SWR Cal do not work on it for some reason. RF goes straight to the top of the green area of the meter (4 watts I assume) when I set the switch to RF and transmit. It also works fine showing receive signal strength. My antenna/coax setup is tuned and my SWR is around 1.2 to 1.4 depending on what channel I'm on, as measured by the SWR meter on my main radio, the above-mentioned unmodified 29 NW LTD.

The other thing I found was a resistor soldered between the external speaker jack's center connection and ground. Judging by the stripe colors on it, I think it's a 42-ohm 1/4 watt resistor. Further investigation leads me to conclude that's how talkback was added.

I don't think it gets out much farther than my stock radio, so I assume any mods were done incorrectly. I think I'll sell or trade it for a virgin radio to play with; I want to spend some time with a radio that I don't care if I destroy, a soldering iron, my oscilloscope, multimeter, etc. learning and modding, maybe ending up spending $350 in parts and dozens of hours on, to get a radio that performs like a $300 Connex or Galaxy radio that I could buy and have a "big radio" out of the box ;-)

 

[mackmobile43]

The talk back feature is most likely just a resistor across the external and PA jacks in the rear.

 

[hotrod]

only thing id change is d11 .put that back in place. then use a power mic
if u want.theres a variable resister that controls the rf/rx meter and swr
my guess would be that was messed with.id KEEP it and add a nice lil amp
with it , like a klv 300p or 500p.

 

[RedBeard]

only thing id change is d11 .put that back in place. then use a power mic
if u want.theres a variable resister that controls the rf/rx meter and swr
my guess would be that was messed with.id KEEP it and add a nice lil amp
with it , like a klv 300p or 500p.

That diode (D11) was clipped right up at the glass of the diode. The info I found for "supertune" on a 29 LTD Classic says to replace that if it's been clipped, with a "small signal switching diode, like a 1n147 or similar".

I was an electronics hobbyist in a previous life, and have a decent collection of components, but no 1n147 diodes. I do have some 1n4148 (similar to 1n914) diodes, and some that have no markings, but are clear glass with a blue band at the cathode end, the same size as a 1n4148.

My handy multimeter says the 4148s have 680mV drop, and these blue ones drop about 620mV. I just installed one of these, gonna go try it out in a bit. Will post here with results.

Oh, in the "supertune" info I found, it says to tune L16 for best modulation. I can't tune it while powered up or hooked to antenna (my antenna cable doesn't reach far enough to be able to get to the bits inside), but I did give the slug in the middle about 1/8 turn counter-clockwise for S&G's. Let's see how that works

 

[RedBeard]

That diode (D11) was clipped right up at the

My handy multimeter says the 4148s have 680mV drop, and these blue ones drop about 620mV. I just installed one of these, gonna go try it out in a bit. Will post here with results.

Oh, in the "supertune" info I found, it says to tune L16 for best modulation. I can't tune it while powered up or hooked to antenna (my antenna cable doesn't reach far enough to be able to get to the bits inside), but I did give the slug in the middle about 1/8 turn counter-clockwise for S&G's. Let's see how that works

Well, it works okay. I talked with somebody about 3-4 miles away, over a pretty big hill (3/4 mile of 5% grade up and down). He said he was on a stock Cobra 29 with a Wilson 2000 antenna, and he pushed my RF meter to 2. I'm using the antenna that came on my Freightliner Century from the factory, and pushed his meter to 3. Now to look at antennae to really get out

 

[hotrod]

glad ya got it working. yes antenna /coax are the biggest part
never skimp on this.that factory coax is junk at best
i like lmr195 [low loss rg58 ]
i like lmr 240 [low loss rg mini 8 gray]
next is antenna i like the browning ,monkeymade, p10k and wilson 5000 trucker

 

[RedBeard]

glad ya got it working. yes antenna /coax are the biggest part
never skimp on this.that factory coax is junk at best
i like lmr195 [low loss rg58 ]
i like lmr 240 [low loss rg mini 8 gray]
next is antenna i like the browning ,monkeymade, p10k and wilson 5000 trucker

Yeah, that'll take a little doing. It's my understanding that the factory antennas handle dual duty for both CB and FM, being co-phased for CB and handling FM through some sort of box/signal splitter. That'll be the real banjo work, getting it all worked out so I can use decent coax and antennae for CB while preserving FM signal for the stereo. I don't want a forest of antennae or coax wrapped around my fip-down sun visors.

 

[hotrod]

i havent seen a setupike that in a big truck.
is this a single or dual antenna system?
if dual keep one antenna run it to your fm stero
then buy a good cb antenna for other side wiuth good coax hooked
up too it.