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COBRA RADIOS & MODIFICATIONS

THE SOUND TRACKER FEATURE THAT IS IN ALL
THE NEW COBRA RADIOS, IS NOT TO MY LIKING, ITS OK IF YOUR TALKING TO SOMEONE WHO HAS A
SIMILAR RADIO.



COBRA SOUNDTRACKER RADIOS

Quite a few of you have wondered, "Just what does the soundtracker circuit do in these radios"? The manufacturers failed to tell their customers about the new technology incorporated into their latest Radios. The problem was a "failure to communicate" between manufacturer and the ad agency.

The benefits of the soundtracker circuitry can only be effective when communications are taking place between (2) two or more units that are equipped with the soundtracker circuit.

If you have one of these radios and you are talking to another user who has a radio that IS NOT EQUIPPED with sound tracker, the benefits will not be apparent. Of course the manufacturer did not explain this to there customers.

Of course they would not mention this because the buyer would not buy the radio if he knew of this feature.


SEPT 22, 2000

NEW INFORMATION ON THE COBRA 148 NW ST.

Cobra has really shot themselves in the foot with this new radio, why you ask? I'm getting to that but first let me say that Cobra has been the industry leader in producing good quality CB radio's down through the years, there was the great line of Mobil "AM" radios such as the 25LTD, 29LTD. Their Sideband radios such as the 146, 148 gtl and 2000 were superb, they were stable and a radio you could be proud of.

The first thing that happened to Cobra was that Uniden wanted to raise the price on there PC boards, Cobra did not like this so they got some other company to make there boards cheeper! The first thing that happened was that they started using 1/6 Th. watt resistors throughout the whole radio, don't know how many radios failed in the field because of this, I had several myself that had a failure of several resistors that had previously been 1/2 watt. Of course this is all history, they fixed that problem.

Now we have this new radio, the 148 NW ST, It looks real pretty at night. I had a customer send me a 148 NW ST for peaking and tuning for better receive and transmit. So before doing anything else, I hooked it up to my antenna to get some radio checks, well, AM was OK, nothing to write hone about but clear and clean. Then I went to Sideband, now people say they can't "clarify" me in, What! So I tried some others on the band with the same results, can't clarify me! So I'm thinking maybe my oscillators are off frequency so I check them and find that they are a little off so I align them. This time is the same as the last, they can't clarify me!

So I really got concerned! After checking the oscilator frequencies I find that they change when I modulate! What the Hell! they are not supposed to change, they are supposed to be rock solid! I placed a volt meter on several test points and found that the voltage to the oscillators is changing with modulation! So I start looking for the reason for this and ran into the cause right away.

The new units don't have the MB3756 Voltage Regulator, They have decided to further cheapen the radio by doing away with it! and installed a 3 pin voltage regulator in its place with a series resistor on the 8 volt pin, on the input pin there is no voltage drop, on the output pin there is.

What this all boils down to is this, the radio is in my opinion now is somwhat "unstable" on sideband and I for one, would not own one!

To my knowledge it could be inproved with some additional filtering and RF bypassing.

Handheld CB Radios

Cobra HH28 Basic low-priced 40 channel CB

Cobra HH33 this model is discontinued!

Cobra HH34 Basic handheld with charging jack and lighter plug.

Cobra HH36 ST this model is (Discontinued)

Cobra HH37 ST 40 chan. with Soundtracker and charging jack

Cobra HH38 ST This compact model includes 10 weather channels Soundtracker and charging jack

Cobra HH45WXST this model is discontinued! (refurbs may be available)

Mobile CB Radios

Cobra 18WXST CB Weatherband Radio

Cobra 19 Ultra 2 Compact Mobile CB Radio

Cobra 25ST (Discontinued) Classic CB Radio With SoundTracker.

**NEW** Cobra 25 NW ST CB Radio With SoundTracker and backlite front panel!

Cobra 25STWX Classic CB Weather Radio With SoundTracker

Cobra 29ST Classic CB Radio with SoundTracker

Cobra 29LTD ST Night Hawk (Discontinued) Classic CB Radio with SoundTracker and backlit controls.

**NEW** Cobra 29 NW ST Deluxe AM CB Radio With SoundTracker and backlit front panel

Cobra 29STWX Classic CB Weather Radio With SoundTracker

Cobra 75WXST Compact/Remote Mount CB Radio With SoundTracker

Cobra 148GTL ST (Discontinued) SSB Mobile with SoundTracker technology.

**NEW** Cobra 148 NW ST Top-of-the-line CB Radio With SoundTracker and backlit front panel

Cobra 148F (Discontinued) Cobra's top-of-the-line SSB Mobile with Frequency Counter.

Cobra Base Station CB Radios

Cobra 93 (Discontinued) Cobra's only AM Base with weather.

Cobra 2010 (Discontinued) The Cobra 2010 is Cobra's newest and best base station ever!


POWER TWEAKS: Cobra 25 & 29 LTD

This is a typical example of mis-information, the resistor changes listed below will not produce more power, they will make the radio sound crappie.

1. Change R76 (3.3k) to a 1k (near the mike socket) <-NOT OK AS IT MAKES THE MIC PICKUP TOO MUCH BACKGROUND NOISE.

THE ABOVE WILL PRODUCE DISTORTION AND
MAKE THE MIC CIRCUIT HARD TO CONTROL.

2. Change R43 (10 Ohm) to a 2.7 Ohm. <-NOT OK
3. Change R108 (1 Ohm) to a .47 Ohm. <-NOT OK

BOTH OF THESE CHANGES WILL PRODUCE OVERDRIVE
IN THE PRE-DRIVER AND DRIVER STAGES, NOT GOOD!

4. Replace stock final with a 2SC1969 transistor. <- THIS IS OK TO DO
5. Re-tune coils L10,L9,L8 for maximum forward Power.<- THIS IS OK TO DO


Select Cobra Power Mods
-------------------------------------
2000 GTL,2010

Cut R-131 Tune L47, L48, L46, L45 and L38 for max RF output

AM Carrier Power VR10

Mod Meter VR7

AM Signal meter VR1
-----------------------------------------
142GTL

Lift the Emiter leg of TR32, and insert a 1K resistor from the leg to ground. Adjust L36,L37 for max RF output

AM Power VR6
SSB Power VR7
VR1 "S"Meter
VR2 Squelch
VR3 TX Freq
VR12 Mod Meter
VR10 TX Power Meter
-----------------------------------------
148GTL+F

Lift the Emiter leg of TR24 and insert a 1K in series with it, to ground.
--------------------------------------------
29GTL/LTD/CLASSIC

Just turn up VR-4 for Max modulation, Adjust L17, L20, L21, L16 and L14 for max RF output.

Modification: Change stock final to a 2SC1969 Change C71 to a 33uf 16volt polarized electrolytic capacitor.

VR1 receive gain
VR2 receive signal meter
VR3 squelch
VR4 AMC
VR5 transmit power meter
VR6 antenna warning light
--------------------------------------------------
25GTL/LTD/CLASSIC

Just turn VR-5 for Max modulation, Adjust L10, L9 and L8 for max RF output. Modification: Change stock final to a 2SC1969 Adjust L10,L9,L8 again Change C91 to a 33uf 16volt polarized electrolytic capacitor.
------------------------------------
Cobra 18RV 23 Plus

Retune L305, L306
-----------------------------------
Cobra 90LTD

Cut D203, Retune L304, L305 and L306
-----------------------------------
Receive Boost Cobra 29

One, Locate pin 14 of the PLL chip, and the Positive (+) side of C-12.
Two, Install a 220k ohm pot between these two points. The smaller the value, the more the receive is boosted.

NOTE: adjust this pot with care as too much gain will SLAM your "S" meter, (This works! I've tried it, Peg Leg)
----------------------------------

Cobra 2000

No Audio on AM Transmit with good Carrier, This problem is more common in the "older" 2000's but is relatively easy to fix. Look for the collector of TR-23, you should read about 4.5 V with a dead key. If you find that this voltage is not present, check on the opposite side of R-120 for 6.5V. Most likely this voltage will not be present; if this is the case, C18 has shorted and needs to be replaced. This is a 330uf, 10V electrolytic capacitor beside L31. To prevent "future" problems, replace with a 16V unit.


COBRA 29LTD RECEIVE PROBLEM ( unit with - 1/6th. - watt resistors )

Problem :

Receive Audio Low.
Transmit Audio normal.
100 uV signal input did not move meter much. (should indicate S-9 on the meter)
Noise Blanker Didn't work.

Found problem: R-31 is supposed to be a 47k 1/6W and it opened up to 137.5k

NOTE : The new units have 1/6W resistors, the quality control is not good, this is the second example I've seen of this. The newer units may have corrected this problem.

Cobra 148 GTL
SYNTHESIZER ALIGNMENT
---------------------------------------------
Input of Freq. Counter To TP-13 (IC-1 Pin 8)
CH-19 -------------------------------- Check for 10.240Mhz.
----------------------------------------------------------
Input of Scope To TP-10
CH-19 AM ------ Adjust L-21 for Maximum RF (3-4vPP Typical).
----------------------------------------------------------
Input of DC Meter To TP-9
CH-40 AM
Adjust L-19 for 3.2 Volts DC - 2.2 Volts DC on CH-1
-----------------------------------------------------------
Input of Scope To TP-1
CH-19 AM -------- Adjust L-20 for Max RF (190 MvPP Typical).
-----------------------------------------------------------
Input of Frequency Counter To TP-1
CH-1 AM ----------- Adjust L-23 for 34.7650 Mhz. +/- 20 Hz.
CH-1 USB ---------- Adjust L-59 for 34.7665 Mhz. +/- 20 Hz.
CH-1 LSB ---------- Adjust L-22 for 34.7635 Mhz. +/- 20 Hz.
-----------------------------------------------------------
Input of Frequency Counter To TP-10
CH-1 AM -------------------------------- Check for .790Mhz.
-----------------------------------------------------------
Input of Frequency Counter To TP-3
CH-19 USB ------- Adjust CT-2 for 7.8015 Mhz. +5 Hz. -0 Hz.
CH-19 LSB ------- Adjust L-30 for 7.7985 Mhz. +0 Hz. -5 Hz.
CH-19 AM XMT
Disconnect TP-7, TP-8 Adjust L-31 for 7.8000 Mhz. +/- 5 Hz.
-----------------------------------------------------------
Input of Frequency Counter To TP-15 (FET-1 Gate 1)
CH-19 AM ----------------------------- Check for 7.3450Mhz.
-----------------------------------------------------------
Input of Frequency Counter To Antenna Input
CH-1 AM ------------------------ Adjust VR-5 for 26.965Mhz.
-----------------------------------------------------------
RECEIVER ALIGNMENT
Set output of Sig Generator to 27.186 Mhz.
Ch. 19, USB ------- Adjust L-14, L-12, L-10, L-9, L-8 and L-7 for Max. output at speaker
-----------------------------------------------------------
Set output of Sig Generator to 27.185 Mhz.
Ch. 19, AM - Adjust L-15, L-13 and L-3 for Max output.
------------------------------------------------------------
Set output of Sig Generator to 27.185 Mhz.
Ch. 19, AM - Adjust L-6, L-5 and L-4 for Max output at speaker.
-----------------------------------------------------------
NOISE BLANKER ALIGNMENT
Set output of signal generator to 27.185 AM, 1000 Hz. @ 30%
modulation. Input of O-Scope to TP-17, (D-2 Cathode) Inject a 100pps,
1uSec. pulse width signal at antenna input.
Switch noise blanker ON
Adjust - L-1 and L-2 for a Maximum while maintaining wave form Symmetry
-----------------------------------------------------------
RECEIVER ADJUSTMENTS
Set output of signal generator to 27.185 AM, 1000 Hz. @ 30% modulation. Output level 1000 uV. Set Squelch Maximum.
Adjust VR-3 so that squelch just breaks.
-----------------------------------------------------------
Set output of signal generator to 27.185 AM, 1000 Hz. @ 30% modulation. Output level 100 uV.
Adjust VR-1 for s9 on signal meter.
-----------------------------------------------------------
Set output of signal generator to 27.186 USB, 1000 Hz. @ 30% modulation. Output level 100 uV.
Adjust VR-2 for s9 on signal meter.
-----------------------------------------------------------
TRANSMITTER ALIGNMENT
Input of RF wattmeter to antenna input
Ch. 19 AM Adjust L-47, L-48, L-46, L-45 and L-38 for maximum output
NOTE: Do not touch L-36 (TVI filter), factory adjustment is O.K.
-----------------------------------------------------------
TRANSMITTER ADJUSTMENTS
Input of RF wattmeter to antenna input, no Modulation.
Ch. 19 USB, Dynamike MINIMUM
Adjust VR-4 for MINIMUM RF output.
-----------------------------------------------------------
Input of RF wattmeter to antenna input, no Modulation.
Ch. 19 USB, Dynamike MINIMUM
Insert a DC AMP meter at TP-8 (Violet wire) Adjust VR-9 for 25 mA.
Insert a DC AMP meter at TP-7 (Green wire) Adjust VR-8 for 50 mA.
----------------------------------------------------------
Input of RF wattmeter to antenna input, Inject a two-tone 500 mV signal at mic input,Dynamike MAXIMUM Ch. 19 USB
Adjust VR-11 for 11.0 watts PEP RF output maximum.
-----------------------------------------------------------
Input of RF wattmeter to antenna input
Ch. 19 AM, Dynamike MAXIMUM
Adjust VR-10 (AM carrier) to 3-4 watts RF output
-----------------------------------------------------------
Input of RF wattmeter to antenna input
Ch. 19 AM
Adjust VR-6 So that it agrees with your output meter.


REPAIR TIPS: 29 GTL:

1. "Motorboating": Locate C242(.001)on pin 5 of TA7222 audio ic and move to pin 4 of ic.

2. Receive on adjacent channel: Bad solder joint to C131 (next to 10.240 crystal).

29 LTD:

1. OVERMOD: Check D11
2. PLL dead, no TX or RX: Shorted C112
3. No TX/RX with Channel LED lit on RX and then goes out when keyed: Bad /shorted C119.

29 LTD Classic:

1. OVERMOD: Check D11. Missaligned VR4.
2. Internittent audio/modulation: Bad solder joints on pins 1&2 or 2&3 of audio ic.

3. TUNE UP TIPS: L11 is the TVI trap. DO NOT adjust!!!
Stretching L12 gets more output. L14 peak is about a turn or two up from factory original setting.


25 GTL

1. Channel display reads all 7's or 8's: Leaky or shorted C104 (470uF 10v). Replace with a 16v or 25v.

2. NO TX: Check for shorted 33uF(10v) capacitor across C87. 3. OVERMOD: Check D19, TR14, VR5

25 LTD

1. OVERMOD: Check D9

2. LED goes out and NO TX when keyed up: Check C118

25 LTD Classic:

1. Burned R89 (15 Ohm 1/2 watt): Replace with a 1 watt.

2. OVERMOD: Check for missing or cut D9, missaligned VR5.


Cobra 25 & 29 LTD Super Modulation

If you want to keep the carrier reasonably high (no AMP) make R1=33 ohm.

If R1 is 68 ohm, will give a Carrier of about one and a half Watt Dead Key and an Audio Swing of about 12-14 Watts PEP. RMS will be one and a half Watt Dead Key to about 5 Watts RMS!

If R1 is 82 ohm, will give a Carrier of about 1 Watt Dead Key and an Audio Swing of about 12-14 Watts PEP. RMS will be 1 Watt Dead Key to about four and a half to 5 Watts RMS!

R1 Must be rated at 2 Watts metal film. C1 is rated at 35VDC.

NOTE: When doing this modification, the AMC should be adjusted back a little, for an audio swing of about 9 or 10 Watts PEP, this will reduce transmit distortion.

NOTE 2: In the diagram, the notation that says add a 68 pF disk cap to C-62, I have found this in some cases does not produce the desired effect so disreguard it.

WARNING! DO NOT cut the limiter or you will have a radio that sounds over modulated!

I have found that on the small Amps, a dead key of three quarters to 1 Watt to be acceptable. After all, Super Modulation is the object, right?

Foot Note: If these Changes are done, you will have changed the Radio to a different Class of Amplifier, a Modulator.


COBRA 29 XLR MOD
CH HF\UP CH HF\UP
** ****** ** ******
5 --- 27.415 21 -- 27.615
6 --- 27.425 22 -- 27.625
7 --- 27.435 23 -- 27.655
8 --- 27.455 24 -- 27.635
9 --- 27.465 25 -- 27.645
10 -- 27.475 26 -- 27.665
11 -- 27.485 27 -- 26.875
12 -- 27.505 28 -- 26.885
13 -- 27.515 29 -- 26.895
14 -- 27.525 30 -- 26.905
15 -- 27.535 31 -- 26.915
16 -- 27.555 32 -- 26.925
17 -- 27.565 33 -- 26.935
18 -- 27.575 34 -- 26.945
19 -- 27.585 35 -- 26.955
20 -- 27.605 39 -- 26.995

THE INFORMATION I HAVE ON THE COBRA RADIOS IS EXTENSIVE INCLUDING THE MODS WILL BE ADDED IT MAY EVEN TAKE TWO PAGES SO PLEASE KEEP CHECKING BACK I WILL BE A POSTING NEW INFORMATION EVERY DAY

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