CB Tune-Up Manual Master Edition/Volume 1/Radio Modifications

Most radios develop more peak power with a whistle than with normal voice, as many of you have noticed. First of all you should never use a whistle when tuning for maximum forward (PEAK) power. Instead use a constant tone no higher than 300 to 400 Hz (Normal Voice Range). This tone can either be generated by most tone generators, or by using a steady ah-h-h-h-h sound as you speak into the mike. Never whistle into the mike when tuning any radio, unless your customer has a very high pitched voice.

The following modifications allow you to obtain maximum peak output power, by increasing normal voice sensitivity. Improvement will vary anywhere from 25% to 50% depending the radio's circuitry. These modifications should be performed after doing a standard radio tune-up.

We have also shown the nominal value of most parts to change, however these may vary from radio to radio. When replacing any parts be sure to change or use only those specified.

Cobra 21 GTL, 21 LTD, 25 GTL, 25 LTD, 25 LTD Classic

1. Change R76 (3.3K) to a 1K (Located near mike socket).
2. Change R43 (10 Ohm) to a 2.7 Ohm.
3. Change R108 (1 Ohm) to a .47 Ohm.
4. Re-Tune L10, L9, L8 for maximum Forward Power.

Cobra 77 X

1. Remove TR219 & C270 (1 uf).
2. Change R269 (560 Ohm) to a 270 Ohm.
3. Change C258 (220 pf NPO) to a 270 pf NPO.
4. Change R264 (1 Ohm) to a .47 Ohm.
5. Re-Tune L211, L212, L214 for maximum Forward Power.

Cobra 78 X

1. Remove TR8 & C49. (1 uf)
2. Change R31 (560 Ohm) to a 270 Ohm.(Located next to TR8 in Mike circuit)
3. Change C83(220pf-NPO)to a 270pf-NPO.(Located between Driver & Final)
4. Change R91 (1 Ohm) to a .47 Ohm.
5. Re-Tune L11, L12, L15 for maximum Forward Power.

Cobra 18 RV, Cobra 23 Plus

1. Change R505 (3.3K) to a 1K
2. Change R310 (2.7 Ohm) to a .47 Ohm
3. Re-Tune L305, L306 for maximum Forward Power

Cobra 19 GTL

1. Remove TR8 & Change R31 (5.6K) to a 1K
2. Change C134 (330 pf NPO) to a 390 pf NPO
3. Change R91 (1 Ohm) to a .47 Ohm
4. Re-Tune L15, L16, L17 for maximum Forward Power

Cobra 21 XLR

1. Change R110 (2.2 Meg) to a 1 Meg
2. Change R35 (10 Ohm) to a 2.7 Ohm
3. Change R33 (4.7 Ohm) to a 1 Ohm
4. Re-Tune L15, L12, L11 for maximum Forward Power.

Cobra 29 GTL, 29 LTD, 29 LTD, 29 LTD Classic, 29 LTD Classic Gold

1. Change R58 (10 Ohm) to a 2.7 Ohm
2. Change R123 (1 Ohm) to a .47 Ohm
3. Re-Tune L14, L13, L12 for maximum Forward Power

Cobra 29 XLR, 89 XLR

1. Change R87 2.2 MΩ to a 1 MΩ
2. Change R49 47 Ω to a 27 Ω
3. Change R42 4.7 Ω to a 1 Ω
4. Change R43 2.2 Ω to a 0.47 Ω
5. Re-Tune L15, L12, L11 for maximum Forward Power.

Cobra 87 GTL, 89 GTL, 1000 GTL

1. Remove C128 0.47 μf & TR16
2. Change R132 1kΩ to a 470 Ω
3. Change C61 220 pF NPO to a 270 pF NPO
4. Change R50 3.9 Ω to a 1 Ω
5. Change R47 1 Ω to a 0.47 Ω
6. Re-Tune L17, L13, L12, L11 for maximum Forward Power.