Super Swing, etc. for Cobra AM Radios

25 GTL / LTD / CLASSIC

Super Swing MODULATOR
PARTS NEEDED: 1- 220uF (16 to 25 volt) electrolytic capacitor.
1- 100 to 200 Ohm resistor. (1/4 to 1/2 watt)

ASSEMBLY:
1. Make sure your radio is peaked out.
2. Unsolder JP6 near final.
3. In JP6 holes, solder the 220uF capacitor. (negative side of cap. faces final)
4. On the solder side of the board, solder the resistor across the 220uF capacitor.
You can use different resistor values to get the desired dead-key.

VARIABLE POWER:
1. Find the two wires that are attached to the RF gain control, follow them to the PC board and UNSOLDER them.
2. Solder in a jumper wire from where you removed the wires on the board. This will restore receive.
3. Place the two wires from the RF gain control in your mod kit by soldering one wire to each of the 220uF capacitors leads.

REPAIR TIPS:
25 GTL:
1. Channel display reads all 7's or 8's: Leaky/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, misaligned VR5.
 

29 GTL / LTD / CLASSIC
Note: These radios can generally be modified for 40 above and 40 below by installation of a crystal-switcher such as the "expo" kits.

POWER TWEAK #2
1. Remove final and replace with a 2SC1969 transistor.
2. Remove R123 and replace with a jumper.
3. Replace C58 with a 180pF capacitor.
4. Re-tune coils.

AM LIMITER: D-11
Super Swing MODULATOR AND VARIABLE POWER
ASSEMBLY:
The instructions are the same as the Cobra 25 above, EXCEPT the following:
1. Unsolder JP36 instead of JP6.

REPAIR TIPS:
29 GTL:
1. "Motorboating": Locate C242(.001)on pin 5 of TA7222 audio ic and move to pin 4 of ic.
2. Weak mic gain: Some raios have a 2SC372 mic amp. Replace with a 2SC945.
3. 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. Misaligned VR4.
2. Intermittent 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 moe output. L14 peak is about a turn or two up from factory original setting.

Cobra 29 NW Super Swing Modification

There are tricks to lower the carrier of the Cobra 29 transmitter while keeping the modulation swing almost to maximum. Say you have a amplifier that requires 25 Watts peak input to deliver full peak output power, but you only want a 2 Watt dead carrier. First the final output transistor must be changed to the 2SC1969. Then the JP-36 jumper is removed. Add a 50 to 200 Ohm resistor and a 220 MFD 16 volt capacitor in the JP-36 location. See the schematic below for the resistor and capacitor location.

Schematically:

Pic of PCB:

The resistance value will have to be selected for the desired carrier level. The lower the resistance, the higher the carrier. For a cleaner signal, don't eliminate the AMC, just adjust it for maximum modulation. Also retune L14 for maximum RF swing then check your carrier level again. It may be necessary to select another resistor value once L14 is readjusted, or a compromise may be achieved to get the proper carrier to swing level desired.