Note: This page is an exact copy (with graphics removed and links changed) of the Cobra mods page found at Wholesale CB Radio, as of 9-24-03.

    By Dave

    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.

    Install Q1(see diagram) where JP6 was.The right leg(emitter) of Q1 in the hole near the final.The Centre leg(collector) in the other hole.

    Solder a wire from the left leg(base) of Q1 to the centre 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

Receive Boost

Cobra 29
1. Locate pin 14 of the PLL chip, and the Positive (+) side of C-12. 2. Install a 22k-220k ohm resistor between these two points. The smaller the value, the more the receive is boosted. 3. If desired, this mod may be wired through a switch.

How to make Cobra 29LTD,Classics slide on TX & RX

1)Remove D-18 , R-110 , C131 and R-112. 2)Put jumper were R-110 was. 3)Take orange wire from delta tune and unsolder it from board then resolder it to Tr-23 (Solder to the pin closest to front of board. It should have around 8 VDC on this pin.) 4)Put radio back together and if all is done right you should slide transmit 5-6 KCs up and down. For more slide change D-16 to a 1N-4007

Cobra 2000

No Audio on AM Transmit with good Carrier

The Fix: 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 replaced. This is a 330uf, 10V electrolytic capacitor beside L31. To prevent "future" problems, replace with a 16V.

Cobra 148/2000GTL
President(Uniden) 'Grant'/'Madison' (new)
L38, TR24-AM, VR11-SSB, VR12-AM PWR

Cobra140/142GTL
Courier Galaxy
Midland 79-900
President 'Washington'/'McKinley'(new)
Realistic TRC450/490
Robyn SB505D(new version)
SBE LCMS-8, LCBS-8
Superstar 360
Teaberry Stalker IX, XV, XX
Tram D80, D300
L36, TR32-AMC, VR7-SSB PWR, VR6-AM PWR

 

Cobra 46/47XLR, 50/55XLR
Midland 63-445
L112, L109, VR105


 

COBRA 142 AND 140GTL

MOD.-R104 AM PWR- VR6 SSB PWR-VR7

COBRA 2000 148GTL PRESIDENT WASHINGTON MADISON

MOD.-R131 AM PWR.-VR10 SSB. PWR-VR11

Cobra 148 GTL-DX/ Superstar 360FM Power

This is for the Cobra 148 GTL-DX late model only (PB010AB)

L44, VR-13-AM PWR, TR32-AMC, VR12-ALC, VR5-FM DEV


 

Cobra HH-40 Handheld Radio

This handheld radio uses the LC7230 PLL, But as it is all surface mount components this mod should only be tackled by persons who feel confident to do so :)..

Summery of mods :- RV-201 - 'S' meter adjust, RV-301- 'TX' meter adjust..
AM Power :- L304,L305,L306,L307

Modulation (This is the hard part)..
Remove R243 (2K2 surface mount, marked '222', black in color, just above the marking 'R243'
Remove C225 (0.22uF surface mount, no number, tan in color, just below C226, which, in turn is just below the 'C226' marking.

Component Access And Identification

    2. Remove the front and rear covers (4 screws). The front cover (with the speaker) stays attached by the speaker and mic leads.

  2. On the back of the rear circuit board, you will find RV201 and RV301, and all the inductor coils, all are identified with numbers printed on the circuit board.
  3. On the front circuit board (the side with the LCD facing you), desolder the 4 pads that ground the front circuit board to the case. Two of these have tangs that must be bent open to release the board. Remove the two screws on either side of the LCD (friction fit only). Fold the front circuit board over and out of the way exposing the front face of the rear board.
  4. Orient the unit horizontally with the antenna jack facing to your left and the bottom of the transformer visable on the right. The printed component numbers should read right -side-up. Locate the markings 'R243' and 'C226' in the area near the lower-left corner of the transformer.
  5. Remove R243, which is black in color, vertically orientated, just above the marking 'R243' and to the left of the 'Q205' marking. It has '222' marked on it.
  6. C225 is not marked. Start by finding C226, which is a horizontally orientated, tan component located just below the 'C226' marking. DO NOT DISTURB C226!. Remove C225, which is another horizontal, tan component located just below C226.

Cobra

Cobra 148 GTL and F-GTL

Extra Channels: Locate the MB8719 chip (usually right behind the channel selector). Wire a single-pole, single-throw switch from pin 10 to ground (SW-2 on chart), and another from pin 11 to pin 12

Tune-Up: Many of you have asked about supertunes for this radio. I do not recommend modifying the 148, due to the type of modulation circuit used. I have found that the best over-all sound and performance comes with nothing more than a standard tune-up, with the mod level set for just over 100%.

Cobra 140/142 GTL

Extra Channels: Replace the 11.1125 mhz crystal with a 11.3258 crystal. Locate the MB8719 chip (usually right behind the channel selector). Isolate pin 10 from ground (Cut the trace). Wire a switch from Pin 10 to ground(SW-2 on the chart), and another from pin 11 to pin 12 (SW-1 on chart). You may need to re-align the frequencies a bit. Click Here for a channel chart.

Tune-Up: Many of you have asked about supertunes for these radio. I do not recommend modifying the 140/142, due to the type of modulation circuit used. I have found that the best over-all sound and performance comes with nothing more than a standard tune-up, with the AMC circuit left intact.

Cobra 29 LTD:

Receive Boost: Locate pin 14 of the PLL chip, and the Positive (+) side of C-12. Install a 22k-220k ohm resistor between these two points. The smaller the value, the more the receive is boosted. Use CARE when soldering to the PLL chip! If desired, this mod may be wired through a switch.

Supertune: Replace the stock final with a 2SC1969 unit. Solder a jumper across R123. Solder a 68pf cap across C62. Solder a 120pf cap across C59 if there is already a 33pf across it. If there isn't, use 150pf instead. Replace R 55 with a 47-68k resistor, and solder a 33-55 pf cap across that resistor. Replace R58 with a 4.7 ohm resistor. Set VR4 just a bit to the right of half-way open. If D11 or C3 have been removed, replace them. If TR18 has been removed or cut, install a 2SA733. Spread Open L12 a bit, and adjust L16 for best swing, using a loud, steady "aaaaaaah" sound into the mic. MAKE SURE to use heat-sink compound (a light coating is sufficient) when mounting the new final, and MAKE SURE the final mount screw is tight!

Channel 9 switch to Channel 19: Cut D32. Hard, huh?

Cobra 25 LTD:

Supertune: Replace the stock final with a 2SC1969 unit. Change stock driver to a 2SC1957,{some model use a 2SC2028 rf driver}. Solder a jumper across R108. Change R43 to 4.7 ohms. Remove C149. Place 100-150pf cap across C 46. Place 33-68pf cap across C42 (select for best carrier). Set VR5 half-way open. Adjust L10 for best swing, using a loud, steady "aaaaaaah" sound into the mic. MAKE SURE to use heat-sink compound when mounting the new final, and MAKE SURE the final mount screw is tight! Optional: For more mic amplification, replace R76 with 1K ohm.

Cobra 19 Plus, 19 DX, 19 Ultra:

Supertune: Replace the final with a 2SC1969 unit. Solder a jumper across the diode going from the Audio Transformer to the Driver and Final. Solder a jumper across the resistor going from the base of the Final to the Driver. If a diode has been cut for modulation, solder it back together, or replace it. Adjust the modulation control POT (VR) for just over 100% modulation (usually around 1/4 to 1/2 way open). Tune output coils for best swing, using a loud, steady "aaaaah" sound into the mic. Note: These radios are superb for clarity of modulation, even on the stock mic.

By Dave

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