Hello there.  Welcome to the SILICON212.ORG CB section.  I am your host, 'Black Lightning'. 

First off, a lot of this information comes from the Copper Electronic web forum, written by user 'Adshar64'.  In turn, he got it from another source.  I've modified this to specifically apply to the Cobra 2000 GTL.  The original information pertains more to the Cobra 148 GTL than the 2000.  I've also added my own tweaks.  This modification is known as the Negative Peak Compression / Reduced Carrier mod aka npc/rc.

Today, we're going to modify a Cobra 2000 GTL base station for BIG AUDIO.

Parts needed: 1 10 microfarad, 25+ volt electrolytic capacitor

              Wire or other suitable material to create jumper shunt

              Blown up, non-operational source radio for parts.

This file has been edited to repair formatting and other screwups.

The Cobra 2000 GTL citizen's band base station radio demonstrates the epitome of contemporary CB base station design.  The radio comes equipped with a 2SC1969 RF transistor in the final output stage, a transistor that can produce 25 watts PEP at 12 volts Vcc.  This radio is manufactured by Uniden in either Taiwan (to mid 1988) or the Philippines (1988 to final production in 1992).  It is well known that these radios can produce 25 watts on single sideband, but on AM they will usually do only 8-10 watts maximum out of the box, even with the carrier set way down to 1-2 watts.  This has to do with the design of the radio in AM mode operation.  What we're going to do here is remove a limitation of the design, and make the radio perform on AM, the way it performs on single sideband.  This is really a two-in-one operation, we're going to perform a negative peak compression (NPC) mod, as well as volting the FINAL.  Don't get scared away - the tandem modification will ENHANCE audio and not ruin it.  It will also keep your neighbors off your back as this is a clean mod.  It is important to realize that the 'volted final' condition occurs in AM only.  In SSB mode, the finals receive full input voltage by design.  We will add this capability to AM.  Also, you will NOT lose the ability to control carrier power, as VR10 will also continue to be used.  VR10 controls the driver, which in turn controls the final.  We will not be volting the driver.

My work on this design makes it completely reversible, so that if the desire is to return it to stock, you will be able to do this.  I am a stickler for detail, and I don't like hacking a radio, so I try to do any work as cleanly as possible.  As such, the original instructions for the AM volt mod involved cutting the TP7 wire, but I do not like to cut wires so we will add a new 'terminal' to attach the existing TP7 wire to, in short - creating a new TP7.  The new wire will use both a TP pin and corresponding wire from a non-working donor radio.

Along with the instructions, I will be using pictures to help the viewer perform the mod, because a 'picture is worth a thousand words'.

The mod limiter will be disabled on the radio.  It will not be needed - with the NPC mod, the harmful effects of overmodulation will not take place - i.e. harmonics, splatter etc.

1. TR24 or Modulation Limitation

Removing this deactivates the modulation limiters in all modes. Use the front panel mic gain (aka Dynamike) control to set the modulation percentage.  Replace the following 10k resistors with 1/4 watt, 4.7k resistors:  R126, R124 (both located near the VR3 Squelch range trimpot).  If you do not wish to remove TR24, you may add a 1.5k resistor in series with the middle leg of TR24 (the collector).  Replace R131 (10k) with a 4.7k resistor (only if you do not remove TR24).  My own note:  Removing TR26 or R131 will disable the AMC in the AM mode but leave the limiter intact for SSB usage.  Since this is a primarily AM mod, and delimiting modulation in SSB is a bad thing for SSB audio clarity, this is the personal route I would take.

2. Add a solder bridge, jumper or shunt to the solder side of the board that effectively jumpers out R196. This is a quick way of replacing R196 with a jumper (reducing its value to zero ohms). This increases the range of VR10 (AM dead-key power) so that the
dead-key can be set to 1.5 to 2 watts later on.

 

3. Add a 10 uF 25 or higher volt electrolytic cap to these points: the positive leg goes to the trace that connects to pin 9 of the IC6 (the audio IC), and the negative leg goes to the R194/D63/R228 junction.  This is the mod that compresses the negative modulation peaks and allows the average power to increase based on the modulation percentage (aka the NPC mod).

 

The picture to the right demonstrates the locations on the backside of the PC board showing the physical locations for the parts in step 2 and 3 above.

 

The positive lead of the capacitor is soldered to the point directly below the capacitor itself.  The negative lead is soldered to that yellow wire, which extends to the point on the PC board where the R194/D63/R228 junction exists. You can see the point directly below the capacitor where the shunt is added that extends the range of VR10.
4. Set the driver bias to 50 mA. (Power up the radio, put it in LSB or USB, set the mic gain at minimum, remove the wire from test point 8, insert a milliamp meter in series between the test point (which is positive) and the wire, key the mic, and adjust VR9 until the meter reads 50 mA.)

5. Set the final bias to 100 mA. (Same instructions as in step 4 except the test point is test point 7, and the adjustment is VR8). On some of the newer radios the final bias can't be set higher than about 50 mA. The reason is that the value of R179 has been increased in order to decrease the effective range of VR8. To solve the problem, replace R179 with a 500 to 1000 ohm resistor.

The picture to the left demonstrates the use of the digital multimeter.  The black probe is touching the yellow wire to the final, where the red probe is being held on TP7.  This is the final bias adjustment - the driver is identical except the wire is violet and the test point is TP8.

 

6. Now, we are going to make a new test point 7.  We will use the old driver/final power leads as well as the test point pins from the donor radio.  First, we will remove the leads from the radio, and also the pins.  In the first picture on the right, the pin from TP8 has been unsoldered and is in the jaw of the pliers.  It, and its companion wire, are shown in the image on the far right.  We will solder the pin to the wire, as demonstrated in the pictures below.

Solder the wire as shown.  Place the short part of the pin inside the wire receptacle.  Add just enough solder to allow a permanent connection.  Let it cool a little, then add a piece of heat shrink tubing over the soldered connection as shown in the below left image.  Heat and then it's ready to go.
On the above left image, you can see the heat shrink tube placed over the joint.  Once you have applied heat, the joint should look as it does in the above right picture.  The other end of this wire will be soldered to the +12v source rail on the underside of the PC board.  This rail will have a bunch of open wire positions on it, all in a row.  It is located near the MB3756 voltage regulator switch, as shown in the image on the right.  You may solder this wire to any one of those holes - that doesn't matter, but do make sure you get the +12v rail!  The rail near it that has a bunch of open connections is neutral (ground) and will result in a radio damaging short circuit if it is soldered here.  Double check your placement with the image.

Once you are finished with the above steps, you will be at the point of the image on the left.  This is the new TP7.  Remove the yellow wire from TP7 on the PC board and relocate it to this new wire.  Use a zip tie to hold it against the wiring harness where the original TP wires are located.  TP7 on the PC board may be left open (in reality, TP7 and TP8 on the PC board are both electrically connected).

This is the mod that converts the RF final stage to linear in all modes.

7. Power up the radio, put it in the AM mode, key the mic, and set VR10 (AM dead-key power adjustment) for about 1.5 watts.

8. Tune the RF chain coils (L38 and L45 through L48) for maximum peak (modulated) output power in the center of the band (that would be Channel 19 on a stock radio and Channel 40 on one that has the popular expanded frequency range of 26.815 to 28.045).  If you have a favorite channel that is more than 30 channels from 19 or 40, do your tuning on that channel.

9. Double check the dead-key power. It should be around 2 watts.  If it is higher than 2 watts, use VR10 to cut it back to between 1.5 and 2 watts. Don't overdo it. Keep in mind that the carrier (aka dead-key) power increases up to 10+ watts with modulation, so there's absolutely no point in having the dead-key power any higher than is required to reliably key an amplifier. Most amps will key reliably with as little as 1/2 watt of dead-key power.

The following numbers are what you should expect. However, since there is a lot of variation in CB test equipment setups, don't be alarmed if you don't see these exact numbers. These numbers are provided as a guideline to make sure you did the mods properly.

The dead-key wattage should be 1.5 to 2 watts. The maximum average power should be 10 to 12 watts. And the maximum peak power should be around 25 watts.

10.  Squish, Hammer, Nail, Mallet, Maul!

This page is for informational/educational use only.  (c) 2008, 2009 gary at silicon two one two dot org