President Washington/McKinley, Cobra 140/142, Realistic TRC450/490, Tram D80/D300, Midland 79-900, etc. (MB8719 single final, single conversion) transmitter modification instructions including NPC and final to linear in all modes mods.

Note: This modification package is very similar to the 148 package. The differences are only in the part numbers. Refer to the 148 mod package instructions for notes. The 148 instructions are included at the end for your convenience.

1. Remove TR32

2. Jumper out R123.

3. Add a 10 uF 25 or higher volt electrolytic cap to the following spots: Attach the positive leg to pin 9 of IC4, and attach the negative leg to the junction of R121/D62/R204.

4. Set the driver bias to 50 mA.

5. Set the final bias to 100 mA.

6. Solder the end of the final bias wire that is furthest from the final transistor to the cathode (banded) end of D60.

7. Set the dead-key power to 1.5 watts using VR6.

8. Tune the RF amp chain coils (L36 and L26 through L29) for maximum peak (modulated) output power.

9. Double check the dead-key power. If it's higher than 2 watts, use VR6 to set it to 1.5 to 2 watts.

Below are the 148 instructions that have notes that apply to each of the above instructions.

148/Grant/2000/Madison etc. (MB8719 single final, dual conversion) transmitter modification package including NPC and final to linear mods.

Note: When I do this I see the radio as having two sides; the solder side of the main board, and the parts side of the main board. The most efficient way to get this done is to do all the solder side work first, then do the parts side work. The following steps are presented in that manner.

1. Remove TR24 This deactivates the modulation limiters in all modes. Use the front panel mic gain (aka dynamic) control to set the modulation percentage.

2. Add a solder bridge 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).

(This is the end of the solder side work. The rest of the work is done on the parts side of the board.)

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).

NOTE: 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.

6. Once the final bias has been set, unplug the DC power cord, put the final bias wire back on the test point, cut the final bias wire 1/4 inch above the connector, strip and tin 1/8 inch of the wire, tin the cathode (banded) leg of D55 (the reverse polarity diode), and solder the wire to D55. This assumes the test point connector is at the end of the wire that is furthest from the final transistor. On some of the newer models the test point connector is at the end of the wire closest to the final transistor. On those models, completely unsolder the wire at the end opposite the test point connector and solder it to D55.

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.

If you have any questions, post them to the CB Tricks forum or e-mail me.

Good luck and have fun.

Bill Eitner (10-6-01 and 4-16-03)