As advertised, here is the mod to put the carrier control on the front panel of a Cobra 2000. It serves four objectives. To set the minimum carrier below a half Watt without having to add a trimpot. To preserve the same peak modulated power no matter how low the carrier is turned. To sound as smooth as the stock radio no matter where the carrier is set, and to preserve the function of the radio's SWR meter.
Bill of materials is short. A 1N4148 or equivalent signal diode, a 10k 1/4 Watt resistor. A trimpot if yours is gone. A 560 ohm 1/4 Watt resistor if R228 is missing or damaged. And if D52 and D75 can't be salvaged, another four 1N4148 to wire in a series string.
To start, if VR10 is missing, you need to replace it. The schematic says 5k. The black trimpot in the pic is not factory. Someone had removed VR10 and poked three wires into the holes. We use a 2k trimpot because I have a box full of them. MAKE SURE R228 IS IN PLACE! Clipping or removing R228 is a popular "NPC" trick. It just screws up the audio. Install a 560 ohm resistor at R228 if it's missing or fatally mangled. And yes, the yellow wire from the final-transistor bias-test socket has been "power jumped" to a jumper wire with full 13.8 Volts DC on it. Just make sure to put your current meter in line with this wire and set the final bias to 60 mA BEFORE you solder it to a full-voltage source.
These two tiny diodes, D75 and D52 are next to the big PLL crystal that's missing from this pic. If they are not missing or destroyed, remove them carefully so they can be reused.
The wiper (center) lug of VR10 gets the trace cut.
The two tiny diodes each have two diode junctions in series inside them. Gives us about 1.2 Volts constant voltage drop on each part. Putting them in series gets us the equivalent of a 2.4 Volt zener diode. They get lap-soldered in series with the anode end (black) soldered to the outboard lug of VR10. The cathode end (orange) goes to a ground foil.
The SWR Cal control on the front panel has three wires on the lugs closest to the inside of the front panel. Clockwise (max) wire is red, wiper is white and counterclockwise (min) is black. First unsolder the red wire from the switch circuit board, just above the SWR cal switch.
The white wire also gets unsoldered.
The black wire is soldered to the far outside edge of the switch circuit board. No pic, you can find that and unsolder it.
Now's a good time to point out that you don't have to totally kiss off the radio's SWR meter. A 5.6k resistor will take the place of the Cal control wired as you see here.
Doesn't have to be strictly 5.6k. Any value from 3.9 to 6.8 probably works okay, just haven't tried them.
Each of our three wires red, white and black now gets added length lap-soldered to the end of each control wire to reach the vicinity of VR10.
The red wire goes to the wiper lug of VR10. VR10 will now set the max carrier with the Cal knob full to the right. The black wire goes to the outboard lug of VR10, where the anode end of our two-diode string is attached.
A 10k resistor now gets soldered to the solder pad to the rear of the cut. Extend a short bit of the lead so we can lap-solder a diode lead to it. The other end of the resistor goes to ground.
The white wire gets a 1N4148 or equivalent diode with the anode (no band) end lap-soldered to the end of the wire. We slide sleeve over the lap-splice and diode to keep them out of trouble.
The free (banded) end of the diode gets lap-soldered to the extended "hot" leg of the 10k resistor.
Here's the power this radio showed with the SWR Cal turned all the way down. 300 milliwatts, or three tenths of a Watt. No trimpot needed to set it, and sounds just as good as 4 Watts.
We've used this setup for more decades than I care to admit. The customers like it. YMMV.
73
Bill of materials is short. A 1N4148 or equivalent signal diode, a 10k 1/4 Watt resistor. A trimpot if yours is gone. A 560 ohm 1/4 Watt resistor if R228 is missing or damaged. And if D52 and D75 can't be salvaged, another four 1N4148 to wire in a series string.
To start, if VR10 is missing, you need to replace it. The schematic says 5k. The black trimpot in the pic is not factory. Someone had removed VR10 and poked three wires into the holes. We use a 2k trimpot because I have a box full of them. MAKE SURE R228 IS IN PLACE! Clipping or removing R228 is a popular "NPC" trick. It just screws up the audio. Install a 560 ohm resistor at R228 if it's missing or fatally mangled. And yes, the yellow wire from the final-transistor bias-test socket has been "power jumped" to a jumper wire with full 13.8 Volts DC on it. Just make sure to put your current meter in line with this wire and set the final bias to 60 mA BEFORE you solder it to a full-voltage source.
These two tiny diodes, D75 and D52 are next to the big PLL crystal that's missing from this pic. If they are not missing or destroyed, remove them carefully so they can be reused.
The wiper (center) lug of VR10 gets the trace cut.
The two tiny diodes each have two diode junctions in series inside them. Gives us about 1.2 Volts constant voltage drop on each part. Putting them in series gets us the equivalent of a 2.4 Volt zener diode. They get lap-soldered in series with the anode end (black) soldered to the outboard lug of VR10. The cathode end (orange) goes to a ground foil.
The SWR Cal control on the front panel has three wires on the lugs closest to the inside of the front panel. Clockwise (max) wire is red, wiper is white and counterclockwise (min) is black. First unsolder the red wire from the switch circuit board, just above the SWR cal switch.
The white wire also gets unsoldered.
The black wire is soldered to the far outside edge of the switch circuit board. No pic, you can find that and unsolder it.
Now's a good time to point out that you don't have to totally kiss off the radio's SWR meter. A 5.6k resistor will take the place of the Cal control wired as you see here.
Doesn't have to be strictly 5.6k. Any value from 3.9 to 6.8 probably works okay, just haven't tried them.
Each of our three wires red, white and black now gets added length lap-soldered to the end of each control wire to reach the vicinity of VR10.
The red wire goes to the wiper lug of VR10. VR10 will now set the max carrier with the Cal knob full to the right. The black wire goes to the outboard lug of VR10, where the anode end of our two-diode string is attached.
A 10k resistor now gets soldered to the solder pad to the rear of the cut. Extend a short bit of the lead so we can lap-solder a diode lead to it. The other end of the resistor goes to ground.
The white wire gets a 1N4148 or equivalent diode with the anode (no band) end lap-soldered to the end of the wire. We slide sleeve over the lap-splice and diode to keep them out of trouble.
The free (banded) end of the diode gets lap-soldered to the extended "hot" leg of the 10k resistor.
Here's the power this radio showed with the SWR Cal turned all the way down. 300 milliwatts, or three tenths of a Watt. No trimpot needed to set it, and sounds just as good as 4 Watts.
We've used this setup for more decades than I care to admit. The customers like it. YMMV.
73
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