Project of the Month

Program out-of-bounds frequencies into an Icom IC-40S handheld.

The Icom IC-40S handheld UHF CB radio can be programmed with an additional 20 channels between 450 MHz and 480 MHz. While the programming software of the IC-40S allows you to program any frequency you like, the VCO goes out of lock above 480 MHz. The display flashes and the radio does not receive or transmit. In this project, we will adjust the VCO lock range to make the radio usable up to as high as 500 MHz.

You will need:

A set of jeweller's screwdrivers.

A digital multimeter.

A soldering iron with a fine tip.

Acetone.

Assorted hand tools.

Step 1

Program the radio.

Just how high in frequency you can go with the IC-40S is different in each radio. I have done this modification to a lot of these radios, and while a few may go as high as 504 MHz, mostly they will only go to 500 MHz, and some will only go as high as 495 MHz. So we will program a range of frequencies into the radio for this adjustment procedure, so that if it won't go as high as you want it to, you can just change channel and tune it to a lower maximum frequency. Program channels 50 to 60 with the following simplex frequencies.

Channel 50 - 490 MHz

Channel 51 - 491 MHz

Channel 52 - 492 MHz

Channel 53 - 493 MHz

Channel 54 - 494 MHz

Channel 55 - 495 MHz

Channel 56 - 496 MHz

Channel 57 - 497 MHz

Channel 58 - 498 MHz

Channel 59 - 499 MHz

Channel 60 - 500 MHz

Be aware that as you move the the upper range of the VCO up, the bottom of the range moves up also. That is, if you adjust the VCO so that the highest frequency it will operate is 500 MHz, the lowest frequency it will operate at could be 460 MHz. After programming, switch the radio on and select the channel of the highest frequency you want to get out of the radio, or just select channel 55 to for a reasonable expectation of success of the adjustment. We select the channel now, because changing the channel once the radio is disassembled will be fiddly.

Step 2

Disassemble the radio.

Remove the battery, the antenna and the speaker/mic dust cap. Pull the volume knob off. The antenna socket nut is held in place with loctite. You should be able to see the two little blobs of green on either side of the antenna socket just at the top of the nut. Dip a small jeweller's screwdriver in acetone, and work the acetone into the thread until the loctite is dissolved. It is worth spending some time doing this to make sure that the nut comes off easily. Be careful not to get the acetone on the radio housing, as it will damage the plastic. Grab the nut firmly with a pair of pliers and undo it. If the nut does not want to move, resume applying the acetone. Take care not to damage the thread of the antenna socket with the pliers, or it will be hard to remove the nut. Once the nut is removed, undo the two screws at the bottom rear of the radio. Prise the chassis of the radio from the housing at the bottom, and lift out of the housing. Unplug the speaker from the circuit board.

Remove the red dust seal from around the chassis. Remove the screw from the top corner of the shield near the antenna socket. Support the radio with a pen or pencil or similar while undoing the screw so as not to crush the battery terminals. Unclip the shield from the sides of the chassis, but don't try to lift the shield away just yet. Desolder the shield from the circuit board at the point indicated in the picture below. WARNING ! There are some small surface mount components near where the shield is soldered down, so use a fine-tipped soldering iron and only heat the solder joint on the shield. Lift the shield away from the chassis as the solder melts.

Desolder the positive battery connection from the volume control sub-board. Remove the screws indicated on the picture below. Support the radio with a pen or pencil or similar while undoing the screws so as not to crush the battery terminals. Remove the nut on the volume control. Desolder the antenna socket connection while lifting the circuit board down and away from the chassis. WARNING ! There are some small surface mount components near the antenna socket connection, so use a fine-tipped soldering iron and only heat the solder joint on the antenna socket connection. The heatsink compound on the output transistor may be acting like glue, and holding the circuit board down, so just apply a little upward pressure.

Solder a red wire onto the positive battery connection of the volume control sub-board. Solder a black wire onto the solder pad where the shield was soldered. Connect the red and black wires to a 10 volt 3 amp power supply. Any voltage between 8 volts and 12 volts will be OK, but if you apply a voltage greater than 13.2 volts, the radio will shutdown and show 'HI-VOLT' on the display. Turn the radio on, flip it over and check that it is on channel 55, with the display flashing.

Step 3

Adjust the VCO's.

The keyways of the VCO adjusters are square drive. This means that if you don't have the proper tool you will have to either file or grind an old allen key into a square profile, or find a jeweller's screwdriver that has a snug fit in the adjusters. Connect the negative probe of your multimeter to the power supply negative. Check the voltage on the test point CP1, indicated on the picture below. If your meter is accurate, it should read 4.96 volts. Turn the RX VCO adjustment clockwise until the voltage starts to drop. The adjuster will require firm pressure to turn, and when you reach the end of it's adjustment range, you will feel a definite stop. Don't force the adjuster beyond this stop. Adjust until the voltage is 4.86 volts. If you cannot  get the voltage to drop to 4.86 volts before you run out of adjustment, you will have to change down in channels until you get a frequency that is in the lock range. Use the keypad membrane from the housing to change the channel. Once you have set the VCO, flip the radio over and check the display. It should now be steady. That's the RX done.

Press the PTT button. The TX indicator should come on and then go off very quickly, and the display should flash until the PTT is released. Turn the TX VCO adjustment clockwise while repeatedly pressing the PTT button until the TX indicator stays on. Adjust until the voltage on test point CP1 is 4.86 volts. WARNING ! while the circuit board is out of the chassis, the final output transistor does not have a heatsink. If you transmit for too long, the final output transistor will overheat and be destroyed. While performing the TX VCO adjustment, keep a finger on the heatsink block and stop transmitting when it gets too hot to touch. You will probably need three hands or an assistant to do this part. I usually rest my left pinkie knuckle on the heatsink block, while operating the PTT with my left pinkie and holding the multimeter probe with my left forefinger and thumb, and adjusting the VCO with my right hand. An external speaker/mic would probably be easier. Once you have the TX VCO set to the same voltage as the RX VCO, that's it.

Check that any low frequency channels you may have programmed into the radio still work. If not, you may have to settle for a lower top end.

Step 3

Reassemble the radio.

Reassemble the radio in the reverse order of the disassembly procedure. Replace any heatsink compound you may have rubbed off the heatsink block. Before you put the chassis back in the housing, be sure to double-check that you have -

resoldered the antenna socket.

resoldered the positive battery terminal.

resoldered the shield.

plugged the speaker back in.

When you put the chassis back into the housing, the rubber seal will have a tendency to roll out of the seal recess. Applying a thin layer of grease to the seal will make it slide back in easier, and it will improve the seal as well. You need not put loctite on the antenna nut.