The IC-705 MARS mod, step by step

What the MARS mod is

Icom radios leave the factory limited to the amateur bands of each country. Which version a radio is gets decided, among other things, by a matrix of SMD diodes on the main board: depending on which diodes are fitted, the radio behaves as one version or another. Remove two of them and the IC-705 switches to the extended transmit configuration, the one used in the United States for the MARS service, hence the name.

Nothing is added and nothing is bridged: you only remove two diodes. The hard part is that they are tiny and surrounded by twelve others that must stay where they are.

The IC-705 opened, seen from above: the green main board inside the black case, with the Texas Instruments WiFi and Bluetooth module at the top left, the Renesas processor in the middle, several metal shields and the flat cable to the front panel board on the right. — The IC-705 opened. The diodes are not on this side: you have to get to the other side of the main board.

Two warnings first

You lose the warranty. Removing parts from the board voids Icom's warranty.
The radio being able to transmit does not mean you may. The mod's frequency table includes the airband (118–137 MHz), marine and professional services. Transmitting there on an amateur licence is illegal and can interfere with safety communications, aviation above all. Outside your bands the radio is for listening, unless you hold the matching authorisation.

If you'd rather not, a radio service shop will do it in no time, and your local club surely has a member with a hot-air station who has already done it. I wanted to do it myself, and it worked first time.

What you need

About €83 in all, and you keep the kit for other jobs.

The kit I used
Tool	What for	What to look for
Hot-air station	Melting the solder on both pads at once	Adjustable airflow and temperature, small nozzles. I used a Yihua 959D, which comes with tweezers and a mat; an 858D will do too.
Gel or paste flux	Even melting and a clean lift	Rosin or no-clean, for electronics. A syringe is handier. Never plumbing flux, which is acidic.
Fine anti-static tweezers	Lifting the diode	ESD-15 type, straight or curved.
Kapton tape	Shielding everything you don't touch	Genuine polyimide, 10 mm wide.
Magnifier	Seeing what you do	A headband one with a light and 3× to 15× lenses. With parts this small, a must.
Isopropyl alcohol and a brush	Cleaning the flux afterwards	An old soft toothbrush works.
A sharp craft knife	Cutting windows in the Kapton	New blade.

With the magnifier, work with the lowest lens (3× or 4×): it leaves some 15–25 cm to the board, plenty of room for the gun and the tweezers. The 10× and 15× lenses are for inspection with the station off. And keep the gun away from the headband: the lenses are usually acrylic and warp.

If you have never done SMD work, practise first on any old board, a router or a broken toy, until lifting a part takes you under 30 seconds. Ten minutes of practice saves you a scare on the 705.

Where the diodes are

On the other side of the main board, next to the DSP, a Texas Instruments TMS320C55 you'll spot at once. To its right is a matrix of 3 rows by 8 columns of diode positions, with fourteen diodes fitted and the rest empty pads.

The board side with the DSP: on the left the square black chip marked TMS320C55, and to its right, inside a red box, the matrix of tiny diodes in three rows, with a large metal shield on the right. — The landmark to get your bearings: the DSP on the left, its text the right way up. The matrix is inside the red box.

With the board that way round, the two diodes to remove are:

Diode 1: bottom row, first column from the left. It only has a neighbour on its right, so it's the easy one: start with it.
Diode 2: middle row, second column from the right. It has diodes right next to it on both sides, so it's the delicate one.

The diode matrix close up, before the mod: three rows of eight positions. A red box numbered 1 marks the diode in the bottom row, first column; another numbered 2, the diode in the middle row, second column from the right. — The matrix before starting, with the two diodes marked.

Don't let the photo fool you: the whole matrix is under 2 × 2 cm and the diodes are packed tight, almost touching each other. To the naked eye you can barely tell them apart.

The matrix map, with the DSP on the left: in grey the twelve diodes that stay, in red the two that go. When done, the middle row has diodes in columns 3, 5, 6 and 8, and the bottom row in 2, 3, 5 and 8.

Beware of reference photos

I followed a photo that goes around the internet, and nearly picked the wrong diodes. It had two traps:

It was rotated 180°. The DSP showed on the right of the matrix, its text upside down.
It was taken after the mod. The red boxes marked empty pads, where the diodes had already gone, not the diodes to remove.

The safe way to check is to rotate the reference until the DSP sits as on your board and compare the matrix diode by diode. Both must match everywhere except exactly at the two gaps. Take a photo of your matrix before touching anything, too: you'll want it for the comparison at the end.

Shielding the matrix with Kapton

Hot air heats everything around it. Kapton is the difference between removing two diodes and moving five.

In a matrix this tight, a few bits of tape around the target aren't enough. What works is covering the whole matrix with one strip and cutting a window with the knife the exact size of the diode and its two pads. The neighbours stay covered.

Cut each diode's window only when you go for it.
The tape must be well stuck and flat. A wrinkle traps air, and the hot stream lifts it and uncovers the diodes.
Press the window's edges down with a fingernail or the back of the tweezers, especially those touching a neighbouring diode.
Also cover anything a few millimetres away: the DSP, inductors, capacitors and any plastic part or connector.

First attempt at shielding the matrix: a strip of yellow Kapton tape over the matrix, with a large wrinkle and air underneath along the top. — First attempt: the wrinkle at the top leaves air under the tape, right over the matrix. Not like this.

Second attempt, with the board upright: two flat strips of Kapton, one over the DSP and one over the diode matrix and the parts around it. — Second attempt, board rotated: flat tape over the matrix and another strip over the DSP.

Removing the diodes

Hot-air station settings
Setting	Value
Temperature	About 330 °C
Airflow	The lowest. With more air the diodes fly off, yours and the ones next to it.
Nozzle	The smallest
Distance	1–2 cm, vertical
Time	About 5–15 s until it melts; if it hasn't let go after 30 s, stop

If your station has memories, store those settings in one. And before opening the radio: switch it off, take the battery out and unplug everything.

A small drop of flux on the diode, through the Kapton window.
Heat with the nozzle aimed at the centre of the diode, in very small circles to share the heat between both pads, without drifting towards the neighbours. On diode 1, which is on the edge, tilt the gun slightly outwards, away from the tape.
Watch the solder: when the flux bubbles and the solder turns shiny, it has melted.
Touch the diode with the tweezer tips while still heating. If it's free, it moves by itself; lift it gently. If you have to pull, it hasn't melted: keep heating, don't pull.
If it hasn't let go after 30 seconds, stop, let it cool for a minute and try again. Don't crank up the temperature.
Move the nozzle away and let it cool for a few seconds before going on.

Diode 1 first, then cut the window for diode 2 and repeat. If a neighbour shifts, it's not serious: with flux and a little air it settles back on its pads by itself, pulled by the solder's surface tension. But watch it, because a crooked or lost diode is a bigger problem than the one you started with.

Cleaning and checking

Remove all the Kapton.
Scrub with the brush dipped in isopropyl alcohol until nothing sticky or brown is left around the pads.
Inspect with the strongest lens: no loose solder balls, no bridges between pads, every neighbour in place and straight.
Take a photo and compare it with the one from before, row by row.

The matrix after the mod, in the same orientation as the before photo: box 1 in the bottom row, first column, and box 2 in the middle row, second column from the right, now show only empty pads; those of diode 2 look coppery. The other twelve diodes are still in place. — Done, in the same orientation as the before photo: the two gaps, and the other twelve diodes untouched.

What I found and what it means:

Coppery pads where diode 2 was. Nearly all the solder left with the diode. No problem: those pads are no longer used.
A black stain between the pads. Burnt flux or Kapton adhesive toasted by the heat. It has to go: brush and plenty of alcohol, and if it resists, a wooden toothpick. Never anything metal.
Yellowish drops. Unburnt flux. It comes off easily with alcohol.

For the inspection photos, hold the phone square over the board in good light. If it won't focus up close, back off a little and use the 2× zoom. A blurry photo with alcohol glare won't show you a bridge.

Closing up and testing

Wait until the alcohol has fully evaporated.
Refit the connectors and flat cables carefully, and don't overtighten the screws.
Switch it on and check that it receives normally.
If the new ranges don't show up, do an All Reset from the settings menu. It clears memories and settings, so save them to the SD card first.
Test transmit on your own bands, at low power and into a dummy load.

The result

The radio receives and transmits normally, and now it also transmits on 27 MHz, the band it lacked. Spot on.

All in all, a quiet morning: daylight, some practice on an old board first and no hurry at all. If I could do it first time, never having touched an SMD part, so can you.