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The Icom IC-M12 is a pretty nice Marine VHF Radio, thought to be from the early 80's (if you know a more precise date, please contact me)

It is programmed via a diode matrix board. This used to be a common way to program radios, it can be pretty complicated if you don't understand it.

The 'truth chart' or 'positions table' as they call it, tells you where to put the diodes to get the frequency you require. It can be a nightmare trying to work out how to program the matrix without the truth chart. Thankfully, I have done the hard part and worked it out for you. (aren't I nice? lol)

This page aims to help with understanding the matrix and being able to program your radio.

If you can think of any ways this page might be improved, or anything I have missed, Please contact me via the link above.

This is a layout of the Diode Matrix Programming Board. Note the channels going vertically, and the N numbers running horizontally. So you need to decide which channel you want to program, then follow along the N numbers to program that channel.

The N numbers are positions for diodes, they are N400, N200, N100, N80, N40, N20, N10, N8, N4 and N2.

The Truth Chart tells you where to place each diode to get the frequency you require.

There are also diodes for Duplex (D) and Receive Only (R), you will need to look up if you need the duplex diode on the channel you are programming. Receive Only is mainly for the weather channel and such, where you wouldn't want to 'accidentally' transmit.

Here is the Truth Chart, it tells you where to put diodes to program the frequency you want.

Be very careful noting down where the diodes go! It's very easy to write down an incorrect placement and then spend an hour trying to figure out why you aren't getting the frequency you should be getting. (and yes, I did do that!)

The original diodes are unknown, it does not say what they are in the service manual (possibly 1n914). You can use 1n4148 diodes, which are very cheap and in almost every ones parts box.

Like me, you might find the channel you want isn't listed on the truth chart. For some reason Icom decided to omit several channels from the Truth Chart, channel 62 in this example. You must always program the radio with the RX frequency, then on duplex channels, the duplex diode subtracts 4.600 MHz from the TX frequency.

The base frequency of the matrix board, with no diodes is 160.000 MHz.

Duplex subtracts 4.600 MHz on TX

N400 subtracts : 5.000 MHz
N200 appears unused
N100 adds      : 1.250 MHz
N80  adds      : 1.000 MHz
N40  adds      :  .500 MHz
N20  adds      :  .250 MHz
N10  adds      :  .125 MHz
N8   adds      :  .100 MHz
N4   adds      :  .050 MHz
N2   adds      :  .025 MHz

So, how do you work it out? Don't panic, you don't need a maths degree, though having a calculator handy can help!

The frequency we want (ch62) = 160.725 MHz
The base frequency           = 160.000 MHz
Add a diode to N40 : +  .500 = 160.500 MHz
Add a diode to N10 : +  .125 = 160.625 MHz
Add a diode to N8  : +  .100 = 160.725 MHz on RX
Add a duplex diode : - 4.600 = 156.125 MHz on TX

That's it! You should be able to work out the correct diode placement for any frequency within the band. Just remember you can only use each N number once in your calculations -like on Countdown! :-)

If there is already a diode in one of the places you wish to use, you can just leave it there and don't need to replace it. If there are diodes in places not in your calculations, they must be removed.

Do not mess around on the Marine Band!

I'm all for messing around and having fun on the CB radio band and even maybe the PMR446 band to some extent, but you must NEVER mess around on the marine band. The Coastguard have to THOROUGHLY investigate every call they receive and that could take time away from a real emergency... Imagine how you would feel if you pranked the coastguard, then on the news, you heard that people had died at sea. The coastguard couldn't get to them because they were busy investigating your prank.

Remember, they can track where you are transmitting from. They have the technology to locate boats via their VHF radio signal and that technology doesn't only work on the sea, it works on land too, so they will find you!

Can I program other frequencies into the IC-M12?

Technically no, but it will be possible to program frequencies just outside of the marine band (out of band). Transmitting out of band is illegal and you will be in violation of your marine radio licence. (or any other radio licence you may hold.)