Maxon PM100 transceiver

This transceiver has been available for amateur use for some time at various prices, and is used as both a normal transceiver for individual use, as well as an internet / radio link or repeater being programmable between 430, and 440 Mhz. The maximum power output is said to be 25W, but in practice it may be lower, and in any event can be turned down as you will see in the following. The deviation may be set to 2.5Khz, but that is also adjustable. The receiver is dual conversion with its first IF being 54.5Mhz, and second IF 455Khz, and it's sensitivity....wait for it....is said to be better than 1uVolt emf for 20dB SINAD. That may not be the best thing since sliced bread, but in practice the receiver seems reasonable, and has proved to be pretty impervious to spurious responses, or de-sensing. Incidentally the two buttons are used for selective call, but only active if that module is fitted, and as far as I am aware none of the ones available has that installed.

The tone squelch module is named FTS-14, and the 5 tone selcall unit is named F5D-9

It has 8 channels which can be programmed using our programmer for simplex, or duplex use. ie each channel can have both tx and rx frequencies the same, or set up with a split ( say 1.6Mhz ). If the unit is fitted with a CTCSS module ( which the test unit was not ) then you can also program any freq for CTCSS, always assuming that you know the codes used for the various ctcss tones, as that is what this unit expects rather than tone frequencies. The programming mode also allows the user to select a tx time out period if required. A screen view of the programmer with typical frequencies programmed is shown below. Remember DO NOT put anything in the CTCSS boxes unless your radio has the CTCSS unit fitted.

I also now have a VHF version SMC2516L8 which is the same appearance as the UHF version. It has the ctcss module fitted, and in case you find difficulty in getting the CTCSS to work on receive, I will point you to pin 4 of the mic plug which is a "mic hanger", This pin being switched to ground ( pin 7) via the mic hanging unit. When switched to gnd it also inhibits ptt, so you need to short this to ground, and then break that connection as you wish to transmit. In use as an rf gateway this can be done with the ptt relay in most cases. So far I haven't found an easier method of enabling ctcss on rx without the use of pin 4.

The programming software is free for amateur radio use, and common to both the VHF and UHF versions. It is not owned by us, and we only supply it as a free item to purchasers of our programmer. It runs in a dos window from windows, and can be used as is on com1, or run with a batch file selecting com2 etc.

The radio is normally off, and the software run. If you have existing frequencies which you want to use you can read from the radio by hitting the READ button, and then switching the radio on. You will see the percentage read running in the screen window. You need to save that as a PRG file under whatever name you decide. Then you can either edit existing entries, or insert new ones, and then save that file. Once having saved the file you can then program the radio. To program the radio, select PROGRAM, and it will tell you to put the radio into program mode. To do this you switch the radio off, and turning it over you will see a small hole. Insert a very small screwdriver into the hole and press and hold the switch pressed whilst switching the radio on. Then hit OK, and the data you have modified will be written into the radio. Switch the unit off, and remove the programming lead plugged into the mic socket. Switch back on, and you will have all those new frequencies available. You can run the software again if you wish, just to confirm they are actually there :-). By carrying out another READ command.

You can see the connections made to the right hand side of the front module. These are tx audio, ptt, and rx audio from top to bottom.

This transceiver has been available for amateur use for some time at various prices, and is used as both a normal transceiver for individual use, as well as an internet / radio link or repeater. It's 4 channels being programmable between 430, and 440 Mhz. The programming software handles both this, and the PM100V2 high band vhf version, which are identical in appearance. The circuitry is pretty well identical also, except the UHF version uses an rf module rather than a driver and p/a transistors.

I required cor from the receiver in order to use it with my Echolink rf gateway, and required it to be available when ctcss was programmed for a frequency, and not when a carrier came on. Although I needed the option for both, and this mod gives me just that.

The mod consists of taking the audio output muting signal which is normally sat at 4.18V with no carrier being received, and dropping to zero volts on receipt of a carrier. To isolate the radio circuitry from any loading problems on that line, or faults which may develop external to the radio I decided to use a cmos inverter and diode to give a 5V cor output when a suitable carrier was received. I say suitable because if the radio is programmed for ctcss receive, then that carrier would have to have ctcss encoded to enable the cor output. But on a frequency that wasn't programmed for ctcss, it would still give a cor output on a non encoded carrier.

The point where the muting signal is taken off is link 12, or LK12 on the pcb. This is located just behind the 14 way connector CON404. With the radio front towards you, looking down on the connector which is near the front of the pcb on the left, LK12 is the second link in from the right. In fact it commons with LK13 which is on its left. So the take off could be from either. If you monitor this point with a meter, with squelch closed, and opened by a carrier, you will see the voltage switching.

Feed that into the input gate(s) of a cmos inverter which can be either mounted on a small pcb, or glued dead bug style on top of CON401, which is the connector feeding the front panel. I opted for dead bug style, and as it turned out it looks pretty neat, as the connecting leads are quite short. The supply volts +5 rail can be taken from the 8 way connector to the right of CON401, which is unused. Pin 1 is designated by the use of a square pad, which is ground, and pin 4 is the +5V rail. So the ic is powered from those, and an output lead taken from the ic via a diode to the output socket ( D Type) on the rear of the radio. I should mention that my UHF version had a 15 way D type fitted with a small harness going to CON404. I removed both, and fitted a 9 way D type instead. On some VHF versions there is a blanking plate covering a D type cut out, and that plate can be removed, and a D type plug fitted. There are other versions which simply don't have provision for the D Type, and removing the blanking plug simply leaves a hole :-). However it is possible to fit a socket with some good old fabrication, and I have since changed my audio in/out and ptt connections to that socket also.

Incidentally these are the pin connections for the PM100 mic. ENSURE YOU CHECK the orientation with a meter. Quite easy as you will see gnd is on pin 1 and 13.8V is on pin 6.

Pin1= GND. PIn2 = CS. Pin3 = CTCSS Hang SWitch. Pin4 = Mic. Pin5 - Ptt. Pin 6 = 13.8V

Okay, well refer to the drawing, and good luck with the mod. I am always available by email if you feel like asking any questions, just my call sign at blueyonder.co.uk . During the course of this mod, I also took off the received audio from the top of the volume control via a 1mfd capacitor, and also brought out the mic input, and ptt to the same 9 way connector.

So, remove the four screws from the base of the radio. Lift the lid, and away you go.

Have fun. Now you can use Echolink RX Ctl with carrier detect (COR) instead of VOX, now that has to be 100% better :-).