Maxon PM100 Modifications

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.

TX audio input can be connected to the interconnecting plug between the front panel and radio, or inside the head mic socket. The RX audio can be taken from the top (hot) end of the volume control likewise within the head unit. The ptt can be taken off the ptt connection in the head also. All three of those connections could come out on the right hand side of the head unit as there is enough room for two 3.5mm jack sockets, and an RCA phono which is exactly what I did.

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 :-).