Using the Motorola StarPlex Channel Modem on 600 Meters

by W5JGV

September 4, 2010

The Motorola StarPlex Modem

The Motorola StarPlex Channel Modem, MLN6287A

Click HERE to download the manual for the modem

Click HERE to download the schematic diagram and parts layout for the modem.

The technical data is somewhat incomplete, so if you come across any additional data about these modems, please let me know so that I can add it to this web page. Thanks!


The Motorola StarPlex Channel Modem was designed to be bused in wired or radio carrier telephony systems. Each modem contains a transmitter and receiver. The transmitter section accepts an audio signal in the range of 300 - 3600 Hz. The transmitter then generates either a single sideband suppressed carrier signal on one of 614 discrete channel frequencies. The channel baseband carrier frequency is set by the use of a programming plug, which may have either a 10 position DIP switch on a plug-in SIP style card, (MLN6406A) or a "scratch-off" ceramic SIP card, (MLN6309A). Setting the appropriate DIP switches or scratching out conductive areas on the ceramic card will send -15 volts to the pins of the divider stages in the frequency synthesizer chain in the modem to select the carrier frequency. Each modem may be set to any one of the 614 channel frequencies by the use of the programming plug.

For use on 600 Meters, we will be concerned with two channel frequencies, 492 and 508 KHz. In this frequency band, the modem generates an upper sideband suppressed carrier output. The ARRL Experimental Project on 600 Meters has assigned QRSS and CW operations in the low end of the 495 - 510 KHz band, and PSK, MSK, and CW in the upper part of the band.

Note that the modems I am working with are the Motorola StarPlex series of channel modems, type MLN6287A, and not the StarPoint MLN6625A units. The MLN6625A are more available on eBay and elsewhere. So far I have been unable to locate any information at all about the 6625's, so I do not know if they are usable on 600 Meters. I suspect that they generate carriers in the 5 MHz range, instead of lower baseband carriers as do the 6287's.


Looking at the card edge connector on the modem. Pin 1 is to the right side of the connector, and pin 24 is to the left side.

Pin numbers are shown on the solder side of the circuit board.

In this photo,pin 1 is at the top and pin 24 is at the bottom.

This modem is set up for receive only, so there are no connections to the RF output pins.

The most commonly used pin connections from top to bottom are:

Pin 1 (red) - TX RF Output Hot, Pin 2 (black) - TX RF Output Shield/chassis ground,

Pins 3 & 4 (red) - TX Audio Input, balanced and isolated from ground, 600 Ohm nominal impedance.

Pin 5 (green) -24 Volts DC, Pin 6 (black), -24 Volt DC return/chassis ground, NOTE: this is a POSITIVE GROUND unit.

Pins 21 & 22 (blue) - Receiver Audio Output, balanced and isolated from ground, 600 Ohm nominal impedance. NOTE: RX AF Gain is adjusted by the small pot that is visible through the front panel.

Pin 23 (black) - Receiver RF Input Shield/chassis ground, Pin 24 (blue) - Receiver RF Input Hot.

The modem requires a positive grounded, negative 24 volt power supply. The allowable hum and ripple is 300 millivolts. The tolerance of the supply voltage is + or - 10 percent. The modem draws about 150 milliamperes.

The chassis of the modem is connected to the positive side of the power supply, therefore the positive side of the supply will be grounded.

A suitable power supply may be made from a voltage regulated "wall-wart" power supply. I have found several of these at second hand stores that were previously used with HP printers. Just remember that the modem requires a POSITIVE grounded power source.


If you are lucky, your modem has one of these handy programming plug assemblies already installed. If not, you can make one or just use some thin wires to make the necessary jumpers.

This one is programmed to put the modem on a channel baseband frequency of 508.000 KHz.

This is an example of a "scratch-off" programming plug, MLN6309A.

Note that "switches" 2, 4, 5, 6, and 9 have been scratched open, so those switches are off. This programs the modem to a baseband carrier frequency of 148.000 KHz.

The chart below shows the DIP Switch Settings for Motorola StarPlex Channel Modem when you are using the Motorola Programming Plug MLN6406A (with switches.) If you make your own programming plug using jumper wires, please use the data in the last column to the right of the page to make your programming jumpers.

Note that the minimum audio frequency the modem will accept is 300 Hz, and the maximum audio frequency is 3600 Hz. Frequencies above and below these limits will be blocked by the modem's internal band pass filters. This means that there is a 1300 Hz gap between the top of one channel and the bottom of the next upper channel.

 492 KHz 492.300 KHz 495.600 KHz 2-4-5-7 1-3-6-8-9-10 1-2-5-7-9-11-12
496 KHz 496.300 KHz 499.600 KHz 4-5-7 1-2-6-8-9-10 1-3-7-9-11-12
500 KHz 500.300 KHz 503.600 KHz 1-2-3-5-7 4-6-8-9-10 1-5-9-11-12
504 KHz 504.300 KHz 507.600 KHz 1-3-5-7 2-4-6-8-9-10 1-3-5-7-11-12
508 KHz 508.300 KHz 511.600 KHz 2-3-5-7 1-4-6-8-9-10 1-2-4-7-9-11-12
518 KHz 518.300 KHz 521.600 KHz 1-3-5-7-9 2-4-6-8-10 1-3-4-7-9-11
148 KHz 148.300 KHz 151.600 KHz 2-4-5-6-7-9 1-3-9-10 1-4-11-11

Please Right-click on the picture above to save it as a high resolution GIF file.

The diagram above is the wiring diagram for the Motorola MLN6406A Programming Plug using the 10 position DIP switch. If you don't have one of these handy programming units, you may make a simple wire jumper setup as shown in the next diagram. 24 or 26 AWG wire should fit into the header socket nicely. This will allow you to set the channel frequency of the modem.

Please note that the header PIN numbers DO NOT correspond to the SWITCH numbers!

Please Right-click on the picture above to save it as a high resolution GIF file.

This diagram shows the jumper arrangement you will need to program the modem to the two most common parts of the 600 Meter band for the ARRL Experimental Project. Note that there is only one switch/wire jumper difference between the two frequencies. You can use a SPDT switch to rapidly QSY between the two channels if you wish, however, besides switching these two jumper wires, you will also have to reset the RANGE SWITCH on the modem. (See next photo.) Since the jumper wires carry 15 volts DC, no shielded wire is necessary. The required switch may be mounted on the front panel of the modem in place of the ALARM LED, since the LED is not required for TX or RX operation.

Please note that the header PIN numbers DO NOT correspond to the SWITCH numbers!

This photo shows the black plastic programing plug socket. It is seen near the top left of the photo. Midway down the picture, below the programming plug socket and the glass fuse, you will see a small, 2 position DIP switch, that is blue in color. Motorola calls this switch the RANGE SWITCH.

This switch is used to select several varicap diodes which act as tuning capacitors in the low pass filter loop in the frequency synthesizer. Depending on the baseband channel frequency you have selected, the settings of this switch will need to be changed. If you set the switch to the wrong setting, you will get little or no RF output from the modem.

The DIP switch settings I have found that work on the modems I have are: for 492 KHz QRSS, switches 1 and 2 ON. For 508 KHz CW/PSK, switches 1 OFF, and 2 ON. Try different switch combinations if necessary, and use the one that gives the highest RF output.


Here is an example of the first-time setup of the modem for operation on 495.028 and 508.800 KHz. These are the assigned frequencies for WD2XSH/6.

1 - Program the modem for the correct baseband channel frequency by using the programming plug or jumper wires and set the RANGE switch to the correct setting.

For the PSK/CW channel frequency of 508.800, you will need switches 2, 3, 5 & 7 set OFF, and all the rest of the switches set ON. This will set the modem to a channel carrier frequency of 508.000 KHz.

2 - Connect a source of DC power to the modem, -24 volts DC to pin 5 and ground to pin 6.

3 - Take the RF signal from the modem on pin 1 (hot) and pin 1 (shield ground.)

4 - With no audio applied to the input of the modem, monitor the RF output from the modem with an oscilloscope.

5 - Set the XMIT LVL pot full clockwise (maximum gain.)

6 -Adjust the CARR LVL pot for minimum carrier level as seen on the oscilloscope.

7 - Connect an audio signal to the modem audio input on pins 3 and 4.

To generate a CW signal on 508.800 KHz, you will need to send the modem an audio tone of 800 Hz. This will cause the modem to generate an USB carrier on 508.800 KHz. To generate the CW signal, all you do is key the audio signal going to the input of the modem. For PSK, you set the audio output of the PSK program to frequency of 800 Hz and send that to the modem.

For the QRSS channel frequency of 495.028, you will need switches 2, 4, 5 & 7 set OFF, and all the rest of the switches set ON. This will set the modem to a channel carrier frequency of 492.000 KHz. To generate the QRSS signal on 495.028, you will need to send the modem an audio tone of 3028 Hz, and then key the audio signal going to the input of the modem. This will cause the modem to generate an USB signal on 495.028 KHz.

8 - Starting at zero level, slowly increase the audio signal until the RF output as seen on the oscilloscope stops increasing. That is the maximum peak audio level you can send to the modem without distorting the RF output. This is important for PSK operation, as excessive audio input will severely "flat-top" the peaks of the PSK signal.

Things to be aware of:

The impedance of the RF output of the modem transmitter is high impedance. Use a short length of low capacity coaxial cable to connect the modem to your amplifier. The longer the interconnecting cable, the lower the available signal at the amplifier end of the cable. You may need to use a cathode or emitter follower amplifier to obtain a sufficiently low impedance drive source for your amplifier.

Note that the maximum usable gain setting of the XMIT LVL pot will depend on the load impedance seen by the modem RF output terminals. This, in turn, will affect the maximum audio signal you can send to the modem without distortion. In other words, the load impedance, XMIT LVL and audio input level all interact with each other. I suggest keeping your oscilloscope connected to the RF output until you have everything working smoothly. This is particularly important for any linear mode, such as PSK or SSB.

By using various settings of the channel programming switches, you can cover the entire band from 495 to 510 KHz - almost. One "Gotcha!!" is that there is a guard band between the channels. That means there is a series of gaps across the band where the modem will not output a carrier due to the internal filters. It may be possible to be remove them, but I have not looked at that yet.

For example, with the modem programmed for 492.000 KHz, you can use an audio tone from 300 to 3600 Hz to generate an RF output from 492.300 to 495.600. Changing the programming switches to a carrier frequency of 496.000 KHz (the next higher channel) results in an RF output of 496.300 to 499.600. In this case, the gap runs from 495.600 to 496.300.

It is possible to increase the audio signal into the modem above 3600 Hz and below 300 Hz to force the modem to output a signal in the guard band, but if you increase the audio drive too much, the RF output will collapse as the internal amplifiers in the modem clip and distort. Be sure to look at the RF output with a scope as you tweak the audio drive and the output amplifier gain adjustment pot.

Be aware that the modem "leaks" a little bit of carrier signal even with no input audio signal. The carrier suppression is pretty good, -55 dB or more, but you will still hear it on your RX when your key is up if the power amp is on and running in linear mode. This is usually not a problem unless you are trying to listen for a signal close in frequency to your carrier. In that case, either turn off the amplifier during RX or bias the amp so that it is just beyond plate current cutoff with no drive. OR power off the modem during RX. That will not work for break-in keying though since the modem takes a couple of seconds to power back up.


Although the setup for using these modems to transmit is pretty straightforward nothing was said so far about the possibility of using these modems to receive with.

Because my earlier use of these modems several years ago was to generate a SSB signal on 166.5 KHz for my Part 5 beacon transmitter (WC2XSR/13,) I had never planned on using the modem as a receiver. When I shifted the modems to the 600 Meter band for QRSS and CW/PSK31 operation, the same mind-set took hold, and I completely ignored the receive side of the modem. I received an email from Pat, W5THT - WD2XSH/6, about receivers for 600 Meters, he queried me about the possibility of using the receive side of the modems on 600 meters. That thought intrigued me, and I decided to run some tests on the modem receiver to see just how good or bad it actually is.

I referred to the modem test procedure, and determined that the RF carrier sensitivity was supposed to be -55 dBm @ 75 Ohms. That is rather poor for the front end of a receiver. Nevertheless, that spec was for the receiver in normal operation, not weak signal conditions. Testing was in order to determine whether or not the receiver was really that deaf.

My test setup consisted of my AIM-4170 acting as a signal generator. The signal went through an 80 dB attenuator, which is switchable in 1 dB steps. From the attenuator, the signal was split and went to one of my HP-3586 SLM's, which would act as the reference receiver. The signal also went to the input of the modem receiver. The output of the HP-3586 and the modem receiver each went to separate ARGO displays so I could compare signals. The bandwidth of the HP-3586 was set for 3100 Hz, which closely matched the bandwidth of the modem's receiver.

The Minimum Detectable Signals (MDS) for faint but solid ARGO traces were:

HP-3586 -116 dBm

Modem -108 dBm

I was surprised, to say the least. That MDS was down below the noise floor on 600 Meters just about any time of year. It looks like the modem receiver should be usable on 600 meters.

Some Things To Be Aware Of if you use the modem as a receiver:

- The modem has no front-end static or overload protection. It would be a very good idea to use some back-to-back protection diodes across the RF input.

- As with most LF and VLF receivers, common mode noise can be a problem. Using a small isolation transformer to break the shield of the coax from the antenna will help greatly to eliminate the noise. The transformer will also protect the front-end of the receiver from static discharges. A suitable isolation transformer may be made by scavenging a dual winding ferrite core common mode RF mains choke from an old computer power supply.

- The RF front-end of the modem receiver has no RF tuning or band pass filter. It pretty much accepts stuff from DC to light. There is an internal band pass filter, but that is after the first mixer in the receiver. It would be a very good idea to add a preselector in front of the receiver. That being said, the unwanted spur and image problem seems to be almost non-existent. Signals about 400 KHz away from the desired frequency had to be increased up to almost -30 dBm in strength, or about 78 dB stronger than the desired signal before spurs started showing up in the ARGO display, and that was with no front-end tuning or filters.

- The front-end of the modem receiver is speced at 75 Ohms. When it was in use, the 75-Ohm termination was really in the backplane of the card cage that the modem was plugged into. The receiver input is quite different. According to my AIM-4170 the impedance of the receiver input looks like 3000 Ohms. Basically, an open circuit. This means that a front-end RF tuner with an impedance step-up to the receiver input would be a suitable addition for both sensitivity and rejection of unwanted signals. That very high front-end impedance also makes the unit very sensitive to static and lightning impulses. Use some sort of lightning suppression on the input, as noted earlier.

- The audio output from the receiver is pretty low at the MDS point. Even with a signal level set to 30 dB higher than the MDS level, the audio signal from the receiver barely tickled the green level indicator bar in ARGO, even though I had the Line Input gain slider in the Volume Control settings box on my computer set for maximum gain. A small, low noise, audio preamplifier might give some weak signal improvement, depending on your computer sound card.

- Remember that this receiver will generate an output audio signal at a frequency that is the difference between its base carrier frequency and the incoming RF signal. In other words, you cannot change the pitch of the CW or PSK signal you are listening to. What you hear is what you get. You have to use your brain or a computer program to sort it out.

General Q & A

Q - About the plastic 24 pin connector to plug the board into the rack, did you unsolder it and replace it? If so did you just wire to it or find another plug?

A - On one modem, I found that the motherboard power plug on an old 386 AT motherboard was exactly the right pin size and spacing, so I salvaged that and used it. For the other one, I simply inserted some short lengths of 14 gauge (I think) copper wire into the connector holes and used some hot melt glue and a strip of plastic to make a quick-and-dirty connector. You only need a few pins anyway. -24 volts, ground, RF out and chassis ground, and two audio input wires.

Q - What about the audio, can I ground one side of the audio going to the modem? What about the audio output from the modem receiver? Does "4-wire T" mean I have to use balanced 600 ohms only?

A - Although the audio input is 600 ohms balanced, it is transformer coupled, so either one of the audio input lines can be grounded if you use single ended audio input.

Q - What is E&M Signaling?

A - The E & M signal for on/off keying a relay on each modem to ring a bell or trigger some external device.

Q - Is "VF" voice frequency?

A - Yes.

Q - What do I do with "E" and "M" pins?

A - Generally, just ignore them, since they are only used for signaling between a pair of modems.

A pair of the Motorola StarPlex channel modems installed at WD2XSH/7. They are simply laid flat on a rack mounted shelf. Cardboard is used to insulate the solder side of the modems from the metal shelf. The power supply for the modems is located on the shelf, behind the push-button switchbox. The switchbox is used to select between the two modems for fast QSY between the QRSS and PSK segments of the 600 meter band. Each modem gets its audio feed from its own CD player which loops the audio times for the various modes of operation.

73, Ralph W5JGV


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