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Converting a MaraTrac to a 6-meter Repeater Transmitter By Robert W. Meister WA1MIK

I started this conversion with a stock MaraTrac mobile transceiver, model number T81XTA7TA7BK, which is a 110w 42-50 MHz radio with an advanced control head. It had previously been converted to receive in the 6-meter band. Since this radio was to be used only as a repeater transmitter, it just needed an audio input, a push-to-talk input, power, ground, and of course an RF Output. There was no need for a control head or any of the accessories.

Control Cable Connector and Interface:

I purchased a chopped-off 19-pin control cable connector from a popular auction site and completely stripped it clean. I rewired it according to the table below, using only the pins that were required. Input signals come from the control head and enter the radio chassis. Output signals leave the radio chassis and go to the control head. Refer to the explanation for each pin below this table.

Pin	Signal Name or Use	Where It Goes To and Notes
4	Ignition Control Input	Tie to pin 19 (A+)
8	Data Input	4.7k resistor to pin 10 (+5V)
10	+5V Output	Supply 5V to resistor to pin 10
12	Push-To-Talk Input	PTT Input: Ground to Transmit
13	Power On Input	Power Input: Ground for On
14	Microphone High Input	Audio Input: Has +9VDC on it
15	Volume Control Wiper Input	Tie to pin 17 (Ground)
16	Xmit Light Output	To TX LED
17	A- (Large pin, large wire)	Main Power and Signal Ground
19	A+ (Large pin, large wire)	Main Power Input (+14VDC)

Why did I use these particular pins? Here's a detailed explanation.

• Pin 4 is normally fed from the vehicle via a fused green wire. This allows the radio to operate, or at least receive. It provides power to everything except the power amplifier (PA). It can go directly to the battery or be run through the ignition switch. In a standard installation, a separate fused orange wire provides power to enable transmitting. This wire is often connected to the vehicle's ignition switch so the radio can only transmit when that switch is on, or it can be tied to the same source as the green wire.
• Pin 8 carries serial TTL data from the control head, which is not present, back to the radio chassis. Terminating it with a resistor to +5V eliminates any noise (phantom data signals) from affecting the radio's operation.
• Pin 10 carries +5V from the radio to the control head and is used here only to pull up the resistor going to pin 8.
• Pin 12 is the PTT input. Ground it to make the radio transmit. The pull-down current is just a few milliamperes and the pin rests at +5V when idle.
• Pin 13 is grounded by the control head's power switch to turn the radio on. This activates a relay inside the radio chassis that provides power to everything behind the front panel. I decided to bring this line out of the connector to a small toggle switch so I could turn the radio on and off if necessary.
• Pin 14 is the Microphone Audio input. Remember this has about +9VDC on it so use a 10uF 16V capacitor in series (positive towards the radio) if your repeater controller doesn't already have one. The input impedance is around 600 ohms, the input level required for full deviation is around 350 millivolts, and the input is pre-emphasized and muted.
• Pin 15 is the Volume Control wiper arm that sends receiver audio back to the radio. Since there's no control head attached, this prevents random noises from getting into the audio amplifier, in case you have a speaker attached.
• Pin 16 is pulled to +5V through a 1k resistor when the radio is transmitting. This could go to an LED to indicate that condition. In the clamshell head this drives a transistor that illuminates a light bulb. There's no other indication that the radio is actually transmitting.
• Pin 17 is the main ground connection to the vehicle's chassis and the battery's negative terminal. Connect this pin to the negative power supply terminal. I used two 24-inch lengths of #12 stranded black wire terminated in crimped-on (and soldered) 1/4-inch ring terminals to attach to an Astron RS-35 power supply.
• Pin 19 is the main power connection to the battery's positive terminal. This is used for the high-current PA. Connect this pin to the positive supply terminal. I used two 24-inch lengths of #12 stranded red wire terminated in crimped-on (and soldered) 1/4-inch ring terminals to attach to an Astron RS-35 power supply.

The image below shows the pin numbers on the radio's control head connector. They are also present on both sides of the mating cable connector. Some radios may have a thin piece of foam weather-strip material over the connector. Make sure you assemble the connector properly; all cables exit on the right (pin 17) edge of the housing.

I wired the heavy power lines first, then the remaining pins, using sleeving over the resistor leads and other short un-insulated jumper wires. I used a 3-foot length of Belden 8723 cable (two separate shielded pairs: black and red, green and white), wired per the color code in the table below. I ran the Audio Input signal in its own pair to reduce noise pickup; you can use a single shielded pair or individual wires if that's all you have. If the Audio Input line will be more than a few feet long, use a shielded cable for it. This radio may replace a Vertex VX-6000 mobile that has a DB-25F on the back for interfacing. The existing controller-to-radio cable (with a DB-25M) should plug right into this DB-25F connector. If this radio is to be used in place of a MaxTrac that's fed via the front panel MIC jack, you can omit the DB-25F connector and wires listed below and just drill a hole in the top cover of the MaraTrac and plug the modular cable directly into the modular jack that's used for programming.

M-Trac	Wire	Signal Name	VX6000
Conn.	Color	or Function	DB-25F
Pin 17	Black	Audio Ground	Pin 18
Pin 14	Red	Audio Input	Pin 13
Pin 13	Green	Power On Input	- - - - -
Pin 12	White	PTT Input	Pin 11
Pin 17	Shield	Ground	Pin 25

I assembled and tested (with an ohmmeter) the control head cable and connector first, as this allowed me to work on the radio and power it up without having a control head, microphone, speaker, and control cable clutter up the bench. I didn't wire up a TX LED but I might mount it in the side of the control connector housing. A miniature SPST power on/off toggle switch is temporarily dangling at the end of the cable; I might try to mount it in the side of the connector housing too.

If you don't have a spare control head connector, you can solder wires onto the back of the appropriate connector pins inside the radio, but then you'll need a way to get some wires out of the radio, by drilling a hole or cutting a slot in the top cover. Of course you can also use a clamshell or advanced control head and the appropriate control cable and attach wires to the mike jack pins, possibly through a used mike cable. I just didn't want to make too many physical modifications to the radio itself, and a control head connector was available.

Radio Programming and Configuration:

The radio was powered up by an Astron RS-35M power supply through the new control cable. The programming software had previously been hex-edited to allow 42-54 MHz frequencies. After connecting my computer, I read the original code plug, saved it to disk, and went through all of the Service Menu alignment and adjustment screens in RSS and recorded all of the tuning data. Even though I'll be fully initializing the radio, it's handy to have these values written down. I also noted the model and serial numbers, crystal and tuning numbers, and measured the 9.6V supply with a digital voltmeter, in preparation for logic board initialization.

I removed the existing firmware EPROM and installed one with amateur frequency ranges in it, so the transmitter would control its power and deviation properly over the 42-54 MHz range. I went through the full Logic Board Replacement procedure including selecting the model, frequency range, and serial number, and performed all of the alignment steps with the test equipment connected. The output power was set to 120 watts but I later reduced that to 100 watts. The maximum deviation was set to 5.0 kHz, which resulted in a PL deviation of 0.66 kHz. At some point the deviation may be raised to 7 kHz; the input audio signal level will limit the actual deviation. This will raise the PL deviation but a single resistor can be added to the logic board to reduce the PL deviation to 500-600 Hz. I will eventually add a pot for an external power control. Surprisingly, the radio transmitted fine over the full 42-54 MHz range, probably because the VCO coils had previously been adjusted to cover the 50-54 MHz part of the band. Since there's no loudspeaker or control head, I didn't check the receiver at all.

After alignment, I still had to program the radio. After the logic board was initialized, the software created two modes. I deleted mode 2 and programmed mode 1 with identical transmit and receive parameters of 53.1500 MHz and 67.0 Hz PL; this was also set as the "Home" mode. The timeout timer was set to 000 seconds. I left the radio capable of working with an Advanced (A7) control head. I thought of changing the control head type to "clamshell" but that really wasn't necessary. All other settings retained their default values.

I decided to test the radio's efficiency over its full output power range by reprogramming the output power and measuring the RF output power and DC input power at 14.0 Volts. These measurements were obtained at the test frequency of 47.6000 MHz and the programmed frequency of 53.1500 MHz. The results are summarized in the table and graph below.

RF	48MHz	48MHz	48MHz	53MHz	53MHz	53MHz
Watts	DC A	DC W	Eff%	DC A	DC W	Eff%
25	9.0	126	20	8.0	112	22
36	10.5	147	25	9.5	133	27
47	12.0	168	28	11.0	154	31
62	14.0	196	32	12.0	168	37
78	15.5	217	36	14.0	196	40
94	17.0	238	40	15.0	210	45
115	19.0	266	43	17.0	238	48
140	21.0	294	48	19.0	266	53

At some point I may reprogram the radio with frequencies at 1 MHz intervals and connect a control head to the radio so I can test the transmit bandwidth over the full 42-54 MHz range.

Final Items:

Since the MaraTrac is a mobile radio, it is not rated for 100% transmit duty cycle, so one or more fans will need to be added to cool the PA heat sink and components. Some means of mounting the radio will also be performed to accommodate the fans.

The harmonic filter in the PA was designed to handle the 42-50 MHz band, so tweaking the coils and other components will be needed to improve the efficiency in the 50-54 MHz band. While the PA is out of the radio, the antenna relay will be replaced with a short length of coax going directly to a UHF female-to-female bulkhead adapter.

Adjustment pots may be added to the radio to allow manual control of output power, overall deviation, and PL/DPL deviation. Even though the firmware was replaced and the radio is fully aligned and functional over the entire 42-54 MHz band, a user may want to adjust the above parameters without having to drag a computer, RIB, and cables to the site. Also, the firmware may still try to reduce the output power if the radio gets too hot, or if the cpu thinks the radio has been transmitting so long that it must be getting hot, so a manual power control may still be needed to deal with that situation.

Test Equipment Utilized:

• Astron RS-35M power supply set to 14.0 Volts.
• Bird 8166 150w dummy load.
• Bird 4111 25-175 MHz 150/15w wattmeter.
• Wayne Kerr AMM257 deviation meter.
• Fluke 189 digital multi-meter.
• HP 5385A 1 GHz frequency counter.
• HP Z3801A 10 MHz GPS-disciplined oscillator (frequency counter reference).

Contact Information:

The author can be contacted at: his-callsign [ at ] comcast [ dot ] net.

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This article was conceived February, 2019 and first posted 25-Mar-19.

Article text, photos, artistic layout, and hand-coded HTML © Copyright 2019 by Robert W. Meister WA1MIK.

This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.