Here's a photo of the inside top of the low-band radio with its shields in place. The UHF radio is similar:

Inside the top we removed a shield over the entire cast-aluminum RF cavity, which exposed a very standard-looking MaxTrac UHF RF board and the VCO shield, which was also easily removed. The usual crystal label and tuning label were present. The RF board's TX output feeds another board that contains the power control circuitry and some filters; this board feeds the input of the PA; just follow the TX coax cable. The RX coax goes directly to the T/R relay. No surprises here; no solid-state switching (unlike a MaxTrac or Spectra). There was also a shield over the entire PA, which was removed for this photo:

Here's a similar photo of the low-band radio with its RF board and PA shields removed:

At the front of the radio (left side of the photo) you find the regular Mitrek T/R relay, complete with an SO-239 antenna connector (these radios still use an SO-239 antenna jack for all models), a black RJ45 jack for programming (barely visible, next to the top cover latch hole) which can use a standard two-wire MaxTrac RIB cable (see note below), and the control head / cable interface. The power amplifier (PA) with a very large heat sink is found at the rear of the chassis (right side of the photo). There's another board between the RF board and the PA (the manual calls this the "exciter" that has the power control circuitry, some band-pass filtering, and an intermediate power amplifier (IPA) on it that produces about 2.5 watts to drive the PA.

NOTE: The RJ45 programming jack is wired differently than what you'd find on a MaxTrac or Radius radio. The programming (7) and ground (4) pins are the same, but +14V is present on another pin (1). Make sure the programming cable you use only has pins 4 and 7 connected. Here's the signal configuration for both the MaxTrac front panel MIC jack and the MaraTrac programming jack:

MaxTrac	Pin #	MaraTrac
Spare	1	SW B+
Spare	2	Spare
Hook	3	Emergency
Ground	4	Ground
MIC Audio	5	MIC Audio
PTT	6	PTT
Programming	7	Programming
Headset Audio	8	Headset Audio

Four T15 flat-head screws holding the bottom cover to the radio chassis were removed, allowing us to pry the cover off (it is well gasketed). Underneath was a MaxTrac-style logic board, complete with the shield over the microprocessor area. This unit had a 5-pin (accessory connector) board. There was nothing plugged into the speaker jack. A few wires were attached to a connector that fit over the 14-pin plug going through the chassis to the RF board. An L-shaped audio and interface board fits into the space around the bottom of the unit; the logic board's 12 and 13 pin connectors were attached to that. Those big silver units just next to the PA are filter shields.

Control Cable:

The radio-end of the control cable uses the same connector that was first used on the early 1960s GGV radio, then the Motrac, Motran, and Mocom-70: a big plastic housing surrounding a captive threaded locking screw, three huge power pins (the heritage of the 6V/12V conversion era), and a bunch of smaller pins in two rows around the outer edge. This cable has a short, black, heavy-gauge, negative power lead and a long, fused, red, heavy-gauge, positive power lead coming out of it, along with the control head cable; standard configuration for Mitrek and earlier radios. The normal configuration of the radio has the large red wire only used to power the transmitter RF PA. These radios require 20-22 amps at 14VDC. An Astron RS35 will more than adequately power any MaraTrac radio.

The control head end of the cable was terminated in what seems to be the same three-row connector as on some remote-mount high-power Spectra radios. Also coming out of this end were the usual orange and green ignition wires plus a Spectra-style two-pin Molex speaker connector. We got the radio to receive, for the photos below, by connecting the orange and green ignition wires to the positive terminal of a small power supply, and the big black wire to the negative terminal of the same power supply.

Control Head and Accessories:

The speaker is the same as would be used on a Spectra. The microphone plugged into the back of the head, just like what would be done on a Spectra A9 head, and it used the same 6-pin connector. A VIP port plug was installed in this unit. Along the bottom of the head was a slide switch that turned the radio on and off. Here's a view of the rear of the control head plus the head end of the control cable:

This radio had the advanced control head that resembles the Syntor or Spectra A9 head without the 12 DTMF buttons. The two-digit display is the same green color like on MaxTracs, but all the buttons operate like they would on a Spectra A9 head. Pressing the VOL up/down rocker adjusts the volume, but there's no indication of the position of it, like there is on a Spectra. There are several LED indicators and the two-digit display for channels 1-99. The buttons are lit from the back, and the intensity of the display, including the backlighting of the buttons, can be changed with the DIM button, again similar to a Spectra. Here's a photo of the head (model HCN1089A) and accessories (a normal microphone is shown; the HMN1022 is a DTMF microphone for the MaraTrac):

Some (later-model) low-band radios have a remotely adjustable squelch circuit. The squelch knob is seen here along the top of this control head (model HCN1090A):

I've seen other MaraTracs with a "clam-shell" style control head (model HCN4034B); it has a rotary power / volume knob, a rotary channel selector (1-8), and one more rotary switch that can handle one option (such as scan on/off or zone). A MON switch is found along the front edge of the head, and a Talk-Around switch is an option. I've also seen a Spectra-style HHCH interfaced to one of these, designated as an A5 model number. The programming software identifies four control head types: a Spectra 99-channel A9-style with scan, a Spectra 99-channel HHCH-style without scan, an 8-channel clam-shell style with scan, and an 8-channel clam-shell style without scan. Any of these can be found on any radio. All radios are capable of 99-channel operation with a non-clam-shell style control head.

Powering It Up:

We only had to connect the orange and green wires plus the big black power wire to a power supply to get it to light up. We never had to use the big red power wire, which only feeds the power amplifier. The green wire powers the receiver; the orange wire enables the transmitter (which allows a mobile installation to use the ignition key as a "transmit enable" switch simply by connecting the orange wire to a switched circuit). Here's a photo of the advanced (A7) control head. The radio was actually receiving a signal at the time I snapped the photo. I had already pressed the MPL and MON buttons just to get something to light up.

On this particular display, there are buttons for the following functions:

• Dir - activates the talk-around or Direct transmit frequency mode
• MPL - activates the Multiple PL choices
• Scan - activates channel scanning
• Mon - puts the radio into carrier squelch mode to monitor the channel
• H/L - activates the Horn, Lights, or both for attention signals
• Rcl - Recalls a previously deleted scanned channel
• Home - rapidly selects the home channel, specified in RSS
• Sel - selects channels to be deleted from the scan list
• Vol Up/Dn - adjusts the volume
• Mode Up/Dn - changes the channel (or mode)
• Dim - dims the display LEDs

There are also LED indicators for the following conditions: MPL, Non-Pri, Pri, Mon, Busy, Xmit, Option, Dir, Scan, VIP. The Xmit and Busy LEDs are along the top of the control head, while the rest are below their respective front panel buttons or next to the legend on the display area.

Out-Of-Band Programming:

The MaraTrac uses RVN-4023/4024 RSS. The M400 uses HVN-9774 RSS. Both of these are, of course, different from what the MaxTrac and Radius use. On the version of software I used, you can not use the SHIFT-NUM method to enter out-of-band frequencies; you need to modify the programming software with a hex editor. The MARATRAC.EXE file in the latest release of the software can be hex-edited to allow entry of amateur frequencies. The values are all 16-bit decimal numbers in 100 kHz increments (i.e. 440.0 MHz is entered as 4400, then converted to hex: 1130, then the bytes are flipped: 3011). The following table, provided by Bruce KC7GR, shows the hex locations, the original values (hex and decimal), and the new values (hex and decimal) that need to be modified. The hex values are already flipped, ready to enter into the file. Remember that there's an implied decimal point before the last decimal digit. Also note that not all values are on word boundaries. As always, before modifying any file, make a backup of the original.

The first group of entries extends the 29.7-36 MHz range down to 28.1 MHz. The second group of entries extends the 42-50 MHz range up to 54.0 MHz. The third group of entries extends the 450-470 MHz range down to 440.0 MHz.

Hex Addr	Original		New
	Hex	Dec	Hex	Dec
53EC8	2901	297	1901	281
53F09	2901	297	1901	281
567E8	2901	297	1901	281
56829	2901	297	1901	281
42C27	F401	500	1C02	540
42C7A	F401	500	1C02	540
53EF6	F401	500	1C02	540
56816	F401	500	1C02	540
53F4C	9411	4500	3011	4400
5686C	9411	4500	3011	4400
56FDC	9411	4500	3011	4400

Converting a 42-50 MHz radio to 46-54 MHz operation:

Hex-edit the programming software as detailed above, so you can enter frequencies in the 50-54 MHz range.

See the MaxTrac 46-54 MHz conversion article for more details on the mods performed to the RF board.

• The two VCO coils on the RF board can be adjusted for a Steering Line (SL) voltage of about 7.4VDC at 54 MHz. They end up with about two turns of the ferrite core exposed.
  
  
• The receiver front-end coils get eight short brass screws inserted into them to increase the sensitivity in the 50-54 MHz range. I found that there's less clearance above the screw heads since the RF board shield mounts closer to the coils, so cut the screws down to 1/4 inch long. (An alternative is to use stainless-steel #6-32 x 1/4 inch set-screws and insert them flush with the top of the coil form.) See the photo below of the modified RF board without its shields.

Nothing else in the radio needs to be modified. The radio's output power (Watts) and receiver sensitivity (dBm) are shown below. Originally, the radio didn't transmit or receive above 50 MHz originally because VCO didn't lock. The output power was also turned down to 100 watts for these tests; when first received it was making in excess of 150 watts on 45.4 MHz.

Freq MHz	Before		After
	W	dBm	W	dBm
45.4	100	-117	100	-108
46	100	-117	100	-111
48	90	-117	90	-115
50	80	-117	80	-116
52	00	---	92	-116
54	00	---	94	-115

Miscellaneous Info:

The radio supports talk-around (or direct) by causing the radio to transmit on the receive frequency, if the control head supports it. Not all do.

According to the sales brochure, the low-band radios support a squelch pot on the control head. I've been told that some older low-band radios do not have an external squelch control or the ability to utilize it. The low-band radio loaned to me did have the remote squelch pot, but a few circuit boards inside the radio had "SP01" after the part number. Also, the low-band MaraTracs (and MaxTracs) have a noise-blanker in the receiver, commonly called an "extender" in Motorola lingo.

Some MaraTracs can support an external Siren/PA unit, connected via the VIP jack on the rear of the control head.

The radio seems to be a marriage of several technologies: Mocom70 / Motran / Motrac / Mitrek (control cable connector / power wiring / antenna relay), MaxTrac (RF and logic boards), and Spectra / Syntor (control head and cable). The PA deck is derived from the Mitrek and has had several bugs from that era fixed. Motorola's web site claims that the radios are discontinued, but they still offer VHF-LO MaraTracs. This is probably the only high power, low-band product line Motorola currently sells. Most of the service and instruction manuals are No Longer Available (NLA).

I did not connect the UHF radio to RSS; I have no idea what unique things the radio is capable of, over and above what a MaxTrac could do. It does seem to have additional features like MPL and Talk-Around that are NOT on a MaxTrac, but are on Spectras.

MaraTrac radios can be altered to cover the amateur bands by following most of the procedures on this and other web sites that deal with MaxTrac radios, since the changes take place primarily on the RF board. In fact, all of the MaxTrac RF boards are used in the MaraTrac radios.

An official VHF sales brochure can be found here as a 1.2MB PDF file.

An official accessories brochure can be found here as a 90kB PDF file.

Remote Squelch Details:

The remote squelch pot was available on the low-band MaraTrac radios with A7 heads. They did this by coming out with revised (SP01) audio/squelch and interconnect boards.

The circuit starts with a 10k squelch pot mounted in the control head. This goes through a two-pin connector to the display board, then through a flexible cable to the main board, where it is present on the 50-pin connector at the back of the head.

The control cable carries the signal to the radio chassis where it goes through the interconnect board and makes its way to the audio/squelch board. Rotating the remote squelch pot changes a DC voltage in the squelch circuit and this adjusts the squelch threshold.

I traced the signal through from the squelch pot to the audio/squelch board. The table below notes path taken and shows the signal names for the standard configuration and the remote squelch version.

Location	Conn.	Pin	Standard	Remote Squelch
Inside Ctrl Head	P1050	14	N.C.	Squelch Pot
Rear of Ctrl Head	P1001	32	Horn In	Remote Squelch
Front of Radio	J1	9	Horn In	Remote Squelch
Audio/Logic Board	J21	12	Keyway	Remote Squelch

Inside the control head, there's a 6.8k resistor in parallel with the 10k squelch pot. One side goes to ground; the other side goes through a 3.9k resistor before leaving the control head. The resistance to ground with the pot fully CCW is approximately 8k and there would be about 2.7VDC present across the pot. The resistance decreases to approximately 4k and the voltage drops to 1.9VDC with the pot fully CW.

Squelch threshold or the lower limit is approximately 0.1uV or 2-4dB SINAD. Tight squelch or the upper limit is approximately 0.3uV or 20dB SINAD.

Acknowledgements and Credits:

All photos were taken by the authors.

Some information came from a VHF-L/VHF-H/UHF M400 Service Manual, 6880901Z56, as well as a VHF-L MaraTrac Instruction Manual, 6880102W95.

Eric WB6FLY provided the list of manuals and documentation.

Chris N1FAA supplied the low-band radio, which was subsequently converted to operate in the 46-54 MHz range.

Mike WA6ILQ provided additional historical information for this article.

Bruce KC7GR provided information about hex-editing the programming software.

MaraTrac, MaxTrac, Radius, M400, Spectra, and all the other model names and numbers mentioned in this article, as well as terms like RSS, MPL, PL, and DPL, are trademarks of Motorola, Inc.

Contact Information:

The authors can be contacted at k_c_single [ at ] digital [ hyphen ] dirt [ dot ] com.

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This page originally posted on Monday 07-Jan-2008