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Modifications To A Motorola Syntor X to Use As A Repeater Receiver or Transmitter
Scott Lichtsinn KBØNLY

Remember, above all, that the Syntor X is a MOBILE radio, and while it can be used quite readily as a system receiver, you cannot use it as a continuous duty repeater transmitter or link transmitter without due consideration to the normal mobile radio limitations on RF power and duty cycle.

That said, the Syntor X transceiver makes a great repeater radio. The modifications are minor, and totally reversible if needed in the future. In this example I will show how it can be done without any control accessories. The Syntor X routinely shows up used without any cables or control heads, as was the case with the radios that I had to work with.

This article assumes that you know how to program the radio, the Syntor X is programmed with a suitcase programmer or the memory module can be removed and the EEprom pulled and changed with a Eprom burner and the appropriate software, it will not be covered here. Another possibly is to use WB6YMH's XCAT product - a board that plugs into the Syntor X in place of the regular memory module. This device allows you to have a frequency agile remote base if you want. Or the Piexx module which replaces the stock memory module but makes it computer programmable without any further modifications to the radio.  Also, it is most useful if you have a Syntor X service manual, however the radio connector J1 is labeled on the personality board so it is easy to identify the pin numbers without one.

You may also use a control head with these radios if you can find one. The internal jumpers will be described with or without the use of a control head. It's easiest to make the connections to the repeater controller using a control cable connector for the radio, however, if it is not available you may also use quarter inch female QD connectors for the supply voltage, and eighth inch QD female connectors for the signal pins. This is not the most reliable method as they could become loose or dirty, but will suffice. You could also solder directly to the radio connectors pins, this is made even easier by using emory cloth, sandpaper, or a metal brush to clean them and rough up the surface slightly for easier soldering. Also, it is possible to take a small drill bit and put a hole through each tab to allow looping the wire through before soldering. If you choose this method to make the connections then cover each connection with heat shrink tubing to prevent accidental shorts. You should also consider covering all the unused pins, which still carry other signals and power, damage may be the result if they were shorted. Another possibility is to use the QD pins for main power and ground, and to add a DB-9 or DB-15 pigtail mounted in the radio for all other connections and directly soldering to the personality board solder pads, but remember you still need to protect the unused pins on the front of the radio from being shorted, a cover could be fabricated from a piece of plastic and a machine screw.

If you have a control cable available you will need to reassign a wire to pin 18, which means disassembly of the radio end. This is very easy, first thing that needs to be done is remove the "E" clip retaining the hold down bolt that goes through the body of the connector, then remove the foam gasket on the radio side, sometimes you can also just pull up each corner one at a time, and remove the four screws under it.

The removal of the crimps in each hole can be done with the removal tool, the Motorola tool looks like a small key (a photo of it is on the Keys Page on this web site). This tool appears to be NLA, as the part numbers I have for it are no longer valid. Or you can use the method that I use, a pin vise and a piece of stiff wire, even a paper clip straightened will do it, but it's not easy without a handle (even a pair of needle nose pliers) to apply the pressure needed. I usually remove all wires except for the main battery leads, the heavy gauge red and black on pins A and B, and start fresh with an empty connector. You can also leave pins 8 and 10 as they are jumpers from B to provide grounds. This Black/Purple wire is also in the control cable. If you're not going to be using a control head then clip the wire from the control cable that gets crimped in with either pin 8 or 10. This leaves you with the main battery leads, and the jumper from B to pins 8 and 10.

The main connector on the front of the radio is J1, which is easily accessible by opening the top cover of the radio. This is where we will do all the modifications needed inside. You can also provide all the jumpers in the connector, however, I prefer to solder the jumpers to the personality board at J1 to consolidate all the wiring to inside the radio. This is a builder's choice, so do it either way that you prefer to.

The jumpers needed are:

J1 pin 32 to J1 pin 10: This is the hook switch lead, also know as HUB or Hang Up Box. It's function is to disable receive PL when the microphone is taken out of the clip (hang up box). This jumper replaces the microswitch in the clip, forcing the PL decoder on. If there is no PL programmed this will not affect operation. If you plan on using carrier squelch for your application then you will also need to add in a squelch control if you are not using a control head. More on that later. You do not have to install this jumper if you connect the green and black wires from the control head to the hang up box when using a control head.

J1 pin 13 to J1 pin 10: This is the M1 select line, when pulled to ground it will select channel 1. You can install this jumper with or without a control head to make sure channel 1 is always active. If you do this remove the channel select lines from the control head connectors and insulate them, or open the control head and remove the channel select lines from the rotary channel select. This will give you a control head to use for squelch and local monitor volume, as well as a local mic, but without the possibility of the channel being changed by accident.

J1 pin 8 to B (power supply negative): This is to duplicate the original control cable connector jumper which ties together pin 8 and ground. Pin 10 is the chassis ground, which is grounded through the screw holding down the personality board via the trace to pin 10. You can also jumper from pin 10 to pin B if your not using a control cable connector but it is not absolutely necessary. If you experience any power or audio ground loop problems be sure that pin 10 has a good chassis ground through the mounting screw near J1 at the front middle of the personality board.

both J1 pin 5 and J1 pin 15 to A (power supply positive): This provides power to the PTT reference line on pin 5, otherwise know as PTT inhibit which is normally supplied by the orange wire at the control head to pin 5. And, it supplies power to the rest of the radio through pin 15 which is normally supplied by the control head via the green wire, aka ignition sense. These can be jumpered directly to pin A, or through a small fuse. I used a resistor to provide some protection against any mistakes in connecting the radio. But you could also use a small fuse from pin A, the factory value is 7.5amps for pin 15 and 1.5 amps for pin 5.

U2-15 to J1 pin 18: This brings out the COR signal to pin 18 of J1. There is a small trace from pin 18 to an empty hole just behind J1, verify that there is no pin header in this location or any jumpers from this hole, if there is remove any connections to this location to free pin 18 for use. I usually just solder the jumper from U2 into this empty hole. From this location it will go high, roughly 5v, when decoding PL. If you plan on using the radio with carrier squelch an alternate signal is available, it is labeled as the Squelch Tail Output on JI-6. Be aware that if you want to use the radio in CSQ without a control head that you will need to provide a squelch control. This is easily done with a 25k trimpot. Connect one end of a 3.3k resistor to J1 pin 2, and the other end to one side of the trimpot. The wiper goes to J1 pin 28, and the other side goes to ground, the control head uses J1 pin 14 for this.

Here is an example of the modifications done to the personality board.

And here is a graphic representation with the jumpers edited into the schematic in blue.

With these modifications made all you have left to do is connect the signals needed to the repeater controller. If your using a control head, the mic is usually wired so that the mic hi line is switched along with the PTT, so the mic will not go hot when the radio is put into transmit by the repeater controller, but verify this by either opening the mic, or by checking to see that the mic is not hot when the controller transmits.

The connections to the radio are as follows.

J1 pin A (power supply positive): Normally fused for 40amps, which is a bit on the high side! They usually draw around 30amps at 100w. Since the power output will need to be reduced for repeater operation to roughly 50% output power, this is done by adjusting the power set control through the hole in the rf board on the top left side of the radio or by adjusting the power set control through the common circuits board from the bottom side of the radio, you should use a meter to check the actual amperage being drawn and fuse the main lead slightly above that. Also, when you adjust the power output you need to set the current limit control. The easiest way to do this is to adjust the power output watching a wattmeter and then while transmitting adjust the current limit until the power output just starts to decrease then set it to slightly higher than this position. This will assure that the current limit is set slightly above the current needed to run the PA at the selected output power.

J1 pin B (power supply negative): Make sure you make a solid connection! Also, ground the radios and the controller to the same location to avoid any ground loops.

J1 pin 2: Detected Audio output to the controller. This is discriminator output and will need de-emphasis added. Most controllers can provide de-emphasis with optional components added, such as the NHRC brand controllers which leave it as an option when ordering the components to assemble it. The S-Com 7K can have a capacitor added.

J1 pin 4: PTT. Ground to transmit.

J1 pin 27: TX Audio. This is the Mic Hi line. Depending on your controller you may need to add additional filtering or audio tailoring to make it sound right. In my case I just adjusted the audio trimpots on my NHRC controller until the audio suited me.

J1 pin 18: PL Decode. Positive going signal, roughly 5v on most of the radios I tested. If your controller needs a higher level, just use a 2N3904 with the Base to U2-15, the Collector to a pull up resistor, anything from 2-10k will work fine, and that goes to pin J1 pin 15, or you may also use J1 pin 5 or J1 pin A, and then the Emitter goes to J1 pin 18 or the empty plated through hole discussed earlier. This will bring the line up to the supply voltage when decoding the PL. I also recommend placing a 20k or 25k resistor from pin 2 to pin 28 on J1 if you going to be using this signal, this keeps the radio squelch closed at all times and prevents squelch tails if your radio doesn't mute the receive audio fast enough, this simulates having a squelch pot in place. Of course you could also take a 25k trimpot and install it, if you decide to do this use a 3.3k resistor from pin 2 to one side of the trimpot, the other side goes to ground on pin 10, and the wiper goes to pin 28, then you can adjust it if you ever program the radio for CSQ only.

J1 pin 6: COR. Alternate to the signal from U2-15. However, this signal is not conditioned by PL. So it only works for CSQ, and you need to add a squelch trimpot if your not going to use a control head. 

J1 pin 10 Ground For RX and TX audio, and to the controller for PTT if needed. I have seen some controllers which require a separate ground for the PTT return. Use J1 pin 10 for any signal grounds needed between the controller and the radio.

Here is an example of a modified interface cable.

If your going to be using a control head and cable you have two options for making the connections. First is that you can open up the radio end connector and tap off each wire needed, dressing them together with heat shrink tubing or tape and routing out the side with the control cable. This allows the control head to be placed farther away in a convenient control position from the radios and controller. You can also obtain all the signals needed, minus the PL Decode on pin 18 at the control head end of the cable. To obtain the PL decode signal at the control head end you can easily reassign one of the wires that will not be needed to pin 18 and then disconnect it from the control head end to isolate it. For example, for repeater use where only channel 1 would be needed you could move any of the mode select lines that are not needed, M2-M8. This gives you seven spare wires for interface use.

With a little more creativity you can wire it up so that both radios share the same control head. You would connect the detected audio and squelch control to the receiver radio, and the PTT and transmit audio to the transmit radio. By doing this with one control head and two control cables you would then have a more convenient local control, with local receive audio and a local mic. This is especially handy for on site troubleshooting and repairs.

Also, when you program and modify the radios do the identical mods to both of them, and with identical programming. This makes the radios immediately swappable to troubleshoot a transmit or receive problem by simply disconnecting and reconnecting the control cables. This is another benefit of using a OEM type control connector for interfacing to the repeater controller. If you go with the solder or QD terminal method then just have an inline connector wired the same for easy swapping. For example, a five pin inline male mic connector from the radio and a five pin female mic connector from the controller. This would make them easy to swap, while also eliminating the chance of connecting something wrong due to the connectors being keyed. Or you can use a common DB-9 or DB-15 connector.

Contact Information:

The author can be contacted at: kb0nly [ at ] mchsi [ dot ] com.

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Article text and all photos Copyright 2005 Scott Lichtsinn KBØNLY
Printed circuit board layout fragment is from the Motorola Syntor X manual, copyright Motorola.

This page last updated 14-Dec-2011

The information presented in this web site, on these web pages and in these modifications and conversions is © Copyrighted 1995 - current by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved.