Making a Repeater, Bi-Directional Link or Remote Base from two or more modified Midland Syntech or XTR model mobile radios Revision August 2007 skipp025@yahoo.com Midland Syntech I, Syntech II and XTR model mobile radios lend themselves nicely to Repeater, Remote Base and Linked Radio Conversion Projects. This article describes the practical application of Midland Syntech and XTR Series Radios toward practical low cost conversion projects. In specific I'll address the physical wiring connections and support circuits best suited to make the radios operate as transmitter or receiver section in a repeater. We will assume for this example you already have the radios you plan to use in your project programmed and aligned for the desired frequency of operation. Locations like the Yahoo Midland Radio Group - Files & Photos Sections and Ebay are a great source of Service Manual radio and part sources. Check those locations first if you need information or parts to complete a conversion, alignment, modification. The Transmit radio is pretty much unmodified and programmed to the repeater output frequency. Midland Syntech and XTR Radios use an in-microphone amplifier circuit, which translates to a higher level of audio drive (voltage) applied at the mic jack. Current Syntech Repeater Transmit Radio Conversions use reconfigured wiring through the rear white Molex connector. Older conversions might have used adjacent wire(s) leads routed through some type of case drilled hole. The added external wiring simply paralleled the front panel mic jack audio and transmit function. Later Repeater Transmit Radios simply make the required mic audio, PTT and connections through the original front panel mic jack. No physical radio modifications are required to a transmit radio. The receiver need only supply receiver audio and COR output. The easy repeater receiver conversion uses the speaker audio output wired direct to the transmit radio mic plug. Don't discount or be quick to say negative things about a speaker audio direct to a mic input connection until you take a moment to think about the actual operation related to the Midland Radios. Speaker Audio can be sourced from the rear Molex connector, terminated into a 3 watt 10 ohm resistor or for simplicity paralleled across the internal speaker. Nothing wrong with using the internal speaker as long as you know the receive audio will be heard (on the local speaker) by anyone at the radio site. The receiver audio volume control serves as the repeater system audio level. The direct type of audio connection is simple, clean and it works well. Advantages are the gated on off receiver audio with valid COR, the ctcss (sub tone) removed and enough low impedance drive power to develop the required audio voltage at the transmitter mic input (jack) with the volume control about 1/3 to 2/3 rotation from the off position. Sure you could use receiver discriminator circuit area tap to source a low level stage for audio, but why bother when the work has been done for you at the speaker leads, which are available on the rear Molex type connector . Depending on which model Midland Radio you have available, two COR (also known as) COS logic function source locations are available. One of the original COR logic sources is a connection to a location on the ctcss/dcs board (OK, but not great). My current and revised location sources rx COR logic functions on the main receiver-transmitter board. Some radio models (like the VHF Syntech 70-340/341 Radios) actually have a very handy unused pc-board location (J-359) where all the desired receiver COR/cos logic is easily sourced. In most cases some type of circuit isolation or buffering is recommended to avoid damage to the receiver circuit when making or adding external connections. The most popular isolated carrier operated relay or carrier operated squelch logic is a simple small 12 Volt Relay, transistor 2N7000 type FET configured to complete a DC ground COR (active low) connection. To use the receiver radio in additional linking projects I use a small 12 Volt DPDT Relay with the coil ground start/active return lead connected to a small 2N7000 FET drain lead. The 2N7000 FET used for relay control provides no noticeable loading of the receiver cos/COR circuit. A properly configured receiver COR activity output line can be wired active high or low with low being the typical industry standard. You are not restricted to placing only the radios in a back to back layout. In fact most Amateurs install and operate some type of external Repeater Controller between the receive and transmit radios. Notes about ctcss and dcs squelch operation: Three types of receiver squelch mode operation are possible. The first selection is plain-jane carrier squelch control of the output COR logic line with a received signal or rotation of the squelch control to the off or full CCW position. The second option is dependent on the relay COR circuit pc-board connection point and ctcss programming. The COR Logic can be wired and programmed for ctcss detection. Specific connection points are described in each radio model text. The third option is related to ctcss detection logic sent to and External Repeater Controller. Controller user programming determines the choice of ctcss or carrier squelch Repeater Operation. The Midland Syntech 70-340/341 series of radios can be wired to provide COR logic based on CTCSS or Carrier Squelch using the front panel Monitor Button as selected by the user. Cheers, skipp025@yahoo.com www.radiowrench.com/sonic The Midland Syntech Repeater and Bi-directional Repeater Link Modifications information now available in the Yahoo Midland Group Files Section.