N6EX Exciter Mods

Description: This document outlines the necessary hardware modifications to adapt a Kenwood TK-931 transceiver for use as a 2-watt exciter for 927 MHz repeater service. The programming of the radio for the desired operating frequency is not included here, as it requires the Kenwood KPG-5D programming software and KPG-4 programming cable or an equivalent. It is assumed that the transmitter was functioning correctly in its original frequency range prior to modification. It is recommended to perform a check-out and alignment of the transmitter after the modification. The current N6EX repeater utilizes the TK-931 as a 2-watt exciter for a 90-watt power amplifier. The original setup used the higher power TK-931HD as the complete transmitter with an output of approximately 30 watts. The modifications described below pertain to the final configuration. The circuit was implemented on a copperclad perf-board, utilizing an LM1481 dual op-amp, of which only one stage was necessary. These low-power amplifiers are widely available and operate on a single supply. The audio interface board is installed in the front left section of the top cavity of the exciter, near the CN2 interface connector. The board is drilled in the right front corner to align with an unused hole in the radio frame, facilitating convenient mounting and access to the audio and DC interfaces. The DC input for the audio interface board can be sourced from the CN1 connector, which connects the front panel to the main circuit board. Switched 13.8 VDC is available on pin 2 (SB), with pin 3 (GN) serving as the return. The audio connection is made by tapping into the cable running from the CN2 connector to the rear panel connector (J4). The brown wire (MIC) is cut, and the audio interface board is inserted at this break. The audio board's input connects to the brown wire leading to the rear panel connector, while the output connects to the wire leading to CN2. Once the audio board is installed and connected to the transmitter interface points, it can be checked and adjusted. For this particular controller (Arcom RC210), a reference level of 2 volts peak-to-peak is used to achieve the maximum rated deviation of +/- 2.5 kHz at a modulation rate of 1 kHz. An audio generator produces the 2-volt peak-to-peak signal, and the trimpot on the audio board is adjusted to yield +/- 2.5 kHz deviation from the exciter. The TK-931 features a Time-Out Timer (TOT) designed to prevent prolonged keying of the transmitter, which can lock up a repeater or channel. This TOT can be programmed in 30-second increments, ranging from 30 seconds to 5 minutes, but it cannot be disabled. While this feature is beneficial for mobile radios, it can pose challenges in repeater service. A typical repeater relies on the controller's timeout timer to manage excessively long transmissions. Setting the TK-931's TOT to the maximum of 5 minutes and the controller's timeout timer to a shorter duration (e.g., 4 minutes) generally resolves this issue. However, it is important to note that the timer on the repeater controller resets after the Carrier Operated Switch (COS) drops. This does not necessarily unkey the transmitter due to controller hang-time. If the COS is reactivated before the TK-931 unkeys, its timer continues to run and may expire unexpectedly, leading to a situation where the transmitter will not resume transmitting until both the controller and transmitter timers are reset. Although this scenario is uncommon in regular repeater service, it became a concern during a broadcast of shuttle audio in July 2006.

The modifications to the Kenwood TK-931 transceiver for 927 MHz repeater service require careful attention to detail and an understanding of the radio's circuitry. The incorporation of an audio interface board is essential for ensuring proper audio levels and modulation characteristics. The LM1481 op-amp serves as the core amplification component, providing the necessary gain while maintaining signal integrity. The physical placement of the audio board within the exciter's structure is strategically designed for optimal accessibility and ease of installation.

The power supply connections are critical for the stability of the audio interface, and thus, the use of the CN1 connector is a practical choice for obtaining the required DC voltage. The modification process involves precise cutting and splicing of the existing audio connections, ensuring that signal paths remain intact and functional. The adjustment of the trimpot on the audio board is a crucial step that directly influences the modulation depth and quality of the transmitted signal.

Furthermore, understanding the Time-Out Timer's functionality is vital for effective operation in a repeater context. The programmed settings must be carefully managed to prevent unintended transmission interruptions, particularly during extended broadcasts. The interaction between the TK-931's TOT and the controller's timeout timer must be monitored to avoid conflicts that could disrupt service. Overall, these modifications enhance the TK-931's functionality as an exciter while maintaining compliance with operational standards in repeater service.This page identifies all the hardware modifications necessary to adapt a Kenwood TK-931 transceiver for operation as a 2 watt exciter for 927 MHz repeater service. Not shown here is the effort required to program the radio for the desired operating frequency. This is accomplished using the Kenwood KPG-5D programming software and KPG-4 programming cable, or equivalent. It also assumes that the transmitter portion of the radio was properly operating in its original frequency range prior to modification. Check-out and alignment of the transmitter after modification is recommended. The present N6EX repeater uses the TK-931 as a 2 watt exciter for a 90 watt power amplifier. The original configuration (prior to the external 90 Watt PA) used the higher power TK-931HD as the complete transmitter (approximately 30 watts output).

The modifications listed below pertain to the final configuration. The above circuit was implemented on a copperclad perf-board. An LM1481 dual op-amp was used. Only one of the stages was needed. These low power amplifiers are readily available and run on a single supply. The audio interface board is installed in the front, left section of the top cavity of the exciter. This area is located near the CN2 interface connector of the radio. The board is drilled on the right front corner to coincide with the unused hole in the radio frame. Not only is this area of the radio available for convenient mounting of the interface board, but it also provides easy access to the audio and dc interfaces of the radio. DC input to the audio interface board can be taken from the CN1 connector (connects front panel to main circuit board).

Switched 13. 8 VDC is on pin 2 (SB) and return is on pin 3 (GN). The audio connection is tapped into the cable running from the CN2 connector to rear panel connector (J4). The brown wire (MIC) is cut and the audio interface board is inserted at the break. The input of the audio board is connected to the brown wire running to the rear panel connector and the output of the audio board is connected to the wire running to CN2.

Once the Audio board is installed and connected to the transmitter interface points, the board can be checked and adjusted. With this particular controller (Arcom RC210), a level of 2 volts peak-to-peak is used as the reference level to produce the maximum rated deviation of +/- 2.

5 kHz at a modulation rate of 1 kHz. An audio generator is used to produce the 2 volt peak-to-peak signal. The trimpot on the audio board is adjusted to produce +/- 2. 5 kHz deviation out of the exciter. The TK-931 (as well as many other commercial radios) has a "feature" which is intended to prevent locking up a repeater or channel by prolonged keying of the transmitter. This Time-Out Timer (TOT) can be programmed in 30 second increments from half a minute to five minutes.

Unfortunately, it can not be disabled. While the feature makes sense for a mobile radio, it can create problems when the radio is used in repeater service. A normal repeater relies on programming of the controller time out timer to handle excessively long transmissions.

Under most circumstances, setting the TOT of the TK-931 to its maximum of 5 minutes and the timeout timer of the controller to something less (say 4 minutes) does the trick. But remember, the timer on the repeater controller resets after the COS drops. This doesn`t necessarily unkey the transmitter (due to controller hang-time). If COS becomes active again before the 931 unkeys, its timer continues to run and could expire unexpectedly.

And when it happens there`s no warning (or alert). It won`t resume transmitting until both the controller and transmitter timers are reset. This situation didn`t happen very often under normal repeater service. It wasn`t until I tried broadcasting shuttle audio in July of `06 that I decided I had to do address the TOT issue. When the shuttle flies, there are often perio