TK-931 Exciter Modifications

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.

See Using the TK-931 As A Complete Transmitter at the bottom of the page for some of the modifications I made to the TK-931 prior to using an external PA.

Audio Interface board addition

An audio interface board was used on the TK-931 to increase noise immunity on the audio input. The high impedance microphone input of the radio normally accepts an the extremely low (millivolt) audio signal produced by a microphone. The harsh environment of a repeater site could result in noise being injected into such an input. To reduce this susceptibility, a high level interface is created.

-----Audio Board Circuit

Receiver Buffer Board Schematic
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This is the schematic of a simple op-amp based circuit used to transform the high level audio signal produced by the controller to the low level required for the microphone input of the transmitter. The gain of the audio stage was chosen to take a 2 volt peak-to-peak, 1 kHz audio tone and drop it to a level necessary to produce a deviation of 2.5 kHz.

Receiver Buffer Board Photo
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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.

-----Audio Board Connection

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 .

Receiver Buffer Board Photo
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This photo shows the mechanical placement and electrical interconnection of the audio interface board within the exciter.

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.

-----Audio Board Adjustment

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.

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PTT Timeout Timer Disable Board Addition

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 periods of many minutes of continuous audio, especially when there's a press briefing being conducted. These periods would make the repeater unusable until the audio source dropped. Either I had to solve the problem, or shuttle audio was out of the question.

-----TOT Disable Board Circuit

I checked with several sources to see if anyone knew how the TOT function of the 931 could be disabled, but received no replies. I then started looking more closely at how the function was implemented. I examined the service manual, but unfortunately it seemed that the radio micro had full control of the function. A TOT value was programmed into the unit and if the transmitter was keyed continuously for longer than the programmed time, the transmitter would be disabled and a warning tone would sound as long as the PTT was enabled. Hmm...a warning tone. This got me to thinking. Could this warning tone be used to my benefit? I did a little more looking at the schematic and did some probing in the radio and found the point on the micro where the warning tone was generated. It was a 5 volt square wave, which seemed like something that could easily be sampled.

With a signal that now can be used as an indicator of when the timeout occurs, a simple circuit can be devised to briefly interrupt the PTT line when the condition is detected.

Receiver Buffer Board Schematic
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This is the schematic of the simple circuit that effectively disables the timeout timer of the TK-931. The PNP transistor conducts under normal conditions, allowing normal PTT action from the controller. When a timeout occurs, the circuit detects and filters the audio tone produced by the radio micro. When the filtered signal is applied to the base of the PNP transistor, it stops conducting, the PTT line is interrupted and the timer resets. The result is a brief (10 - 15 ms) interruption of transmission during the event. This short interruption sounds like brief click when listening to the output of the repeater.

The diode and relatively large resistor values associated with the tone detection line are used to assure that the circuit has no impact to normal operation of the radio.

The PNP transistor is placed inline with the PTT signal (transmit when pulled low) from the controller to the radio.

Receiver Buffer Board Photo
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The above circuit was implemented on a small perf-board. A 2N2907A PNP transistor was used for the switching component.

-----TOT Disable Board Connection

The TOT disable board is installed in the front, center section of the top cavity of the exciter. This area is located to the right of the CN1 interface connector of the radio. The board is drilled on the left front corner and is aligned with an unused hole in the radio frame.

Receiver Buffer Board Photo
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This photo shows the mechanical placement of the TOT disable board within the exciter. It also shows the connection of the TOT warning tone sense signal from the radio to the TOT disable board. The sense line (green wire) connects to the "high" side of R31, which is the same point as pin 33 (ALT2) of the microcontroller.

Receiver Buffer Board Photo
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This photo shows the interconnection of the TOT disable board with the PTT circuitry within the radio. The purple wire (PTT) normally connected between CN2 and the rear panel connector (J4) is cut and the TOT disable board is injected between the two connectors. The purple wire running to J4 (Controller Side) is connected to the collector side of TOT disable board PNP transistor (red wire on TOT disable board). The PTT line on the CN2 connector (Exciter Side) is connected to the line (purple wire) going to the resistor feeding the emitter of the PNP transistor on the TPT disable board.

Although not shown here, the ground connection for the TOT disable board connects to the same pin (pin 3 - GN) of the CN1 connector used to connect ground on the Audio Interface Board.

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2 Watt Amplifier Modification

The N6EX repeater uses a modified TK-931 (15 watt) radio as an exciter. The TK-931 serves as a driver for the separate 90 watt power. The PA requires approximately 1.7 watts of drive to produce the rated 90 watt output power. Accounting for cabling and isolator losses, a 2 watt exciter output power is needed. To efficiently produce this low output power, the TK-931 is modified to remove the Motorola MHW820-3 power brick. In its place, a custom single stage MOSFET power amplifier is installed to amplify the 300 mW of the IC400 driver to the desired 2.0 watts at the rear panel output connector. The custom amplifier is built on a copper heat spreader which both physically and electrically matches the mounting configuration of the original power brick.

-----2 Watt RF Amplifier Circuit

Exciter 2 Watt Amplifier Schematic
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This is the schematic of the microstrip single stage RF amplifier. The amplifier is based on the M/A-Com LF2810A MOSFET (Thanks to Gary, WA6MEM for the parts). The transistor is actually a 10 watt, 28 volt part, but provides adequate gain for this application at 12 VDC. The amplifier provides greater than 40% efficiency at the desired output power. Trimmer capacitors are used for input and output matching. Output power is adjusted by a trimpot which sets the gate bias voltage. The amplifier was designed and fabricated as a joint project with Dave, WA6CGR. The amplifier was designed to provide similar functionality within the Kenwood TK-931 and Motorola Spectra 900 MHz radios.

Exciter 2 Watt Amplifier Layout
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This is the layout for the microstrip circuit board. The board material is 0.030 inch Duroid. The circuit board is compression soldered onto a similarly sized copper heat spreader. The board and spreader are milled and drilled to accept the flange-mount transistor. The spacing of board mounting holes matches the dimensions of the original Motorola power brick.

-----2 Watt RF Amplifier Installation

Original Motorola Power Brick
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This photograph shows the original configuration of the TK-931. The Motorola MHW820-3 power module is mounted to the rear heatsink with two screws. The five pins of the module are soldered to the main circuit board. To begin the modification, the module needs to be unsoldered and removed from the radio. Retain the two mounting screws, as they will used later.

RF Amp Mounting Surface
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This photo shows the power brick mounting surface once the module has been removed. The heatsink mounting holes and the printed circuit board interface holes are clearly visible.

2 Watt Power Amp
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This photo shows the completed 2 watt amplifier installed in the radio. Heatsink compound should be applied between the module heat spreader and the radio heatsink. The input port of the amplifier should be connected to pin 1 of the PCB, the output to pin 5, and the +13.8 VDC input to pin 2. Additionally, a short jumper should be installed between the ground on the amplifier and the host radio. Once installed, the trimmer capacitors should be adjusted for maximum power and the trimpot adjusted for desired output power (nominally 2.0 watts).

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External Radio Interface

To interface the TK-931 Exciter to the Controller (and Power Amplifier), the accessory connector (J4) on the rear of the radio may be used. If the mating connector (J5) did not come with the radio, a standard 15 pin Molex connector (Housing P/N 03-06-2152, Pin P/N 02-06-2103) can be used.

Rx Buffer Audio Plots
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The J4 accessory connector is routed to an internal circuit board connector (CN2) via a multi-conductor cable. This diagram shows the factory wiring between the two connectors. The colors reflect the color coding or the wires in the cable.

Interfacing the exciter to the controller and PA requires the following five signal lines:
Other than the modifications described above, (connection of the audio interface board, TOT disable board and 12 VDC bias supply) no other changes are required for interface to the rear panel connector.

Modified J4 Connector for TK-931 Exciter
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This is a modified diagram of the J4 to CN2 interconnections. It shows the modifications implemented to incorporate the audio interface board, TOT disable board and PA biasing line. It also highlights pins of the J4 connector which are used to connect the TK-931 Exciter to the Controller and External Power Amplifier.

The exciter can be connected to the controller by constructing a cable which interfaces to the rear connector as follows:

J5 PinSignal
2 Ground (PA Bias Supply Return)
3 Transmit 12 VDC (PA Bias Supply)
5 Microphone Audio
6 Ground
8 PTT (Active Low)

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Using the TK-931 As A Complete Transmitter

The transmitter initially used for the N6EX repeater was a modified TK-931HD (high power version) transceiver. All the previous described modifications (less replacement of power brick with the 2 watt amplifier) apply to this stand-alone transmitter configuration. This section briefly describes the additional changes performed to make the radio more suitable for high transmit duty cycle repeater service.

The TK-931HD is rated at 30 watts power output. While the radio is actually capable of higher power (the MRF847 final transistor is rated at 45 watts), the components and cooling system were designed to support the rated 30 watt output under normal (i.e. low transmit duty cycle) mobile operating conditions. To operate in repeater service, a nearly 100% transmit duty cycle operation must be assumed and designed for. The TK-931HD modifications shown here include component changes and mechanical improvements to improve cooling of the unit.

TK931HD PA Mod Photo
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The components of the power amplifier section of the TK-931HD were changed to handle the harsher duty cycle conditions. All capacitors in the RF path were replaced with high quality ATC porcelain (B Size) components. Additionally, the choke coils were rewound from larger diameter solid wire. The limit on the wire size was dictated by the clearance available under the finals section shielding plate.

Top Cover Fan Mod
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To improve cooling of the transmitter, a 4 inch muffin fan was added to the radio case. The fan was positioned to allow some air to flow into (and through) the rear section of the radio and some to flow across the heat sink. Holes were drilled in the top case to serve as the air inlet.

TK-931HD Sidewall Mod
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A matrix of small holes was drilled on the right side of the chassis at the back of the radio. This series of holes allows forced air to enter from the cover, flow over and around the transmitter section, and exit out the right side.

TK-931HD Fan Photo
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The photo shows the modified TK-931HD in service. The 4 inch muffin fan was installed on the top cover to blow air into and over the transmitter section. Although not clearly visible, a screen was installed between the fan and the top cover to prevent larger particles from being blown into the radio. The fan was actuated by the system controller. This limited fan operation to only times when the transmitter was active, thereby increasing its lifetime.

The repeater operated in this configuration for six months. The output power of the transmitter was set to 30 watts. As was stated earlier, the transmitter is capable of more output power, and the temptation to maximize the repeater output power is always present. However, I strongly recommend against going past the 30 watt level, even with the improved cooling approach. Higher power testing during the modification phase showed that component temperatures rose dramatically as power was increased. Components were observed to desolder themselves from the circuit board after long periods of transmission when the transmitter was set to maximum power (40 - 45 watts). The 30 watt level, with improvements documented herein, appears to deliver an adequate safety margin and should provide reliable operation.

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