The Repeater Builders Friend
by Matt Krick K3MK

Concept: Wouldn't it be nice to have a simple diagnostic tool for use on repeaters in the field, Or a device that would allow you to program a controller off the air? Test a transmitter or find Carrier Operated Signal (COS) in a receiver? With a few dollars of parts from a swap meet and the local Hack Shack, we can build said device.

Description: The Repeater Builders Friend (RBF) consists of a COS source/indicator, Push to Talk (PTT) source/indicator, Touch  Tone audio source, And a Audio amplifier for driving a pair of headphones.

COS can be selected as active high or low and the indicator LED will illuminate green to signal COS on the line. So you can use the indicator to probe a radio to find COS for your controller, or signal the controller that COS is present.

PTT is active low. The indicator LED will glow red to indicate when this is occurring. Use this to see if your controller is actually keying the radio, or key the radio your self.

RX audio is a simple keypad that almost everyone has in their junk box. If you do not, get down to the next swap meet and find some one who does. The pad that the prototype used had 3 wires, Vcc, Ground, Tone out. This is the type you want. Toss a AC voltmeter on there and adjust the pot for 250 mV AC. This is a perfect signal source for adjustment of your levels. Using a deviation meter on the output of your TX, you can adjust your levels for 2.5 kHz of deviation.

TX audio can be monitored with a pair of head phones. You can also use this amplifier to trace audio in your receiver path. and listen to the controllers audio prompts when programing.

Advantages to use it: So you don't embarrass your self on the air. When your transmitter wont fire. When the receiver is dead. So people don't figure out your control codes. As a calibrated signal source. Locate problems in audio circuitry. Find COS in a new radio. Test your PTT point.

Design Considerations: The circuit was intended to be built by anyone with access to parts available from Radio Shack except for the touch pad and the 4049 might be a little hard to get these days. CMOS technology is used so that the indicators can handle up to 16 volts.  You might want to make a Printed Circuit board so it looks all nice and pretty and you can show it off at your next repeater party.

Part Quantity Notes
2N2222 3 2N3904 can be substituted
Red/Green Dual Color LED 1 Indicator for COS and PTT
Green LED. Blue or White would be nice  =) 1 Power Indicator
CD4049 1 CMOS Hex Inverter, Be sure to ground unused inputs
7805 1 Voltage Regulator, Can also be a 78L05
3.9K 5
10K 1
300 1
10K Pot 2 Used to adjust audio levels.
1 uF Capacitor 2
220 uF Capacitor 1
Push Buttons N.O. 2 Used to Force COS and PTT
DPDT Switch 1 Used to select Active High or Low COS
SPST Switch 1 Power Switch
LM386 1 400 mW Audio Amplifier
9 pin D style connector, Male 1 Used to interface to the outside world
1/8" phono jack 1 Connect to your head phones, Or if you have room in the case you can put a speaker in there.
Touch Tone Pad 1 Find one at your local swap meet
9 Volt battery, Connector, Holder 1 Power for the unit
Perf Board 1 Something to build it on, a Printed circuit board would be better
Plastic Case 1 Find One that the Touch pad will fit on okay and make sure there is enough room inside for all the circuitry, switches and the 9 volt. Also a label with your call on it is a nice touch.



Interfacing to Controllers: Make your self a few plugs so you can go right in to your controller. Use clip leads on the end of one of the plugs to keep versatility at a maximum.

RBF DB-9  1, Ground 2, COS  3, PTT 4, TX audio 5, RX audio
Controller Connector 
FREEMARS Heavy Industries
TCX-50, TCX-100, 
DB-9 1 2 3 4 5
TCX-150,  DB-25 5 2 3 4 1
Computer Automation Technology
DB-25 20 6 10 11 13
MOLEX 8 Pin 8 2 3 5 4
NHRC-2 DB-9 1 6 3 4 5
Creative Control Products
MOLEX 20 Pin 2 5 20 9 7
Pacific Research
DB-15 9 - 4 3 2
RI-310 DB-25 (ACC) 1 12 13 5 6
18 Pin Edge Connector 18 11 12 8 4
Pigtail Clip leads DB-9, Clip leads Black Green Yellow Red White

This circuit was designed and built by  Matt Krick, K3MK in the fully equipped FREEMARS laboratory