This document describes how I modified a Bosch kf452 transceiver can be modified to work as a 70cm repeater. The repeater is used to test the DRL network (It actually is the first DRL repeater ever :) but can also be used as a stand alone system.
Not finished yet..... Just some photos and little pieces of schematic for now.
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| Front with new switches | Back |
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The KF452 has two bandstop filters. The signal comming from the transmitter goes through a filter set to the receive frequency and the signal going to the receiver passess through a filter set to the transmitter frequency. With a shift of 7.6MHz the filters have a rejection of about 60dB. |
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| The KF452 can operate on both 24 Volt and 12 Volt supplies. Some versions have a switch to select either voltage. Mine didn't have this switch and was hardwired for 24 Volt. (The switch should have been under the holes in the lower left corner of the supply PCB) Rewiring the supply for 12 Volt is relativly easy. In the picture you can see the wires that have to be placed. One blank wire to short a resistor and three wires to connect the coils. At place were the three wires are placed remove the two wires on the other side of the PCB. |
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Top
Controller
Bottom
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The microprocessor communicates with the computer over an 9600 baud serial link. The TTL levels of the microprocessor are inverted and buffered. The Squelch signal from the receiver is buffered by a single PNP transistor. PTT is inverted twice. This is done to prevent the transmitter from being keyed while the processor is in reset state (P1 high). It also has the advantage of being able to draw higher currents. |
| Besides the microcontroller PTT is also controlled by Squelch and the alarm button. The alarm button puls it down with a diode and enables a tone. This tone can be very usefull for adjusting the transmitter. When Squelch becomes active PTT will immediatly follow, but it deactivates with a delay. This and connecting the audio output and input makes the basic repeater. |
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Datasheet for the MT8870 DTMF decoder.