Yaesu FT-736R AGC


Posted on Feb 7, 2014

The FT-736R is a V/UHF all mode transceiver. A picture of the FT736R can be seen in figure 1. This article will show how you can modify the AGC (Automatic Gain Control) circuit to disable it. When you are doing radio astronomy it ’s vital to be able to disable the AGC device so that it doesn ’t influence your measurements. The FT736 has a three po


Yaesu FT-736R AGC
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sition AGC selector. The selector changes the decay rate for the AGC. The positions are Slow, Medium and Fast. However there is no way to turn the AGC off completely. Note also that the AGC only works in the SSB and CW mode. In figure 2 you can see the circuit diagram of the modification. The component names are they of the original circuit and the ones used by Yaesu technical manuals. You will find the corresponding component names printed on the circuit board, see figure 5. The location is on the RX-unit Printed Circuit Board (PCB). The Yaesu schematic is: RX UNIT F2887101. You will find this PCB on the most left side of the transceiver when looking from the front. What we have done is that we have cut the direct connection between capacitor C129 and the emitter of transistor Q37. We have put a two way switch in this location and connected C129 to the center pole and one branch to the emitter of Q27. The other branch is connected to a 2. 2 kilo ohms resistor that is grounded. We have also connected a capacitor Cx to the emitter of Q27 to the J2 connector plug. Via this plug we can connect to the IF (Intermediate Frequency) of the receiver. Both the plug J2 and the switch J1 is mounted on the back of the transceiver as can be seen in figure 3. It very important that all cables are shielded and kept as short as possible. The reason that we connect C129 to a 2. 2 kilo ohms resistor to ground when we switch of the AGC is that...




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