Yaesu FT-736R AGC

Description: The FT-736R is a V/UHF all-mode transceiver. This article demonstrates how to modify the AGC (Automatic Gain Control) circuit to disable it. Disabling the AGC is essential for radio astronomy to prevent it from affecting measurements. The FT-736 features a three-position AGC selector, which adjusts the decay rate to Slow, Medium, or Fast. However, there is no option to completely turn off the AGC, which operates only in SSB and CW modes. The modification circuit diagram is shown in figure 2, with component names corresponding to the original circuit and those used in Yaesu technical manuals. The relevant components can be found on the RX-unit Printed Circuit Board (PCB), with the Yaesu schematic labeled RX UNIT F2887101. This PCB is located on the left side of the transceiver when viewed from the front. The modification involves cutting the direct connection between capacitor C129 and the emitter of transistor Q37. A two-way switch is introduced at this point, with C129 connected to the center pole, one branch leading to the emitter of Q27, and the other branch connected to a 2.2 kilo-ohm resistor grounded. Additionally, a capacitor Cx is connected from the emitter of Q27 to the J2 connector plug, allowing access to the Intermediate Frequency (IF) of the receiver. Both the J2 plug and switch J1 are mounted on the rear of the transceiver, as shown in figure 3. It is crucial to ensure that all cables are shielded and kept as short as possible. The connection of C129 to a 2.2 kilo-ohm resistor to ground when the AGC is switched off is intended to prevent interference with the circuit following C129, which includes components D32, D31, Q38, etc. The functionality of the modification can be tested after assembly by tuning into a station and toggling the AGC switch on and off, noting a significant difference in the speaker output and S-meter readings. The IF signal from plug J2 can be routed to a detector circuit similar to that of D31 and D32, with the output connected to a PC-controlled Digital Multi-Meter (DMM). With an appropriate antenna setup, such as two stacked Yagi antennas and a good preamplifier, a sensitive noise meter can be established. The antenna should be directed toward the celestial coordinates where Cassiopeia passes the antenna loop every 24 hours. Cassiopeia A is a prominent radio source with approximately 11,000 flux units, being the strongest in the sky (excluding solar system objects like Jupiter and the Sun). The coordinates for Cassiopeia A are 18Hr00min Right Ascension and Declination -24 degrees. Recording for at least a day should yield detectable radio signals from the stars.

The FT-736R transceiver modification involves a meticulous approach to the AGC circuit, which is critical for applications in radio astronomy. The AGC system is designed to automatically adjust the gain based on signal strength, which can introduce unwanted variations in measurements when observing celestial radio sources. By implementing the described modification, the AGC can be effectively bypassed, allowing for a more stable and accurate reading of weak astronomical signals.

The modification's schematic indicates the precise locations of components on the RX-unit PCB, which is essential for both understanding the original circuit and ensuring accurate implementation of the changes. The introduction of a two-way switch provides the flexibility to toggle the AGC on and off while maintaining the integrity of the surrounding circuitry. The grounding of the 2.2 kilo-ohm resistor serves to stabilize the circuit when the AGC is disabled, preventing any potential interference that could arise from the AGC circuitry.

Furthermore, the use of shielded cables for the connections to the J2 plug and switch J1 minimizes the risk of electromagnetic interference, which is particularly important in radio frequency applications. The ability to feed the IF signal into a detector circuit allows for further processing and analysis of the received signals, enabling the use of advanced equipment such as a PC-controlled DMM for precise measurements.

In summary, the modification of the FT-736R's AGC circuit is a valuable enhancement for radio astronomy, allowing for improved signal detection and analysis. The careful consideration of component placement, grounding, and shielding ensures that the modification is both effective and reliable, making it an essential procedure for enthusiasts and professionals engaged in celestial radio observations.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 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 the circuit after the capacitor C129 with D32, D31, Q38 etc not shall be effected.

You can easily determine if the circuit works one it have been assembled. Just tune in a station and then switch the AGC switch on and off. You will note a considerable difference in the speaker as well as on the S-meter. You can easily take the IF from plug J2 and feed it to a detector circuit similar to that of D31 and D32. Then take the output from the detector and feed that in to a PC controlled DMM (Digital Multi Meter).

If you have a suitable Antenna arrangement like a two stacked Yagi antennas and a good preamplifier you will have a sensitive noise meter. Now pint the antenna against the celestial coordinates that makes Cassiopeia pass the antenna loop once each 24 hours.

(Cassiopeia A is a radio source that has about 11 000 flux units and is the strongest in the sky (objects within our planet system like Jupiter and the Sun not considered). The coordinates is 18Hr00min Right Ascension and Declination -24 degrees. If you do a recording for at least a day you will surly detect the radio signal from the stars. My first attempt looked like figure 6 below.