3/15/2021 0 Comments Icom 756 Pro Iii Serial Numbers
The waveform below right shows the transition back to receive at 2.5mS per division. Notice that the HSEND line goes inactive 4.5mS before the RF stops which possibly could be a big problem.Since almost everyone uses the same antenna for both transmitting and receiving, an external amplifier must be switched in and out of the circuit when going from transmit to receive.The timing of this switching is important if we expect relays to not be damaged and if we want our transmitted signals to be spectrally clean.
The amplifier (or any other devices that may be switched in or out) must be in the circuit BEFORE any RF begins and must not be switched out of the circuit until the RF has totally ceased and then need to be switched out quickly. The drawing below shows the basic timing external to the radio. The HSEND line is both an input and output but when it is low the radio is in transmit mode and when it is high the radio is in receive mode. The CW key line is shown but is not important to understand the timing as it follows the HSEND line very closely and obviously has is not applicable to any mode other than CW. Amplifiers are generally controlled via the Send Control jack which is a bare relay contact but may also be controlled via the HSEND output driving an external buffer device. As the following drawing shows the amplifier is given time T3 (or T3-T2) warning to get in the circuit on transmit and the radio goes back to receive T7 after the RF stops. If the amplifier is not ready when the RF begins relays may be damaged and the transmitted signal will have an extremely fast rise time producing spurious signals (splatter or key clicks) and if the amplifier is too slow when switching out of the circuit the radio will be sluggish on having the antenna connected for receive and you will miss the first syllable of speech or the first character on CW. Icom 756 Pro Iii S Full QSK ModePlease note that T7 is negative for the IC706mkIIg and the IC7000 in full QSK mode. Time was hard to measure since CW key line had slow rise time due to capacitance on line inside IC7000. For these 3 waveforms, the orange trace is the CW key line, the cyan trace is the HSEND line, the purple trace is the Send Control relay output connected to a 4.7k ohm resistor to 13.8VDC, and the green trace is the RF envelope. The waveform at the right shows the transmit delay at 2.5mS per scope division. The waveform directly below shows the same event but at 250uS per division to show the relay bounce and the small delays between the CW key line, HSEND, and the Send Control relay. The waveform below right shows the transition back to receive at 2.5mS per division. Note that the 756 Pro has a faster RF rise time even though both radios were set the same. Also note that the 756Pro shuts off the RF slightly quicker than the 746Pro. Note that the 706 has no Send Control relay output and an interface must be used to connect from the HSEND line to the amplifier if not using an Icom amplifier. For these 2 waveforms, the orange trace is the CW key line, the cyan trace is the HSEND line, and the green trace is the RF envelope. The waveform below left shows the transmit delay at 2.5mS per scope division. Notice that the HSEND line goes inactive 4.5mS before the RF stops which possibly could be a big problem.
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