The new Heil HM-i electret will fully drive the Pro with Mic Gainat around 9 o'clock. The transmit speech amplifier of the Pro has a little less gain than that of the 756,requiring a slightly higher MIC GAIN setting when using a Heil microphone with a dynamicinsert (HC-4 or HC-5). Incidentally, you can observe thespectral content and occupied bandwidth of your transmitted signal by setting "Scopeduring Tx = ON" in the "Scope Set" menu. The CAL control on the lower rightside of the chassis will center the marker correctly. The horizontal (frequency) display is inthe digital domain, and thus never goes out of cal.
The only alignment procedure for the IC-756Pro spectrum scope isvertical (amplitude) alignment and calibration. A signal of less than 1 µV is visible, whilst the 756 requires at least 20 µVto produce a spike. The vertical sensitivity of the 756Pro Spectrum Scope is significantly higher than thatof the 756.
#ICOM 756 PRO II CAT PROBLEMS MANUAL#
The fact that all the DSP IF filters, including the Manual Notch(but excluding the Auto-Notch) are inside the AGC loop sets the Pro (and the late,lamented Kachina 505) apart from all other amateur HF transceivers on the market. Also, the DSP IFfilters are inside the AGC loop, so strong signals outside the DSP filter bandwidth willnot swamp the receiver. With the Pro, you can optimise the IF bandwidth by tailoring it to the occupiedbandwidth of the received signal, thus yielding optimum S/N ratio.
The manual notch is alsohelpful in improving the SNR of the received signal. The ability to tailor thefilter passband to the received signal (using the Twin PBT or the filter tables) also provides a superb tool for pulling out the "weak ones". I am able to copy easily SSB signals which do not move the S-meter.Those signals would have been barely intelligible on the 756. The measured sensitivityon 20m with Pre-amp 1 on, and 500 Hz bandwidth, is 0.1 µV for 10 dB S+N/N (using an HP8640B generator). Overall, the Pro pulls the "weak ones" out of the noise noticeably betterthan the 756 (or any of its other predecessors in my shack) did. These are clearly due to the NB gatingon signal peaks, and are eliminated by switching the NB out. I observe significant artifacts under strong-signalconditions only when the noise blanker is enabled. The combination of the DSP-NR and noise blanker renders night-time 40m listeningmuch more pleasant and less fatiguing. The DSP-IFfiltering, including a tuneable notch filter, is all inside the AGC loop (unlike theIC-756). It makes an S9+20 undesired tone disappear off the S-meter. The manual pre-AGC IF notch filter (70 dB deep) isdynamite. Also, read George's "Notes on roofing filters" (below).
George, W5YR's IF Filter Page dramatically illustrates this point. The DSP IF filters have much steeper skirts than the analogue crystalfilters in the older rig, and are much more effective against adjacent-channel QRM than analogue filters. I find the Pro a big improvement over its predecessor, the 756.The Pro receiver seems much quieter than that of the 756 - probably due to a cleaner DDSLO implementation. In July 2000, I sold my IC-756, and bought a 756Pro from a local dealer. Adam, VA7OJ/AB4OJ's IC-756Pro User ReviewĪdam, VA7OJ/AB4OJ's IC-756Pro/Pro II User Review
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