ICOM - IC756ProIII - HF - 50MHz

Report updated 19.04.2007

Even in the first quarter of 2007, the performance of the  PRO series' third generation still successfully  challenges equipment of other makes in this market sector (such as the VS FT-2000, which is its direct competitor in this price class.) The main ergonomic aspects of the two radios are shown below:

(click for large images)

Apart from its specific qualities or differences as compared to the FT-2000, the IC756ProIII is distinguished  by the following: Simplicity of operation -- TFT Color Display --  1st IF Roofing Filter at 64.455 MHz --  IF DSP at 36 kHz final IF  -- Built-in Real time Spectrum Scope, independent of main receiver -- Soft & Sharp filter shape factor selection -- Digital Twin Passband Tuning - 32-bit IF-DSP + 24 Bit ADC/DAC -- Built-in RTTY decoder for RX and TX (with 8 memories) - Dual Watch.

IC-756Pro III - RF-B Unit with BPF-A and BPF-B Board (on two separate boards and levels)

RF A Unit: The new BPF2 board, with larger inductors, selects the 5 lower HF bands (B1: 1.60 ~ 1.999 MHz. B2: 2.00 ~ 2.999 MHz. B3: 3.00 ~ 3.999 MHz. B4: 4.00 ~ 5.999 MHz. B5: 6.00 ~ 7.999 MHz.) The filters are diode-switched, using Toshiba 1SV307 epitaxial diodes. 3 new SDS Model ATN203 double-contact relays (RL101/121 and RL122) switch the RF input and the attenuators (6, 12 & 18 dB). These relays are similar to the FTR-B4CA relays in the 7800. For the 8.00 ~ 60.00 MHz range, the RF signal passes through the 6 sections of BPF1, which is implemented in SMD components on the base RF A Unit board. Note: VS' design engineers have chosen a similar layout in the FT-2000 with 2 separate relay-switched BPF boards. The passband of each BPF is wider than the respective HF amateur band, but the VRF Unit (preselector) precedes the BPF group in the receiver's RF signal path.

Report as of 20.03.2007

This page presents an examination of my IC-756Pro III Transceiver, which will provide additional information on technical and construction/packaging aspects of the radio. (Specifications are subject to change without notice or obligation. please check with your Icom Dealer for details.)

IC756Pro3: The new transceiver has been opened very carefully for the technical tests, and for the descriptions of the changes (relative to the IC756Pro2) illustrated in the photos below.

IC756Pro3: Overall underside interior view. Note the die-cast chassis, which encloses the circuit modules in shielded compartments with very neat and logically-arranged intercabling. The vent slots (left rear) improve PA-stage cooling to prevent excessive temperature rise, even whilst receiving.

5" TFT Display: Real-time spectrum scope (1 kHz resolution bandwidth), with the "mini-scope" option allowing the operator to monitor the frequency spectrum whilst using another menu. Example: Multi-Function Meter in lower zone; Spectrum Scope in centre zone; other data (Main & Sub VFO,  Filters, BW, SFT*, Twin PBT, local & UTC time) in upper zone. *SFT = Twin PBT Shift.

Filter Unit - 100W PA: New picture of the other end of the transmitter LPF board. Note also the new toroidal RF filter chokes (white cores) in the main 12V DC power feed, and the ferrite beads on the red (+) and black (-) power leads.

Filter A Unit - PA 100W: The PA RF output signal is filtered via 8 LPF sections switched by 14 Taiko TB160 relays for the HF bands, and 2 Nais AG201344 relays for the 50 MHz band. The filters have been modified (with respect to the IC-756Pro II: Trans.) and cover the following bands: (L1: 1.9 MHz. L2: 3.5 MHz. L3: 5 and 7 MHz. L4: 10 MHz. L5: 14 MHz. L6: 18 and 21 MHz. L7: 24 and 28 MHz. L8: 50 MHz) (Note: New band, 5.33 - 5.40 MHz assigned channels: USA and UK versions only)

Roofing Filter: The new 64.455 MHz roofing filter with 15 kHz BW (identical to that of the IC-7800) is located at the 1st IF output of the balanced quad (1st) mixer. This yields superior selectivity against adjacent signals, thus reducing the distortion of the received signal. In the photo, the roofing filter, marked N^o 89, can be seen between the two IF impedance-matching transformers L1701 and L1702.

RF B Unit - First Mixer & IF: The mixer circuit (Quad FET analog switch array) employs four 2SK1740 JFETs (Sanyo N-junction silicon FET) and has been completely redesigned. As can be seen in the photo, the circuit and the components, whether discrete or SMD, are completely new as compared to the Pro II mixer. Similarly, a quad switch array is used in the IC-7800, but is implemented as an SD5400CY chip (High Speed DMOS Quad FET Analog Switch Array), while the separate 50 MHz mixer uses an MMBFU310 N-FET (similar to the J-310).

RF A Unit Preamp: The two new Preamps are constructed on a separate board (B6185A) with components similar to those in the 7800. Preamp 1 utilizes two Sanyo 2SC5551's in push-pull with feedback. Preamp 2 is a single 2SC5551 transistor. The circuits follow the same design principles as those of the IC-7800. The 2SC5551 is an NPN high-frequency planar epitaxial silicon transistor (f_t = 3.5 GHz).

Tuner Control A Unit (#38xxxx Italy version): Now the two toroidal filter coils are installed in the Antenna input line, between T/R switching relay RL4 (Matsushita AE5343) and the LPF (box MP1) towards J101 (Output to RF Unit).

RF B Unit, J1651: On examining the schematic and photo of the RF B Unit in-depth, I have identified an  Output J1651 (an unused mini-coax jack, top centre right.) J1651 is connected via a 5 pF capacitor to the 64.455 MHz first IF signal path between L1018 (Dual Watch Rx A & B 1st-IF combiner) and L1701, the roofing-filter input matching coil.

J1651 is the Mixer Balance check point documented on pp. 4-8 and 4-9 of the IC-756Pro III service manual. A spectrum analyser can be connected to J1651 to adjust first-mixer balance during alignment.

Note: The technical data and specifications on which the above information is based are those provided in the official IC-756Pro III brochure issued by Icom or by the authorized Italian Icom distributor. Please read the Instruction Manual carefully before operating the IC-756Pro III.

Please send comments to a lidioperugia+gej@gmail.com - thanks!