IC-756 Modifications
Note - some diagrams *Copyright © 1996 Icom Inc.
Spectrum Scope Adjustments
ACC1 Audio Output Increase
General Coverage TX Mod
Spectrum Scope Adjustments
Access to the RF Unit is required.
Remove bottom cover and then remove cover to RF Unit - see below.
To increase overall Scope gain - lower the value of R963 (100k as fitted).
I put a 56k 1/4w through-hole resistor across R963 - see above - making the combined resistance approx 36k.
(One resistor leg was soldered to pin 2 of IC-961 and the other leg to the junction of R966 and R964). To be neater you could remove the small 0603 SMT resistor and replace it with a 39k 0603 part.
To adjust Scope base-line: Trimpot R966 will adjust the base-line of the Spectrum Scope.
ACC1 Audio Output Increase
Access to the Main Unit is required.
The "Line Level" audio output on pin 5 of Rear panel connector ACC1 is very low, with the manual stating 300mV. Normal Line Level is regarded as about 700mV. Although my MiniDisc Recorder had a gain control for its Analogue I/P it still couldn't make 0 VU.
Here, by adding two resistors we can increase the gain of the audio appearing on pin 5 of ACC1.

  • Remove the bottom cover.
  • Remove 6 screws holding down Main Unit.
  • Remove screw on rear panel inbetween ALC & Send RCA sockets.
  • Remove heatsinking clip holding transistor to chassis.
  • Remove flat ribbon cable to J3 in far right corner.
  • Carefully pry PBA loose and fold over see picture at left. (I actually had to remove RF Unit shield and loosen a shielded cable to allow the Main Unit PBA to fold out).
  • Locate IC-503, a NJM2058 OPAMP in an SMT SOIC14 package. 
  • Remove W3 0603 SMT link.
  • Fit an 10k 0603 SMT resistor in W3's place.
  • Fit a 1/8w 5k6 through hole resistor from IC-503 pin 9 to ground.
We have changed OPAMP IC-503c from being a unity-gain follower into a non-inverting amplifier with a gain of approx 2.5.
*  Block Diagram of Audio Stage - Note AF AMP IC-503C which provides "AFO"
*  Before modification
*  After modification. W3 replaced with 10k and resistor to ground added to pin 9
Close-up of IC-503 - note copper area of "Ground" where 5k6 can be soldered to
General Coverage H.F. TX Mod (1.6 - 30.0MHz)
VHF remains 50.0 - 54MHz TX only.
Modification summary:
Remove the W805 jumper chip on MAIN unit by soldering iron.
Modification Procedure:
  1. Remove bottom cover of IC-756.
  2. On MAIN unit board you will see I.F. filters and daughter board (DSP PBA) on the right hand side.
  3. W805 jumper and IC803 chip located under DSP PBA.
  4. Remove DSP PBA (lift up carefully!)
  5. Look for IC803 and line of jumpers (0603 sized SMT links).
  6. Remove W805 jumper - as indicated below.
  7. Re-install DSP PBA
  8. Re-fit bottom cover.

                --------------

        [W801] |              |

        [W802] |              |

        [W803] |    IC 803    |

        [W804] |              |

Remove=>[W805] |     CHIP     |

        [W806] |              |

        [W807]  --------------

        [W808]