Modifications for the ICOM - RogerK

19-07-1998(IC-475) IC-475 Freq. steps modTitolo: Tuning steps change for all mode Icom IC-275/475/575/1275The IC-275/475 FM-Step selectmatrix use these diodes on the front unit: (type 1SS53) D41, D42, D43, D44, (D45), D46Note: diode D45 is not present but solder holes exist for it on the board just between D44 e D46installed diodes are marked with X :(*) (*)D41 D42 D43 D44 D45 D46 D48 Steps DB4 DB5 DB6 DB7. . . . . . . 10.0 Hz. . . X . . . 100.0 Hz 1 0 0 0. X . . . . . 1.0 KHz 0 0 1 0X . X . 5.0 KHz 1 0 1 0. . . . X . . 10.0 KHz 0 1 0 0. . . (*) X X . 12.5 KHz (*) 1 1 0. . . . . . X 25.0 KHz 0 0 0 1X . X . . . . 1.0 MHz 0 1 0 1(*)To set a step of 12.5 KHz in FM mode (TS button off) it is not necessary disconnect the diode D44, just insert the diode D45(IN4148 or similar) near D44 and "voil? les jeux sont faits".Unscrew the two lateral screws and unlock the front panel.Remove also all the connectors of the front board and after that unscrew ALL the screws of the metallic shield covering the frontboard. It seems too much complicated insert the diodes from the component side of the front board, so it is better insert D45from the solder side.This modification is read 545 times.top of page27-10-1998(IC-475) Power control modificationProblem:The power control circuit in IC-x75 is an ALC circuit. ALC circuits suffers from several drawbacks:●●slow reaction time due to time constantsinsufficient regulation due to low feed-back factorThese factors gives the problem that if the average power has been set to a low value (e.g. for driving a HPA), peak power canstill be in the order of 3-4 dB higher. This leads to either considerable side band distortion or a low average power output.note: If you are always using your transceiver barefoot, without connecting any PA or transverter, this modification will offer nobenefit.Solution:The solution to these problems is to use a simple forward gain regulation without the feed back circuit of the ALC. The originalALC is still kept for SWR protection only.The ALC is working on gate 1 of Q13, which is a IF amplifier (9 or 10.75 MHz). The new power regulation uses gate 2 of Q13.Therefore the two circuits can exist in parallel.The modification should also give you a bigger dynamic range in the power regulation (lowest power < 5W on the IC-x75H).Implementation:●●replace R87 (470k) with 100kcut "POCO" wire (pin 3, P1/J1, MAIN, orange) at the plug (P1)● connect the "POCO" wire to gate 2 of Q13 (e.g. via top of R87)