Modifications for the ICOM - RogerK

27-10-1998(IC-1275) 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)●●●●adjust R250 (SWR protection), R256 (low power level) & R259 (high power level) fully counter-clockwise. (this will give youfull SWR protection, but cancels the ALC in normal operation).put the front plate RF PWR in full output position (clockwise) and adjust R82 (Q13 output attenuator) until you have thenominal output power (initially the output power will be higher because you removed the regulation, but the PA will besaturated).the balance between FM, CW & SSB output can be adjusted with R137 (CW carrier level) & R105 (FM carrier level).other interesting adjustments: R135 (mixer balance i.e. SSB carrier suppression), R133 (TX BFO output level).This modification is read 535 times.top of page27-10-1998(IC-1275) Restoration of pass band tuningProblem:ICOM has chosen to remove the PBT in the later models, and replace it with a "data level adjustment". This change removes thepossibility to profit from the PBT in case of QRM or to use the PBT as a cheap CW-filter.Furthermore the two extra IF-mixings is done with only a minimum of filtering, making the receiver very susceptible to spuriousIF breakthrough. This is often heard as high-pitch tones or as a high IF noise level.Solution:The solution is to re-wire the "data level" potentiometer so that it adjusts the PBT VXO-oscillator, and to re-install FI3. FI3 is a455kHz ceramic SSB filter and can be bought under the type number: MURATA CFJ 455 K6.Implementation: