Modifications for the ICOM - RogerK

27-10-1998(IC-575) SSB RF GainProblem:The RF GAIN function is different in FM and in SSB/CW modes. In FM mode the RF GAIN controls a PIN-diode at the input of thepreamp. In SSB/CW the RF GAIN simply activates the IF AGC.The use of the preamp input attenuator (the PIN-diode) can be useful also for SSB/CW e.g. if you have connected a mast headpreamp with excessive gain. In this case the attenuator will help you to adjust the signal levels at the mixer, and thus reduceintermodulation and blocking.Solution:The solution is to remove the SSB/CW RF GAIN for SSB/CW (this function is of little use anyway), and enable the FM RF GAINeven in SSB/CW.Implementation:●●Cut the lead (FM8V) conneted to (pin5, J6, potmeter PCB, green) about 4 cm from the plugConnect the short plug end with R14 (on the potmeter PCB) by soldering it to the end that is connected to the plug (+8V)This will enable the FM RF GAIN in SSB/CW.●cut R30 openThis disables the SSB/CW RF GAIN, and you now have a variable input attenuation with a dynamic range of 10dB (IC275 withMI308/1nF) to 16 dB (IC475).Note: The IC-275 might have a shared PIN diode for FM RF GAIN and TX/RX switching. If you choose to split up the RX/TXpaths, you must install a new PIN diode (MI301 or equivalent) on the RF YGR PCB to implement this modification. See the noteunder the section dealing with spliting the RX/TX paths.This modification is read 546 times.top of page27-10-1998(IC-575) Tuning speedThe tunning speed is set to a defaults of 100Hz, 1kHz, 5kHz or 1MHz depending on front plate setting and mode. These settingscan be manipulated by D41 - 46 at the front plate. At least a setting of 10kHz is also available.This modification is read 542 times.top of page27-10-1998(IC-575) Frequency range & CTCSS tonesThe IC-275 have capability to cover 138 - 174 MHz and the IC-475 has capability to cover 430 - 450 MHz. Because of hardwarerestrictions they might not be fully up to specifications in all of the band, but an extended range can be interesting for e.g. usewith a transverter. It seems that the IC-275 has a parallel wide band preamp, that can be switch via the WBT pin on RF-YGRPCB.In the European versions the transceivers are wired for 1750 Hz repeater call, and they have no CTCSS capability. Thetransceivers have a built-in CTCSS encoder, so these functions can be restored.Both of these version dependent functions can be set by the diode matrix: D20 - D24 in the logic unit. I do not know the exactmapping, but a lot can be derived from studying the different frequency ranges and the diode matrix coding table found on theschematic diagram.This modification is read 576 times.top of page