Modifications for the ICOM - RogerK

26-06-2001(IC-575) Pass band Tuning ModificationAuthor: Adam MAURER, VK4CP - vk4cp@bigpond.com.MODIFICATION.NET(for those who have that silly Data Level control)This article appeared in "Six News" Issue 40Although specifically for the IC-575, this modification should apply to both the IC275 and IC-475.17 June 1993 (Revised 15 October 1996)The early versions of the Icom IC-x75 rigs had Pass Band Tuning (PBT), a feature left off later models, but with minimal effortthis feature can be retrofitted. An improvement in 2nd I.F. selectivity also results.The PBTless models use the “PBT” potentiometer as a "Data Level" control which adjusts the level of the external audio inputfrom the accessory socket on the back of the rig. The most obvious use for this is for a RTTY/packet modem. In my opinion the"Data Level" control does not need to be on the front panel because once it has been set correctly for a particular TNC etc. itshould not require tweaking.Interestingly, the "Data Level" pot is still a centre-detent type like the original PBT control. Examination of the schematic revealsthat it's the same value, 10K, as the original. Furthermore, the PBT line from the D.A.S. unit is now connected to a fixed-valueresistive divider, made up of two 4k7 resistors. Hmmm!A comparison between the "old" schematic and the "new" schematic also shows that a ceramic filter, FI3, has been deleted fromthe newer radios, being replaced with a 0.0047uF capacitor. Other than that, everything else is still there. When you think aboutit, it's the obvious and cheap way out for Icom to remove this feature by making minimal changes to the design and to "hardwire"the PBT to the centre position, rather than making major changes to its production run.The "missing" filterYou will need to purchase a Murata CFJ455K5 filter. The specifications of this filter are;Centre frequency:Bandwidth:Passband Ripple:Input Impedance:Output Impedance:455kHz2.4kHz @ -6dB2dB2k Ohm2k OhmPicture 1 - The Murata filter fitted to the “Main Unit” PBA after the modification.Modification procedure1. Remove all connections to the set, eg; power, antenna, mic, accessory leads etc.2. Remove the 9 screws holding on the bottom cover.Position the radio so the front of it is towards you.All the mods are done to the Main Unit PCB (the largest PCB).