Msg Nr. : 56622 Da : DK2GO Per : ICOM @WW Tipo/status : BF Data/ora : 1025/1225 Dimensione : 5117 Titolo : Alcune modifiche per l' IC R70 Bid (Mid) : 24A310HB9EAS de DK2GO @ HB9EAS.CHE.EU to ICOM @ WW Icom IC-R70 modifications ------------------------- The selectivity of the original IC-R70 in the AM mode suffers from the poor shape factor of the ceramic filter in the 3rd IF stage (455 kHz). It can be improved by 1. Replacement of the ceramic filter and/or 2. Adapting the 2.4 kHz SSB filter for AM Caution: These modifications require some technical experience. Have the circuit diagram and the circuit board layout ready (both included with the IC-R70) should. It is necessary to remove the top cover of the receiver as well as the MAIN circuit board. The author cannot accept liability for any damage that might be due to these modifications. 1. Replacement of the ceramic filter ------------------------------------ There are replacement filters available for the original FI5 (CFW 455 HT) on the MAIN board. The following table lists a few of them (available from Murata or other suppliers): CFW 455 HT CFK 455 H CFK 455 I ------------------------------------------- min. bandwidth @ -6dB 6 kHz 6 kHz 4 kHz typ. bandwidth @ -6dB 9.5 kHz 7.8 kHz 5.5 kHz max. stop bandwidth 18 kHz(-50 dB) 15 kHz(-80 dB) 10 kHz(-80 dB) Input and output impedances are 2000 Ohms. The only problem is the size and the different pin layout for the replacement filters: +---------------+ 3*ground -> I o o o o o I <- input I I output -> I o I CFW 455 HT +---------------+ +--------------------+ 3*ground -> I o o o I I I CFK 455 H / I output -> I o o I <- input +--------------------+ CFX 455 H/I are smaller in size than the CFK filters, but hardly available. In any way, it is necessary to mount the replacement filters slightly 'elevated' and connect them to the main board by short pieces of 0.5 mm wire which are attached to the replacement filter. Keep the additional wire as short as possible in order to avoid 'IF leakage'. It is also possible to put two replacement filters in series (mounted on a separate small board). Signal loss by the additional filter is still acceptable. 2. SSB filter used for AM reception ----------------------------------- The FM button (which is useless unless the FM adapter has been installed) is used to switch the 2.4 kHz filter (FI 3) in place of the 6 kHz filter (FI 4) during AM reception. Some re-wiring is necessary: a) Disconnect the wire labeled "FM" from R67 and connect R67 to Ground. b) Connect the FM wire through a diode (D76,1N4148) to R75. c) Replace the connection from Q12 to R75 with another diode (D75,1N4148). I D31 R75 -->I--+ I I Q12 ----+------>I---+---I<---- "FM" wire R67 I D75 D76 I ground d) Cut the connection from D45 to R50 and connect the cathode of D45 to R44 instead. AM D46 ------------->I---+-----R44--- I FM D45 I ------------->I---+ The selectivity will be drastically improved, especially if the original ceramic filter for FI 3 has been replaced by the crystal filter FL-44. 3. Sensitivity improvement -------------------------- If overloading by strong medium wave broadcast stations is not a problem, you can make the HF preamplifier switchable for the 0.1-1.6 MHz portion. Just cut the red wire that connects L15 with R59 on the main board. 73 es 55 de Uli DK2GO @ HB9EAS.CHE.EU