Modifications for the ICOM - RogerK

The difference as measured on an audio spectrum analyzer was quite significant.I'm not sure if this helps for the Mk II and Mk IIG - maybe Icom have already adapted them....Thanks to http://www.qsl.net/vk4cp/ for the modification.This modification is read 3702 times.top of page25-08-2001(IC-706) Improved VHF recieve mod (see also yellow wire mod)Caution: This is quite involved. If you are not too good you might be advised to get someone who has the experience to handlethis mod! This mod requires the service manual, a propane torch for desoldering the SMDs, and desoldering braid for cleanup.The two VHF Rx filters are on the PA unit, between the EXT SP jack and L29 L30 (two tunable metal boxes). The wide filter isused if the B8W signal is active ([60--144[ and [148--200] range).L29 and L30 form the 144-148 band pass filter, with a 2 dB loss.The wide filter works from 60--120 MHz, see plotrxold.pdf. Since transmitting below 120MHz is impossible due to spuriousemission at ftx+70MHz, I decided to modify the rx filter for a 110--180 MHz operation.Note that the low pass VHF input filter has a -7dB roll-off at 160 MHz.New component values:C57 1nF L26 150nH L40 39nHC56 47pF L25 120nH L49 33nHC55 18pF C52 0pF C51 1nFC54 32.6pF (27+5.6) C53 15pF C153 12pFC151 15pF C152 15pF C154 1.8pFCaps in 0603 size, Farnell or Johanson. Inductors in 1008CS size, Coilcraft. Results are in plotrxnew.pdf and p_nf.pdf.Noise figure data.This modification is read 3583 times.top of page25-08-2001(IC-706) To expand the band on the VHF portionVHF is controlled by the signalsT8 high on Tx (IC11 pin 16)R8 high on Rx (IC11 pin 17B8 [144--148[ MHz (IC31 pin 14)B8W [60--144[ or [148--200] MHz (IC31 pin 13)L8 [60--200] MHz (IC32 pin 11)Out of band Tx is impossible due to two reasons.a. Transistor Q38 realizes B8W and (not R8) to short circuit the VHF signal into the HF low pass filter.b. The IC38 TA4001F VHF pre-amp is only activated if B8 and T8 holds.1. Remove the main board from the unit. Keep the cutout hole away from you. This I consider the top of the board.2. Flip the board over its vertical axis and look at the underside. The cutout hole is now on the left. Locate IC32 on the right.There are two IC chips, and IC32 is a bit more to the bottom and to the right. The chip is at 3 cm from the right edge andcentered vertically. Pin 1 is the top right pin.16 15 14 13 12 11 10 9| |