Modifications for the ICOM - RogerK

70% AM.My personal findings are that this do MW listening much more interesting and enjoyable and I have no problems with to strongsignal distortion what so ever.SM4RNA AndersThis modification is read 1124 times.top of page19-07-1998(IC-781) IC-781 General coverage transmit mod1. Remove power and ant.2. Remove screws open case.3. Locate and cut resistor r80 on logic b board.4. Reset cpu.5. Reassemble radio.This modification is read 1007 times.top of page05-03-1999(IC-781) 781 ALC Circuit problemIcom 781 Date 13/05/97The ALC circuit in the 781 is too slow to respond. This results in the following phenomenon: If you set the power level to 30W,for example, there is a leading pulse that can be more than 200W, particularly if the drive control is advanced. This results in aserious problem if you are using an amplifier. It makes it very difficult to limit drive to an amplifier properly.This behavior is clearly seen on a scope and to some extent on a good peak-reading meter. However I suspect many hams areunaware of the problem as it does not show up on RMS meters due to the short duration of the spike.I have used three 781's over the years ( ser# 19??, 21?? and now 2589) and they all exhibit the same behavior. This behavior isnot exhibited by other Icom radios I have checked such as the IC751A, IC735 or IC765.When I had my previous 781, the tubes in my amplifier, a Ten-Tec Titan using a pair of 3CX800A7's, developed shorted grids,even though they were only a few years old and lightly used. I suspected the 781 caused the failure, but sold it before theamplifier was repaired. I recently bought another used 781 from Burghardt radio, serial # 2589, and within 3 weeks of obtainingthe 781, my tubes (less than 2 years old) are shorted again. I do not believe this is a coincidence! I run the amp conservatively,and I doubt the tubes have 50 hours transmit time on them.I believe the problem is vaporization of the gold plating from the grid of the 3CX800 causing the tubes to become gassy, whichcauses arcing.I spoke to Technical Support at Icom USA and was informed that the 781's just work that way and nothing can be done. That isa ridiculous answer! Of course it can be fixed, the attack time in the ALC circuit has to be shortened.This turned out to be easy to accomplish....Locate the following components connected to the base of Q34 in the ALC amp, R107, R108, C47 & C48. These determine theattack and decay time of the ALC amp.Values as per Service manual:R107 1MR108 100C47 2.2ufC48 .47ufValues as per Production ser # 2589:R107 1MR108 100C47 .47uf