ICOM IC-718 Modifications (SWR Protection, AGC Defeat, CI-V to RS232 Interface)
I’ve been very happy with the IC-718 that I purchased last year. I also purchased a TCXO so I could work 60m someday and a 500Hz filter for the digital modes which are my primary interests at this time. Despite the lack of tubes it’s a great rig.
There are three things I didn’t like. The SWR protection circuit is much too aggressive, there’s no way to shut the AGC off and the TCXO oven turns off when the rig is shut off. I’ve fixed the first two items but haven’t decided on the TCXO. Having it shut down is good if you’re running on a battery… I may add a switch to make shutdown optional.
First some schematic errata. In my radio the resistors forming the divider that set the SWR protection comparator’s reference (IC1710B) did not match the values on the schematic provided with the manual. R1714 was 560K not 330K. R1715 was 33K not 12K.
To tame the SWR protection circuit I changed R1714 by adding a 1M in parallel with the existing 560K. Now the fold back starts at 5:1 rather than 2:1. Yes, I did have the nerve to key up a few times into an open and a shorted line. The finals did not go poof. As with any modification, do this at your own risk.
To provide a means of defeating the AGC I added an NPN transistor across Q1805. The collector of the added transistor is is connected to the base of Q1805 and their emitters are connected together. The added transistor is controlled by a small toggle switch mounted on the back of the rig which applies 8V to the base of the added transistor through 22K resistor. There is also a 100K resistor connected from the base to emitter of the added transistor to keep it off when the switch is open. When the switch is closed the added transistor conducts shunting the signal away from Q1805 which defeats the AGC.
CI-V TO RS232 INTERFACE
Although not a mod it’s a very handy thing. I’ve tested this interface at 19,200 baud for 5 days of continuous commands while running a scanner program and there were no errors. No problems with any HAM software I’ev tried. I’ve very happy with the results. Here is the schematic CI-V TO RS232 Interface (PDF).