Icom IC-746 Faq page 44

This continues to be a process of discovery, so I'll present what we know in that form.
Instruction Manual
Page 79 of the instruction manual says that the SEND jack is rated for 16 Vdc at 2A. Some
rigs have failed while well within those specs. Others seem to survive conditions far in
excess of that rating. Obviously, the ratings in the manual don't tell the full story.
Homebrew explanations (and solutions for) the problems abound. Some talk about bypass
capacitors while others talk about suppression diodes. Folks debate inductive vs resistive
loads and ratings. Surge currents and hot switching usually get brought up, as well as AC vs
DC loading. For years, the topic keeps coming up, but with no consensus or dependable
solution; except that using an isolation relay/circuit seems to prevent any overloads.
Tech Support
So, many of us contacted Icom technical support. Unfortunately, we received different
answers at different times. For example, in the icom@qth.net archives for January 2000, we
see that tech support told AB2BK that there was a misprint in the manual. The true rating
was 12 Vdc at 20mA and Icom recommended an external isolation relay. While that would
explain the failures, it makes some of the successes almost unbelievable. On the other hand,
the archives for February 2002 show that tech support told N1JM that "The real scoop is UP
TO 16V -OR- UP TO 2 amps." Again, Icom recommended an isolation relay.
While the techs did not agree upon the ratings, they did agree upon using the external relay.
Unfortunately, none of the answers from tech support have adequately explained the odd
successes and failures.
Service Manual
All of this boils down to how much abuse relay RL1271 on the Main board can take. The
parts list on page 6-12 shows the relay as an OMR-109F. The obvious solution is to look up
the parts and see what they are rated for. As of January 2002, nobody had revealed any spec
sheets for the relay.
Internet Search
Searching for "OMR-109" on the Internet did turn up a curious tidbit. Icom has used the
OMR-109 before, as the SEND relay (RL2) in the IC-761. It had problems too. There is a
Service Bulletin (#24287-001A) entitled "Larger relay for keying non-ICOM amplifiers"
which recommends:
Replace RL 2 (currently labeled OMR 109) on the PA Connector Board with a SY-12
relay (ICOM P/N 921-04696). This relay is larger but can be squeezed into place. Glue
this relay to the board with RTV.
For the full bulletins, check out:
http://www.qsl.net/kd4sai/mods/icom/ic-761.txt
http://www89405.temp.w1.com/Modifications/B2502
IC-746 FAQ
Page 44 of 113
2003-02-08