IC-746 FAQ
Many people use an external ATU along with their IC-746. Usually, they leave the external
ATU switched to "bypass" and let the internal ATU do its thing.
For troublesome
bands/antennas, they turn off the internal ATU and engage the external one.
Others use an external ATU to match their antenna to the middle of the desired band and let
the internal ATU handle the rest. In theory, the ATU doesn't know or care what's hooked up
to it; it simply tries to match up to the impedance it sees.
In most cases, either of the above approaches work fine. Many hams have been applying
both for years with no problems.
However, some IC-746's have had their ATUs go out from fried detector diodes and driver
transistors. On many repairs, Icom has blamed the failure upon external tuners. One ham
was told that external tuners can cause problems even when they are switched to "bypass".
Nobody has been able to adequately explain why an external tuner would cause such
damage, particularly when bypassed. Some speculate that tuning with low power is safer
than tuning with high power (if nothing else, its at least more polite).
We need more information on this, but at least you've been warned.
The best procedure we can suggest at this point is:
•
Turn off the internal ATU.
•
Reduce power to about 1/4 (25w).
•
Adjust the external ATU using the rig's SWR meter. (The two meters usually disagree.
It is what the rig sees that is important.)
•
Restore full power.
•
Turn on the internal ATU.
Do not adjust the external ATU at full power.
Do not adjust the external ATU while the ATU is on.
Do not change frequency while the ATU is searching for a match.
3.2.7
Does the internal ATU work on receive?
Yes. The internal ATU is enabled while receiving. Of course, it can only "match" while
transmitting.
It's unclear if the internal ATU acts as an additional preselector when enabled.
3.2.8
Any other "gotchas" with the internal ATU?
We've already mentioned above the possible hazard of using the internal ATU and an
external ATU at the same time.
We've also mentioned above that operating the internal ATU at the limits of its abilities
might overtax it over time.
Page 30 of 113
2003-02-08
