How Do I Repair The Detector Diodes In The Internal Atu; How Do I Switch Between More Than Two Antenna Ports - Icom IC-746 Faq

3.2.17

How do I repair the detector diodes in the internal ATU?

???? (Mike) describes his experience at fixing blown detector diodes in the internal ATU.
The problem was that the Auto-Tuner would no longer tune up the antennas that it used
to for me, and the SWR didn't change on the antennas (as measured by my MFJ-259
antenna analyzer). It would either try for 6-10 seconds and then drop out, or try for only
one or two seconds and drop out. When it "dropped out", the Tuner indication would go
away on the display.
Rather than sending it in for repair, I managed to figure out the bad parts were the D8
and D9 Diodes on the Control Unit. They are HSM88AS diodes. They are a 3 lead
device, and look like a surface mount transistor. They have "C1" indication on the top.
These diodes are located NEXT to the orange "L9" inductor close to the speaker, just
below the 820pf caps near the open core inductors.. If you have trouble locating it from
the description above, you'll either need the manual from Icom to locate the two diodes,
or I have a picture that I took is available showing the location.
Looking at the specs for these diodes
http://www.hitachisemiconductor.com/sic/jsp/japan/eng/products/standardics/diod/lin
eup/diode/eHSM88AS.pdf
They have a minimum 10 V (sub) R voltage rating (not too high) which may explain why
they blow so easily (but then I don't know the signals that they receive under normal
circumstances).
Notice also from the specifications that each part really has 2 internal diodes. By looking
at the schematic for the radio (that comes with your Owners Manual, D8 and D9 both
have open leads where one diode is NOT used. They are on OPPOSITE sides!! What
you can do (one time only) is to swap D8 and D9 and the radio should work again. I
found that the D8 diode was leaky. I never did find the D9 diode to be able to measure it
(notice the warning below!)
HOWEVER, this repair procedure is not for the faint hearted.
These are surface mount parts and are about the size of 2 grains of sand. To replace
the parts requires a very small tip soldering iron, a fine tipped pair of tweezers and a
very steady hand.
What I did was to use some solder wick to remove much of the solder. I was then able
to remove the parts with reasonable ease. Just as a warning, when I changed mine out,
once the solder joint released, my D9 when flying. I never did find it. Lucky I had
already ordered spares.
If you're brave enough to try it, I'll wish you the best of luck, and add that I give no
guarantees that this will fix your rig, but it did fix mine.
3.2.18

How do I switch between more than two antenna ports?

With a switch, of course.
Actually, you're probably looking for an "automatic" way to switch between different
antennas based upon your current operating band. For that, check out "Automatic
Antenna/Amplifier Selection by Band" mods elsewhere in this FAQ.
IC-746 FAQ
Page 35 of 113
2003-02-08