Icom IC-746 Faq page 71

variable by the dimmer function on the radio, then this transistor is probably OK. Q303 and
Q304 are the next most likely candidates. These 2 transistors are in push-pull configuration
to drive the X-former (base is 180deg phase shift by the X-former)
Refer to pages 6-1, 9-1, 10-1, and 11-2 in the "Service Manual" for more details.
Several folks at:
http://groups.yahoo.com/group/IC746/database?method=reportRows&tbl=2
have successfully performed these repairs themselves.
OZ5ACI feels that the way the Q302 transistor is mounted is a design flaw. Icom used the
cooling fin of the transistor as a soldering point, thus if it gets too hot, the solder might melt.
That slowly deteriorates the soldering material resulting in a lost connection and a cooked
transistor. He has a feeling that future cooked transistors can be prevented by simply
unsoldering the transistor cooling fin to lift it and lengthening the center leg to establish
connection at the cooling fins soldering point.
AH6LE counters that if the transistor gets hot enough to melt solder (300 degrees or so), then
there are other more serious problems. Most solid-state devices won't survive such
temperatures, thus we're seeing the result, not the cause.
The repair
One of the techs at MTS (a company that repairs Icom gear) told K8UT that the repair is not
difficult, but he does not recommend it to those without soldering/desoldering experience.
The repair involves disemboweling the rig to get the S-logic board out; removing a metal
shield that covers the transistors (about six solder connections); and then replacing the
surface mount transistor. And, of course, reversing the whole process to put the rig back
together again. The part itself is cheap - around $3.00 (Feb-2002).
If you're tempted to unsolder the control pots from the S-Logic board, stop and think; you're
probably doing things the hard way. The affected transistors are on the side of the board
facing the front of the radio. More extensive "mechanical" disassembly will eventually gain
you access. Start by removing the front panel and work your way back. Page 7-2 of the
Service Manual should help.
Icom UK told GW3WSU:
"The transistor (Q302) is available from stock but is a 2SB1201, it is only 47p + carr and
vat, but unfortunately is below out <A3>10 minimum order charge. You do not have to
unsolder the pots, remove the plastic front panel, then undo the pot fixing nuts from the
metal sub-chassis, and it should all just come away. When you come to replace the
transistor, its best to remove the screening can, this gives better access to the device.
After replacement I would advise you to put a generous drop of heatsink compound
across the transistor and spread it over to the metal of the screening can, this will keep
the transistor from getting too hot again in the future."
OZ5ACI used an "overkill" transistor from his junk box and had to cut a bit of the cooling fin
to make it fit under the shield. The original transistor is not very tall and can be left at an
angle so no cutting is needed (or wanted!). He says it's all fairly simple. The only trouble is
that the front section of the 746 has to be taken totally apart and getting it back together with
IC-746 FAQ
Page 71 of 113
2003-02-08