Icom IC-746 Faq page 72

no dust behind the plastic screen is a pain. Get yourself a small can of workshop compressed
air or CO2 if you can.
VK4TZL (Glenn) reports:
"The actual repair is relatively easy...assuming that Q302 is the problem. The whole
repair took me one hour. Removing the front panel from the radio, by removing 4 screws
and the one ribbon connector lets you work on the front panel by itself as an assembly.
You can remove the S Logic Board by removing all the large knobs, the tuning knob, the
tuning knob friction break and then two ribbon cables from the display board.
On removal of the tin lid on the shielding box housing Q302, it was obvious that this
thing was getting HOT. It had unsoldered itself from the pad under the collector, and I
think this was the cause of the "flashing" as the collector was left floating. I decided to
mount the new transistor vertically, and add a small L shaped brass strip from the
collector to the pad on the board, in an attempt to keep it cooler. I removed the tin plate
shield to enable this to be done and have some room to solder."
The part
So far, all reports from heftier transistor replacements have been positive.
From page 6-1 of the service manual, we can see that Q302 is a 2SB1201. At one point, the
folks at MTS suggested a 2SB1202 as a replacement. Examining the specifications:
http://www.nedis.com/datasheets/datasemi/2SB/2SB1201.pdf
http://www.qsl.net/kb9sss/IC-746/Parts/2SB1201.pdf
http://www.nedis.com/datasheets/datasemi/2SB/2SB1202.pdf
http://www.qsl.net/kb9sss/IC-746/Parts/2SB1202.pdf
We can see that the suggested replacement is physically identical to the original part, but
with 50% higher current capacity and 25% higher power dissipation. Its switching time is
about 10-20% longer (slower), but that should be unimportant for this application.
K8UT has successfully installed a 2SB1202 replacement.
The Transformer
In some cases, the transformer is also fried. Watch out for that when you make your repair,
otherwise you'll just blow the transistors again.
Further Analysis & Possible Prevention
WA0KWK observes that we appear to have a DC-AC converter circuit with Q302 operating
as a current carrying switch being turned off and on by Q301. (a light dimmer).
When the backlight is dim, the switch gets hotter because it is dissipating the current rather
than the backlight. When the backlight gets brighter, more power is required by the display
and the transistor has to sink less current by itself.
This leads to the counterintuitive deduction that brighter displays put less strain on the
transistor than dimmer displays. Thus, the natural reaction (turning down the display
brightness) actually makes the problem worse.
IC-746 FAQ
Page 72 of 113
2003-02-08