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IC-475.IC-275 owners



Hi Guys,

I have been chasing a problem here in my IC-475 that may or may not be a
problem.  It originally started as an intermittent receiver sensitivity
problem.  I began checking out the RX 8v power to these stages and
eventually worked my way back to Q55, a 2SD468. It is a regulator pass
transistor, located almost next to the three terminal 7808 regulator which
is mounted on the big heatsink in bottom of the radio.  

I noticed that the emitter lead solder pad was discolored and the solder
appeared crystalized. The board around the bottom of Q55 was darkened a bit
due to the heat.  Q55 was too hot to keep a finger on for very long.  I
finally found that Q55 was passing a total of 139ma and it was rated for an
Ic max of 1 amp.  It didn't look like an excess of current drain.

Then why was it getting so hot.  Perhaps because of the resistive emitter
to pad joint.  Maybe it was always a poor solder joint, maybe not.  Anyway
I resoldered the Q55 emitter pad and thought that would fix the problem. 
It did fix the sensitivity problem, but now, what about the hot Q55?  It
was still hot.  I just couldn't button it up again without finding out why
it was so hot.

I have now obtained a second IC-475 to make some comparisons.  When I
opened up the bottom, I find that Q55 in it is also hot!  The board is not
darkened, but it seems just as hot to the touch.  After all this had I
already fixed the problem when I resoldered the Q55 emitter in my radio?  I
don't know.

I have a photo (not attached) that I will send to anyone wishing to check
out his own IC-475 or IC-275, that shows the location of Q55.  If you do, I
would be extremely interested in what you find.  Just open the bottom cover
(5 screws on the bottom and 2 on each side).  You will be looking at the
Main Board and a smaller YGR board with the long aluminum heat sink between
them.  Let the radio run in receive mode for 10-15 minutes.  Then go feel
Q55.  Is it cool?  Just warm?  Or too hot to touch for any length of time. 
Is the board discolored around its bottom?  You may want to check out the
solder connections, but this will require removing the entire Main Board, a
fairly easy job, but complicated by having to remove 15 cable plugs which
you may not feel comfortable doing.

Again, I would like to have any reports, cool or hot, that you can send
me.  Let me know if you want the photo with a pointer showing the location.

---
73, Roy

Internet: w0sl@amsat.org
Home Page: http://home.swbell.net/rdwelch
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