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Re: IC-475.IC-275 owners



Roy,

Have you checked to see if there is adequate thermal paste under the
transistor.  If it is a lot hotter than the heat sink, I would think there
was not adequate heat transfer.  I have run into this many times in
commercial equipment where there was little to no paste used.  Have you
checked the voltage drop from collector to emitter to determine the power
being dissipated?  I am sure it is normal but there could be several watts
dissipation.

73,
Harold Reasoner, K5SXK

----- Original Message -----
From: "Roy Welch" <rdwelch@swbell.net>
To: <amsat-bb@AMSAT.Org>
Sent: Friday, January 14, 2000 2:11 PM
Subject: [amsat-bb] 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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