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[Yaesu] FT-890 PA heating up--in constant TX

To: Yaesu List <yaesu@contesting.com>
Subject: [Yaesu] FT-890 PA heating up--in constant TX
From: Nate Bargmann <n0nb@n0nb.us>
Date: Sun, 3 Jun 2018 20:12:36 -0500
List-post: <mailto:yaesu@contesting.com>
Hi All.

Hopefully there is someone quite familiar with the FT-890 on the list.

I am looking at a friend's '890 and discovered that that the top panel
above the heatsink is very warm.  A quick Google search found someone
else with the same problem and most suggestions were to measure the
current on receive.  I did so and after watching it for a good while it
had run up to about 2.9 Amps.  The factory spec is 1.5 Amps.

I opened it up and checked the PA idling current and it was running
around 9.8 mA when it should be around 6 mA.  I spent some time
adjusting it to be around 6 mA at its warmest, so I put it back together
and measured the receive current and it was around 2.28 Amps once warmed
up.  That is quite a bit less, but still about 700 mA more than spec.

I spent some time tracing things around the PA (100W version) and have
come to assume the PA is fine as the T9 line coming from the Local Unit
is resting at 1.3V on receive when it should be 0V.  On TX it rises to
the nominal 8.8V shown on the PA schematic.  As I understand it, this is
keeping Q4009 on and activating the PA for transmit mode.  Other
voltages I measured around the PA are consistent with the TX values on
the schematic.

The T9 line seems to be sourced from several places on the Local Unit.
What might be the likely culprit?  The 1.3V on the T9 line at the PA
unit is constant regardless of mode selected.

I am working from a scanned copy of the Technical Manual on my computer
and the resolution becomes poor as I zoom in, especially for the Local
unit schematic.


- Nate


"The optimist proclaims that we live in the best of all
possible worlds.  The pessimist fears this is true."

Web: http://www.n0nb.us  GPG key: D55A8819  GitHub: N0NB
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