[Elecraft] One of my 2SC1969s keeps overheating...

[John A. Magliacane]

Hello All.

One of the 2SC1969 RF power amplifier transistors keeps overheating
and dying in the process.  I thought I would bounce a problem I've
been having with my K2/100 with folks on the list before I purchase
yet another set of 2SC1969s...

First some background:  The rig (s/n 3563) went together without a
hitch.  The KPA100's RF power level circuitry was calibrated against
a Tektronix 400 MHz scope (rather than a wattmeter), and seems to be
quite accurate.  The KI6WX mods (2.5 kHz bandwidth and 10 db extra gain)
were performed on the SSB module.  The rig sees most of its use on 75
and 40-meter SSB at the 100-watt level.  I have my current limit set
to 3.5 amps, and the "Hi Curr" warning never displays at that setting.
The KPA100's fan cycles on and off normally while transmitting.

Everything seems to work just fine, howeever several weeks ago I
noticed my maximum available RF power is considerably reduced on the
higher HF bands.  (About 50 watts on 20 meters, about 37 on 18, and
about 25 on 10 meters).  It took a while to discover the cause was
a dead 2SC1969, since the rig works fine on the lower bands without
any sign of a problem.  Apparently, there's enough gain in the
transmitter on the lower frequencies that full power can be
obtained on 40-meters and below even if one of the 2SC1969s
is "open".

I ordered a replacement set of 2SC1969s, and upon opening up the
rig, discovered one of the transistors may have not been mounted
properly.  The transistor in question had a metal backing with a
"grainy" texture.

The replacements went in using the same thermal pads as those used
originally.  The only other item replaced was a shoulder washer,
which had melted slightly to the overheated transistor's metal tab.

I made *EXTRA* sure the transistors were mounted correctly when they
were replaced.  I tested the rig on 10-meters, and with the KPA100
bypassed, got very close to 10-watts out in "tune" mode.  After about
one minute of key-down time, I felt the temperature of the bottom
heatsink, and the areas around each transistor felt equally warm.
Neither mounting screw felt overly hot.

The rig worked fine on 75 and 40, but after making a few QSOs on
these bands, I noticed greatly reduced power output on 17.  One
of the transistors appears to have overheated again!  :-(

I looked over the circuitry, and have not noticed anything wrong.
The base bias voltage on the push-pull pair of 2SC1969s is correct.
The components associated with the base driving circuitry is
correct (according to their marked values).  The output transformer
is wound correctly according to the manual.

I *THINK* the same transistor (Q7) overheated again, but I'm really
not sure.

Has anyone else run into this problem?  The only anomaly I observed
was a few small metal fragments on the adhesive side of the thermal
pad of the affected transistor.  Could that have caused the premature
failure of the transistor?  I'm at a loss to explain this.

Any help/insight would be greatly appreciated.

Thanks!


73, de John, KD2BD
K2/100 #3563 (Temporarily QRT)



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