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

[Don Wilhelm]

John,

One of the K2s I built had a problem with unbalance in the output stage -
traced it to a slight difference in value between RFC8 and RFC9.  Everything
in the base circuit should be balanced, so you can be suspicious of the
pairing of R53 and R54, R55 and R56, C127 and C128 as well as RFC8 and RFC9.

With your 400 MHz 'scope, you could check the RF at the base of each output
transistor to make certain that each one is receiving an equal share of the
drive signal.  I would use one probe on each base when making the
measurement as a precaution (just to keep things balanced).

73,
Don W3FPR

----- Original Message ----- 

| 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)
|
|