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

[John A. Magliacane]

Hello Everyone.

I wish to thank Allen, KA5N, Don, W3FPR, and Bill, NA7Y for
offering suggestions in finding the reason behind the repeated
overheating and subsequent destruction of one of the 2SC1969
RF power transistors in my K2/100 S/N 3563.

After considering the suggestions, I have some doubts that the
problem is the result of a parasitic oscillation.  It's easy enough
to check, but I think the "Hi Curr" warning message would have
displayed if oscillations severe enough to cause the transistor
to "cook" were to take place.
  
A power, current, or dissipation imbalance seems a more likely
cause.

I examined the RF board very carefully, much like a detective
trying to crack a tough case.  I began to suspect something was
awry with the mounting hardware.  I discovered that the phenolic
insulator (RAF #1122-4-PH) associated with the overheated
transistor was slightly shorter than the insulator used with
the complimentary transistor.  In fact, it's shorter than
the replacement insulators received in the recently ordered
K2PAKIT!  (Lesson: Size DOES matter!)

This would certainly explain the heat dissipation problem
associated with only ONE of the RF power transistors, and
not the other.

But it gets even more interesting...

One of the problems with the transistor mounting and heatsinking
method used in the K2 is that tightening the mounting hardware
does not necessarily force the transistors flush against the
heatsink, as would occur if the transistors were mounted
directly TO the heatsink.

Careful examination of the transistors' height indicated that it
was slightly LOWER than that of the nearby 2-D fastener.  Since
both the transistors and the fastener are secured to the same
panel, this situation would not result in a solid, even,
mechanical contact between the heatsink and the transistors.

The reason the 2-D fastener is slightly higher than that of the
transistors is because the lockwashers associated with the 2-D
fasteners were mounted between the bottom of the RF board and
the fasteners.  However, according to some of the photos in
the back of the Owner's Manual, the washers are apparently
placed between the screw head and the TOP of the RF board.

In my defense, the instructions on Page 33 do not indicate
where the lockwashers need to be installed:

        "Secure each fastener from the top side of the board
         using two chassis screws and two #4 lock washers." 

(I never thought a lock washer would be placed beneath a screw head,
but then again, I'm an electronics guy, not a mechanical guy.)

Removal of the 2-D fastener near the RF power amplifier circuitry
requires removal of T4.  All other fasteners should be fairly easy
to remove and re-install.

Mystery solved.  (Hopefully!)

*Sigh*


73, de John, KD2BD
K2/100 #3563


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