[Elecraft] K2 Finals - Chronic Blow-Outs

[w7asa]

This K2 was a bought from someone on the internet as a working unit & etc.
Once received, it blew the finals in about 2 - 3 hours of QSO time.  I
really like the K2 but it looks it's mine to solve.  I've done a lot of
radoi building and RF troubleshooting down through the years, and have
looked for all the usual suspects of shorts, debris shaken loose during
shipment & etc.

BTW: The condition of the board and mounting hardware for the finals and PA
bias transistors STRONGLY indicates that they've been changed more than a
few times *before* the rig reached me . . .   so , in all probability, I was
probably the only one surprised by it's sudden demise.

Here's what I am seeing:

It has fried three sets of finals in two weeks - I changed them per the
manual twice ( am a glutton for punishment )  and VERY carefully re-checked
my work and metered for shorts each time. After =< 2 to 3 hours of 'normal'
operation each time, the finals blew.  This happened on three separate bands
with an SWR of 1.2:1 (on average) at 10 Watts indicated both on my SWR/power
meter and the K2 digital display. I did not notice a current warning during
the last two failures and now am not certain whether I did during the first
failure. 

Most recently: - I had finished the most recent ragchew then switched to
calling CQ using a Vibroplex (as always) and heard a small 'crack' of a
transistor case rupturing when immediately the 3 Amp automotive fuse in my
power cord blew. The smell told me where to look.

This is a relatively simple circuit and it seems to me that there is not a
lot to go wrong:  T4 with bad insulation ?  Something in the common parts of
the LGP/Band switching but not band specific? I am mystified why it would
run flawlessly for a few hours of CW then let the smoke out.

Anybody have ideas? I have a ton of Elecraft accessory boards on the way and
that's a lot of assessories for a radio that drops dead continuously.


>Ray
W7ASA ..._ ._



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