Alan,
I trust that Q12 in your post is a typo and should have been Q13. Q12 is
in the receive Post Mixer Amplifier circuit and has nothing to do with Q11.
Q11 and Q13 often fail as a secondary failure when one or both PA
transistors become shorted. Yes, I have seen Q11 explode.
Q7, Q8, Q11 and Q13 replacements are provided in the K2PAKIT, and I
suggest you order one from Elecraft (the PA transistors are a matched pair).
That path to ground that you are measuring is through the PA transistor
bases.
When you get the PA kit, replace Q11 and Q13 first, then without Q7 and
Q8 installed, do a TUNE while measuring the DC voltage at the base
solder pads for Q7 and Q8. You should find between 0.62 and 0.64 volts
if all is well.
Once you have made that determination, install Q7 and Q8, check for
shorted collector to ground after you install the heatsink and then see
what the output power is - all should be correct.
73,
Don W3FPR
On 8/10/2013 1:35 PM, Alan Whitman wrote:
I have searched the archives in vain for an answer (might just be my
limited knowledge, however). My K2, which had been operating
properly as a basic 10W, CW only with only the 160/RX antenna option
installed, suddenly began having virtually no output on transmit.
When I opened it up, I found that Q11 was completely split in half.
After checking everything I could, I replaced Q11 and Q12 (this was a
long, involved process). I checked Q7 & Q8 for shorts to ground and
found them to be OK. I applied power but did not turn on the
radio. OK so far. When I turned on the K2, Q11 immediately began
to smoke and burst into flames within a half second. I turned it
off as quickly as I could.
When I checked Q11 and Q12 for short to ground I found that all leads
of both transistors were shorted to ground. I removed the devices
and checked the pads in the same way. Now only the emitter of Q11
showed short to ground, which is correct.
I checked Q6, Q7 and Q8 for shorts to ground. Emitters of Q7 and Q8
show short to ground which is correct. Q6 showed 1.5 ohms, which is
also correct.
I also found that the resistance to ground from the emitter of Q13 to
ground showed 756 ohms, even when I disconnected R63 from the junction
of R60 and R62. I expected to see about 2.8K ohms. From what I can
find on the schematic, there should be no other path to ground, but
I'm not sure about this. I don't know if I should be looking for a
short or looking elsewhere.
Can anyone advise if this last measurement is correct? Where should
I look for the overall problem if this last resistance is not the
problem.
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