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K2 Q11 Repeated Failure

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alan7114p

K2 Q11 Repeated Failure

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.

Thanks and 73
Alan K6ZY

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daleputnam

Re: K2 Q11 Repeated Failure

From: [hidden email]
To: [hidden email]
Subject: RE: [Elecraft] K2 Q11 Repeated Failure
Date: Sat, 10 Aug 2013 12:28:44 -0600

Hi Alan, That is the scenerio for the description of a static induced failure. And the reading is Q13. You didn't mention the ser. number, but it seems a K2PAKIT would be in order, to replace all of the stressed parts. Q11 and 13 become martyrs in order to save the rest of the radio, but the other components, Q5 and 6, have been stressed, and will likely fail in the future. In this case, Q13 was damaged, and Q11 took the hit. It's been said, that this is the season, for the static hits. (I didn't hear it either).

Have a great day,

--... ...--
Dale - WC7S in Wy

> From: [hidden email]
> To: [hidden email]
> Date: Sat, 10 Aug 2013 10:35:57 -0700
> Subject: [Elecraft] K2 Q11 Repeated Failure
>
> 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.
>
> Thanks and 73
> Alan K6ZY
>
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html

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Don Wilhelm-4

Re: K2 Q11 Repeated Failure

In reply to this post by alan7114p

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.
>