K2 - Q7 and Q8 failure

[K and K Stuber]

While operating my K2 (SN 7581) on CW I heard a loud pop! I traced it to
blown Q7 and Q8. They are physically blown apart. The K2 was operating
normally before this happened. Everything else seems to be OK except for no
power output. The driver seems to be working; I can key the radio and hear
it in another RX. The K2's RX is working normally. Any thoughts as to why
this happened would be appreciated.

Ken/N4KS
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[daleputnam]

Depending on the antenna in use, I have had this happen.. to the exact detail.. what happened to my K2, was simple.. wind built static on the antenna, there was no path to ground, but through the K2 finals.. so when the potential reached high enough to break down the resistance in the path.. it did.
  How to solve this issue so it does NOT ever happen again? Provide a dc path to ground from the antenna.. both sides of a dipole.. if balanced line fed.
Some modicum of relief can be had by adding a 100k resistor to the center conductor of the antenna port to ground.. this will only help.. it is NOT a cure all.

Have a great day,
 
 
--...   ...--
Dale - WC7S in Wy
 
 

> Date: Mon, 28 Dec 2015 20:52:42 -0500
> From: [hidden email]
> To: [hidden email]
> Subject: [Elecraft] K2 - Q7 and Q8 failure
>
> While operating my K2 (SN 7581) on CW I heard a loud pop! I traced it to
> blown Q7 and Q8. They are physically blown apart. The K2 was operating
> normally before this happened. Everything else seems to be OK except for no
> power output. The driver seems to be working; I can key the radio and hear
> it in another RX. The K2's RX is working normally. Any thoughts as to why
> this happened would be appreciated.
>
> Ken/N4KS
> ______________________________________________________________
> 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
> Message delivered to [hidden email]

     
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[Don Wilhelm-4]

Ken,

That is normally the result of unmatched transistors for Q7 and Q8, but
Elecraft has been supplying matched transistors for the last 3 years.
I suspect that you have some incorrect components in the Q7/Q8 base circuit.

After checking everything in the Q7/Q8 base circuit, check the PA bias
voltage before installing the new Q7/Q8.
Do a TUNE and monitor the voltage at the base solder pads for Q7 and
Q8,  the voltage should be between 0,60 and 0.64 volts.  If it is not
replace Q11 and Q13 and then recheck.

Check all the components in the Q7 and Q8 base circuit for proper values
and good soldering.

73,
Don W3FPR

On 12/28/2015 8:52 PM, K and K Stuber wrote:
> While operating my K2 (SN 7581) on CW I heard a loud pop! I traced it to
> blown Q7 and Q8. They are physically blown apart. The K2 was operating
> normally before this happened. Everything else seems to be OK except for no
> power output. The driver seems to be working; I can key the radio and hear
> it in another RX. The K2's RX is working normally. Any thoughts as to why
> this happened would be appreciated.
>

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