Blown finals

[artus]

Folks,

I have a K2 that's a couple years old.  A friend and I were working with it a couple weeks ago and switched an antenna connection -- the K2 went blank. After a bit of trancing it was apparent that one of the finals went.  It was not in transmit.  Pulling the bad final resulted in the K2 coming back to life.  We ordered the final replacement kit and my friend installed the kit -- triple checked his installation and wrote: "Got everything back together, checked to make sure the cathodes weren't shorted to ground. Attached the heat sink. Double checked topside to see if the cathodes were shorted.  Triple checked. Attached the top cover.  Set PS voltage limit to 10v and current limit to 1 amp. Dummy load. Punched the on button and voltage immediately dropped below 1 volt. No smoke, no noise. Cathodes are shorted to ground. I assume the finals are blown."  I'll get another final kit on its way -- but need to find the issue before installing.  What is the best approach?  I haven't written Elecraft, yet.

73,
Jack (W0FNQ)Folks,

[wa9fvp]

Check the bias voltage. If it's too high, it can blow the finals.  Sometimes the circuit that provides the bias can have a shorted transistor and drive too much current into the base of the final transistors.

Jack WA9FVF
Willco Electronics

[artus]

Thanks Jack,

That sounds like a very reasonable starting point.  We'll first check to see it the finals are indeed blown; because my friend had a current limited supply set at 1 amp, perhaps not.

73, Jack (W0FNQ)

[Don Wilhelm-4]

Jack,

Forget about the current limited power source, there is another way.

Since your new finals are obviously blown, remove them and then power on
the K2 and do a TUNE while monitoring the voltage at the solder pads for
Q7 or Q8.
If the voltage is not in the range of 0.62 to 0.64 volts, replace Q11
and Q13 and then remeasure.  If the voltage is still not within that
range, look for other problems in the PA transistor bias circuit.
Only after the bias voltage is correct, install the new PA transistors.
The K2PAKIT contains replacement transistors for Q11 and Q13.  I
recommend that you use the K2PAKIT for replacement of the PA transistors
because the PA transistors are matched - if you obtained the
replacements from another source, they may have not been matched.

BTW - the "C" markings on the PA transistor solder pads are for the
Collector (Cathode has no meaning for transistors, only diodes).

73,
Don W3FPR

On 5/26/2015 9:19 AM, artus wrote:
> Thanks Jack,
>
> That sounds like a very reasonable starting point.  We'll first check to see
> it the finals are indeed blown; because my friend had a current limited
> supply set at 1 amp, perhaps not.
>
>

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