K2 3 amps on receive

[Lennart]

When I powered on my K2 today it shut down after a few seconds. The 2
final transistors are warm but no power out. radio has been standing
without antenna or PS connected since last time I used it. Seems like
some bias level or driver is dead.  Last couple of times I have used the
radio has been during tests of my new RS232 interface that worked very
well, but that is removed now during fault finding.

Any tip on where to start looking? Serial no 6403.

Regards
Lennart
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[Julian, G4ILO]

Lennart wrote

When I powered on my K2 today it shut down after a few seconds. The 2
final transistors are warm but no power out. radio has been standing
without antenna or PS connected since last time I used it. Seems like
some bias level or driver is dead.  Last couple of times I have used the
radio has been during tests of my new RS232 interface that worked very
well, but that is removed now during fault finding.

Any tip on where to start looking? Serial no 6403.

I vaguely remember (it was a long time ago) that I had a similar problem when building my K2. There is a component, an inductor I think, that provides negative feedback to the driver stage. It was O/C so the PA went into oscillation when the rig was powered up, and blew the driver transistor.

Why yours should suddenly go O/C after being OK I don't know, so it could well be something else. It could be failure of the device that switches the supply to the TX stages, so the TX side is getting power during RX when it shouldn't be.

[Lennart]

More information: where R55 and R56 are joined (and Q8 base), I have 0.87 volts, but on Q7 base I have 0.93 which make me suspect that Q7 is bad. Both Q7 and Q8 should have the same dc level, right?

This is all on receive with 0 volts on both sides of R61, the "bias" voltage on receive can not come from Q13.

What is the proper voltage on Q7 and Q8 on receive? I would expect 0 volts and never above 0.6?

Regards,
Lennart

[Don Wilhelm-4]

Lennart,

You are on the right track, but do not dismiss Q11 and Q13 yet.
I think your analysis about Q7 may be correct - but:
I would suggest that you remove Q7 and Q8 and again check the voltages -
if there is a problem with Q11/Q13, you will still have voltage at the
Q7/Q8 base locations - OTOH, if the voltage is zero (or close to that),
then the problem is collector to base leakage in either Q7 or Q8.  I
would replace them both.  The PA replacement kit from Elecraft contains
replacements for Q7, Q8, Q11 and Q13.

Another thing to check before going much further is to see if both 8R
and 8T both on during receive (only 8R should be present).  You can
easily check at the anodes of D6 and D7.  If both are present during
receive, the problem is with Q1 or Q3 on the Control Board.

Good luck with it.

73,
Don W3FPR

Lennart wrote:

> More information: where R55 and R56 are joined (and Q8 base), I have 0.87
> volts, but on Q7 base I have 0.93 which make me suspect that Q7 is bad. Both
> Q7 and Q8 should have the same dc level, right?
>
> This is all on receive with 0 volts on both sides of R61, the "bias" voltage
> on receive can not come from Q13.
>
> What is the proper voltage on Q7 and Q8 on receive? I would expect 0 volts
> and never above 0.6?
>
> Regards,
> Lennart

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