[Elecraft] K-3 Transmit Problem: no RF Output

[Don Wilhelm]

Mike,

K3support can give you a couple places to make measurements to determine 
whether the LPA transistors are good or bad.
I forget what those measurements are, but I suggest you contact them.

73,
Don W3FPR

On 3/26/2016 7:30 PM, Mike Cizek W0VTT wrote:
> Greetings -
>
> My K3 has had low output for quite some time, maybe 60-90w depending on the
> band.  I always meant to look into it and fix it someday.  Now it has no RF
> output at all. (Just in time for VK0 and FT/J!)  I believe I have traced the
> problem to two transistors on the LPA board (driver stages), Q4 and Q5, but
> am not confident they are the only parts at fault.  I'm hoping someone else
> has had the same problem and found the solution.
>
> I tried re-seating all the connectors, with a drop of DeoxIt on the pins,
> and running TX gain cal.  5w and 50w failed immediately, but 100 mW is OK on
> all bands.
>
> KPA-100 was removed, the PA jumper installed between P67A & P67B, and the
> config menu was set to KPA not installed.  Measurements were taken with an
> old Tek 2465.  DC voltages are all correct.
>
> Measuring with a scope - I get a nice clean sine wave going in to the LPA
> board at P71A pin 3.  Most bands are 1.2 to 2.2v P-P, but 6m is only 0.6v.
> The output of Q1 is a little distorted.  The output of Q2 varies from
> 10-25v, depending on the band, and is also a little distorted.  Signals at
> the base of Q4 and Q5 are back down to 3v.  Output of Q4 and Q5 is a few mV.
> Changing the drive control has no affect on signal levels.
>
> Is 1 to 2 v P-P the correct drive level at P71A?  What is an acceptable
> variation in power level between different bands?  Should the drive control
> change signal levels here, or does that happen later in the circuit?  I am
> also concerned about the distortion on the wave forms.  Q4 and Q5 are
> available from RF Parts for $5 apiece, so replacing them will likely be my
> next stap.
>
> Thank you.
>