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.
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