A couple of weeks back I sought help for a problem encountered at the first
stage of 40 Meter Transmitter alignment (Page 75).
The problem was uncontrollable output power.
Specifically:-
Holding 'Tune' after requesting 2.0W output yielded a 'Hi
Current' display and an indicated power of 19.1W.
Don / W3FPR came to my assistance explaining how the K2 compares requested
power with detected power to develope the required signals to adjust the drive
to get the output power to the requested level. With Don's guidance we explored
all the circuitry associated with this process but found everything to be OK
which led Don to the conclusion that the high output was resultinfg from
spurious oscillation.
At this juncture Gary took over and the first issue was to determine whether
the oscillation was inherent in the transmitter chain itself or a fault in the
t-r switching circuitry. The answer to this question was provided by removing
link W1, which immediately stopped the oscillation --- almost certainly the
problem lay in the t-r circuitry --- a systematic 'trouble shoot' around
Control Board Q1, Q2, Q3, Q4 and RF Board Q23 established that Q4 (2N7000) was
the culprit.
Tiresome though it was for a while, the nice part of this exercise was that by
taking it slowly and getting good advice from Don and Gary I was able to
achieve a result without any unnecessary 'invasive surgery' --- I only had to
'pull' one component, the faulty one --- left to my own devices I don't think
the boards would have still looked as attractive as they do.
A final word --- this is the second 2N7000 that has brought me undone on this
build --- earlier on Q5 deprived me of sidetone for a while, obviously these
mosfets are rather fragile devices --- whilst I did not use a mat during the
build I did diligently wear a wrist band whilst assembling sensitive devices
although at the time I thought that with our relative humidity being quite high
that this was probably 'guilding the lily' --- I shall be more circumspect in
the future.
With continuing appreciation of this reflector and particular thanks to Don and
Gary I''ll stop chewing up bandwidth.
Bill Scovell / VK4SQ
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