K2 a problem solved

[Bill Scovell]

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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[Ron D'Eau Claire-2]

Bill, VK4SQ, wrote:
...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 ...

-------------

It's good practice to use ESD precautions whenever handling individual
boards as well as when handling components. Depending upon the circuit,
ESD-sensitive terminals of a transistor or I.C. may be open at the board
connector until the board is plugged in, leaving it as exposed to ESD damage
as if one were handling the part itself.

Ron AC7AC


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