In April this year, John Zbrozek, KJ4A, and I collaborated on modeling the QDX PA.? The models showed a large voltage spike when the transmission was terminated at the end of a message.? This was verified with an oscilloscope to exceed the 60 volts rating of the BS170.
Practical experience has not shown this to be an issue, as there are thousands of QDX in operation and not a very large failure rate that can be verified as caused by this overvoltage.
Per Hans, the Zener pads were added for the QMX in case you want to add the protection.? Several other kits use Zener protection for the finals.? It was unknown what side effects the Zeners would add to the output signal.? To the best of my knowledge, no one has performed that addition and posted the results of a spectral purity check.? There was also concern about the power output and efficiency of the PA if the Zeners were added.
I have not modified any of my QDX as of this date.? I use the QDX as an exciter for a WA2EBY amp and run it at reduced voltage.? I have not experienced any issues with that setup.? I do not run my QDXs portable.
When I do build one of my QMX kits, I will add the Zeners and do the purity test.? I have been waiting to do that for the operations manual and the final firmware updates that fix the current issues of the QMX CW operation.? ?I have been spending my time on other projects.
To date, I have read one post that indicated the Zeners protected a QDX after repair from a subsequent fault caused by an open antenna connection.? The model does show that transmission into an open circuit does show a high voltage on the BS170 drains that the Zener could protect (for a time, depending on the power rating of the Zener).? It does not show that a shorted antenna has the same high voltage issue, so only 1/2 of the possible extremes are covered with the Zener.
The takeaway for me is that monitoring the antenna SWR is important to watch.? That, and running below the design voltage.? Both of these will be more effective than adding the Zeners.
The above are my opinions based on my experiences.? Please do not take them as absolute facts and investigate before you make any choices.? I would appreciate any feedback to help me learn more.
73
Evan
AC9TU
