John Zbrozek started the development of an LTSpice IV model of the QDX PA that he and I expanded to test a theory on what happens when the transmitter is turned off at the end of an FT8 cycle.? The model and subsequent bench testing verified a voltage spike at the end exceeding 60 volts.? I measured over 100 volts for a very short period on my Rev 1 QDX.? This can be eliminated with a "commutating" diode to dissipate the built-up magnetic field.? Subsequent tests indicate that the single diode does eliminate the spike.
Because of the minimal duration and normal variance on devices compared to specifications, the likelihood of your QDX suffering from this problem is minor.? There are many QDX in service that have not exhibited the failure.? The ones that are noticed are the ones that have failed.? Consider that thousands of these radios are in operation, and we are seeing tens reported with a failure that could be power-related or severe operating conditions.
For myself, I have installed the diode on my QDX.? This is not needed for the QMX, as the key shaping circuit removes the sudden cutoff of the PA supply.? The QMX would not have the issue.
I have done some simulations of short and open loads on my most recent model of the QDX PA.? I can not get an overvoltage with either of those conditions.? This leads me to believe that the Zeners to protect the BS170s are most likely unnecessary.? That is not to say that a high SWR would not cause a failure.? It indicates that it is likely not voltage but a power or current-related issue.? I have not looked into that failure mode.? Since a voltage failure does not seem probable, adding the Zeners is not a fix for the problem.? The Zeners would not hurt, but I doubt it will add much protection.
It is still good practice to check the antenna for high SWR before the first transmission in a session.? I would also avoid applying full power to an ATU connected to a QDX.?
A request to Hans would be a lower power tune mode for the QMX that could be done in software with the key shaping circuit already part of the QMX.? This would need a hardware mod to work on the QDX.
The above are my observations and conclusions.? Please let me know if any data goes against any of the above.? I am ALWAYS open to constructive feedback.? It is one of the methods I use to learn.
73
Evan
AC9TU
