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Re: New assembled High Bands QDX went into smoke

 

Evan is correct on all his points. I will add a bit more.

The end-of-transmission spike is caused by an inductive Ldi/dt kick produced by L14 when all four BS170 transistors turn off.
It can under some conditions exceed 100 volts. It is correct that field failure has not been tied directly to the existence of that spike, but forensics are quite a problem!

There is an easy alternate way to protect against this potential source of damage that does not use Zeners. It requires? that a high speed silicon commutating diode, like a 1N4148 or 1N914 be placed across L14 as one would across a relay coil to protect the relay driver. I have run my QDX, modified this way, for several months now without failures. This solution does not address any SWR problems and will not confer immunity to mistakes there!

Regarding QMX, yes the pads for Zeners exist but are currently unpopulated.
Hans has a truly elegant solution to the above problem in that the PMOS modulator that is present in QMX and is under DAC control can be used to gracefully wind down final amplifier current, avoiding the Ldi/dt problem altogether. Hopefully it is to be used in all transmission modes.

Zeners in QMX might pose a brand new problem when SSB arrives.
The EER style of SSB generation to be used in QMX does not appreciate voltage variable capacitance at the drains of the finals. That introduces distortion that must be reduced through audio pre-compensation. The BS170 transistors already have such capacitance. The Zeners would add even more.

73 and good luck! JZ KJ4A?

On Mon, Jul 17, 2023, 6:14 AM Evan Hand <elhandjr@...> wrote:
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

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