Re: QCX tuning


Alan de G1FXB
 

Hi Glenn,

I'm in defence of all Hans views as well.  https://groups.io/g/QRPLabs/message/26421
(I keep pointing out his statement that he has never experienced a failure of any prototypes and beyond to production testing.
Last was my Post https://groups.io/g/QRPLabs/message/26498)

What I'm trying to understand is the conditions that lead upto what appears to be a one or two a week report of PA failure,
Is it always user abuse, and / or is there any easy solutions ?.
My posts:-
https://groups.io/g/QRPLabs/message/26420
 https://groups.io/g/QRPLabs/message/25533
https://groups.io/g/QRPLabs/message/25822

Putting it into perspective the QCX has celebrated it's first birthday
Best case, its perhaps 52 out of 5500 units built, equates to just over 1% failure(?)
of the most heavily stressed and abused components of any transmitter.
(it's probably fair to assume that failure is likely to be under rather than over reported,
many would just replace the components rather than post a question about it here?)

It's academic, we will move on and soon have a new unit to talk about.  :-)


Alan


On 02/09/2018 16:44, Glen Leinweber wrote:
In defense of Hans,
I don't recall seeing a brag about QCX able to handle infinite-SWR conditions. (ie: antenna short or open-circuit).

 I can see a dangerous scenario....
if you're using an open-circuit type outdoor antenna - one that measures open-circuit resistance between driven
element and ground. An extreme example might be a kite-elevated long wire. This type of antenna can attain
a considerable charge, and rise to very large DC voltage levels.
Then you attach this antenna to your radio. Could even be an un-powered radio....
Ka-blooy!
The QCX has an output filter that nearly floats as far as DC is concerned. It has a path through R43, a 120k resistor
to one of the four 74ACT00 logic gates - most likely in a logic "high" state. If you measure the DC voltage at the
antenna connection terminals with a high-Z DC voltmeter, you'll measure close to +5V.
I would ensure that any antenna has a DC path to ground, either through a RF choke, or a resistor, to bleed off any
static charge. The QCX's 120k resistor path is too feeble to serve as a reliable discharge. A charged antenna can
blow the finals nearly instantly when connected. There is no over-voltage antenna protection in the QCX.

A loop-type antenna is safer from this scenario, but I'd still include a discharge path from the loop to earth.

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