Re: QCX tuning


jmh6@...
 

Hi Alan,

Academic??

ALL failures are an opportunity to learn and make a product more robust.

I am surprised no one has talked about bringing up the power in steps? Measuring things as we go up to full power.

From where I look, this would give up real data to look at in the event of failure.

Lots of fun :).

John Concord, NH

On Sun, 2 Sep 2018, Alan de G1FXB via Groups.Io wrote:

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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