RE: QCX PA Failures:-
there comes a time where you cannot protect for every
(If you not already seen it, have a look at the PA circuitry
in the NC2030 design,
I guess something similar was not included for reason of
cost, both component and also PCB size increase?)
It's an example of when potentially it costs more for the
protection circuit than the likely failure scenario,
In the QCX It's Q1->3, Q6 and possibly IC3, they add up
to something sub $4 to replace??
(Q6 Should never fail spectacularly as often as it's being
reported, it's rated at 2A dissipation.
The QCX on TX consumes circa 500mA TOTAL. Why doesn't a fuse
the failure of the si5351 is 1 in 5,500+ QCX's that have
being built occurrence?
The best protection is afforded by the operator him / her
self, perhaps think before pushing a QCX hard..
Remember there is no fancy SWR mismatch or over temp
protection on long duty cycle modes.
"Without any protection circuits the QCX will try it's very
best to fulfil it's purpose,
right up to the point of failure."
27/08/2018 08:38, Alan G4ZFQ wrote:
;-) I think a narrow BPF
improves the performance.
Yes, I do not dispute that. But may I suggest the
difference would not usually be noticed without careful
It won't work... if you capacitatively couple IC3 pin 3
to the PA gate, then the peak-peak squarewave will be 5V
BUT, it will be centered on 0V.
I would suggest isolating IC3
pin 3 with a capacitor.
Oh yes, I see...
But it seems there have been a couple of cases where PA
device failure has resulted in 12V being sent back
through IC3. One of those affected have reported failure
of the Si5351 and another has said IC2 required replacing.
If that really is the case then I'm wondering how to
buffer IC3 to avoid this rare occurrence.
73 Alan G4ZFQ