Re: Pushing the QCX (T1 question) #qcx

Hans Summers

Hi Alan 

Yes, protection circuits would have added cost and complexity. And it is hard to protect against every possible failure mode. In my opinion it wasn't necessary. During all my QCX development, I never managed to break any IC3, Q6 (the earlier MPS2907 0.6A version), or any of the BS170s. Not by mismatched load, nor open load, nor short-circuit, nor continuous duty-cycle. Not even by temporarily increasing the supply voltage to around 20V and getting 10W out. Not a failure on any of the six bands. Not with a detuned Class-E resonant circuit either. If I couldn't cause any failures while I was working on the development... I didn't see the need to unnecessarily increase the cost and complexity if it was so hard for me to break something.

73 Hans G0UPL 

On Mon, Aug 27, 2018, 17:27 Alan de G1FXB via Groups.Io <> wrote:
RE: QCX PA Failures:-
there comes a time where you cannot protect for every eventuality?

(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 save it?)
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."


On 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 comparison.

I would suggest isolating IC3 pin 3 with a capacitor.
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.

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

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