#qcxmini Ideal BPF Peak and 5v on coax center pin? #qcxmini


Vance.Langer@...
 

Hi everyone,
I finished up my QCX Mini (what a fantastic kit!) and noticed a few things while doing the alignment. Firstly, I was only able to set the BPF Peak to high 7, almost 8. Is this too low? Is there anything I can tweak within the hardware to improve this? The rest of the alignment went well.

Secondly, when trying to determine the power output with a DMM and the 50 ohm dummy load I noticed I could get just over 5 watts output with 13.5V in, reading off the charts in the manual. However there's a lingering 5v on the center coax pin. What's the deal there?

I got it hooked up to my EFHW for some listening, there didn't seem to be much happening on the 20m band but my antenna placement was less than ideal, I'll try again later. I did manage to listen in on a couple of QSOs and the built-in decoder seems to do well! Does anyone have recommended decoder settings that work best?

Thanks a lot everyone for the help and advice, this is an awesome group!


Ronald Taylor
 

Vance, that high 7 range reading is probably fine, as long as you are not at fully open or full meshed on the C1 trimmer. 

I checked my QCX just now with DC reading DVM on the BNC connector and find 5 volts on the center pin as well. But when terminated with 50 ohm dummy load it goes to zero. My guess is we are seeing the local oscillator signal from the mixer but that's just a guess. Anyone else have any ideas?? 

73 .. Ron

On Wed, Aug 18, 2021 at 10:49 AM Vance.Langer via groups.io <Vance.Langer=protonmail.com@groups.io> wrote:
Hi everyone,
I finished up my QCX Mini (what a fantastic kit!) and noticed a few things while doing the alignment. Firstly, I was only able to set the BPF Peak to high 7, almost 8. Is this too low? Is there anything I can tweak within the hardware to improve this? The rest of the alignment went well.

Secondly, when trying to determine the power output with a DMM and the 50 ohm dummy load I noticed I could get just over 5 watts output with 13.5V in, reading off the charts in the manual. However there's a lingering 5v on the center coax pin. What's the deal there?

I got it hooked up to my EFHW for some listening, there didn't seem to be much happening on the 20m band but my antenna placement was less than ideal, I'll try again later. I did manage to listen in on a couple of QSOs and the built-in decoder seems to do well! Does anyone have recommended decoder settings that work best?

Thanks a lot everyone for the help and advice, this is an awesome group!


Curt wb8yyy
 

Vance

nice work, just built a 20m mini here. 

the 7 level on BPF peaking means little - it changes as you adjust volume control.  as long as you get a pronounced peak, all is well. 

the other nulling calibration is more important, and sounds like that went well.  note it takes several iterations and each menu setting and knob to get it optimized. 

20m propagation has not been great recently.  i managed a few WAE contacts this past weekend, but not much since.  so hang in there. 

5 volts on coax center pin has me spooked - check it again with no antenna attached.  I confess I have not looked into that, check the schematic and see who else comments.  there should be a blocking cap that isolates the center pin from the circuitry - and with EFHW likely introducing a DC short - I am saying how can this be if the blocking cap is shorted - check it again. 

73 Curt wb8yyy
fellow QCX traveller


John Pagett G4YTJ
 

Gents,

Don’t worry about the 5V on the coax centre pin. It comes from the output of IC3C (pin 8) - at least on the QCX+. It’s possible that Hans has used different pins on the mini, I haven’t checked, but the principle remains the same.

This output produces the alignment signal for setting up, but is attenuated by a 120k resistor (R 43). When the alignment signal is off the gate output sits at 5V but the 120k resistor will mean that you can connect a dummy load or antenna without issue.

Regards,
John
G4YTJ


Vance.Langer@...
 

Curtis: I was under the impression that the peak level on the bpf isn't affected by the volume. I guess I could check it again and make sure. I was able to get a peak that is between fully open and fully meshed.

Ron: I think I measured 5v on the dummy load while  doing my output test, but ill double check that.


Arv Evans <arvid.evans@...>
 

Curt and Vance

Re. +5VDC on the coax connector,

If your DVM has fairly wide bandwidth it is possible that it is reading the
average of the RF above ground.  This happens when the RF is floating
above ground level (usually when a capacitor is used for DC isolation).

Some rectifier circuits for dummy loads also measure the average RF
voltage in a way that adds this to the actual DC and give an impression
of more power than is really there. 

Arv
_._


On Wed, Aug 18, 2021 at 12:06 PM wb8yyy via groups.io <wb8yyy=yahoo.com@groups.io> wrote:
Vance

nice work, just built a 20m mini here. 

the 7 level on BPF peaking means little - it changes as you adjust volume control.  as long as you get a pronounced peak, all is well. 

the other nulling calibration is more important, and sounds like that went well.  note it takes several iterations and each menu setting and knob to get it optimized. 

20m propagation has not been great recently.  i managed a few WAE contacts this past weekend, but not much since.  so hang in there. 

5 volts on coax center pin has me spooked - check it again with no antenna attached.  I confess I have not looked into that, check the schematic and see who else comments.  there should be a blocking cap that isolates the center pin from the circuitry - and with EFHW likely introducing a DC short - I am saying how can this be if the blocking cap is shorted - check it again. 

73 Curt wb8yyy
fellow QCX traveller


Gary Bernard
 

Sounds to me like you did a great job. 20 has been dead lately. Try late afternoon and early evening. 
GL, Gary W0CKI 


On Aug 18, 2021, at 12:06 PM, wb8yyy via groups.io <wb8yyy@...> wrote:

Vance

nice work, just built a 20m mini here. 

the 7 level on BPF peaking means little - it changes as you adjust volume control.  as long as you get a pronounced peak, all is well. 

the other nulling calibration is more important, and sounds like that went well.  note it takes several iterations and each menu setting and knob to get it optimized. 

20m propagation has not been great recently.  i managed a few WAE contacts this past weekend, but not much since.  so hang in there. 

5 volts on coax center pin has me spooked - check it again with no antenna attached.  I confess I have not looked into that, check the schematic and see who else comments.  there should be a blocking cap that isolates the center pin from the circuitry - and with EFHW likely introducing a DC short - I am saying how can this be if the blocking cap is shorted - check it again. 

73 Curt wb8yyy
fellow QCX traveller


Ronald Taylor
 

Vance, you are correct, the BPF adjustment is independent of the volume control setting. The IQ Bal and Hi and Lo Phase null adjustments are the ones that are dependent on the setting of the volume control. If you are peaked at the high end of 7 on the BPF and are not at max or min setting on C1, then you are good to go at that level. 

73 .. Ron

On Wed, Aug 18, 2021 at 12:00 PM Gary Bernard via groups.io <garybernard2=aol.com@groups.io> wrote:
Sounds to me like you did a great job. 20 has been dead lately. Try late afternoon and early evening. 
GL, Gary W0CKI 


On Aug 18, 2021, at 12:06 PM, wb8yyy via groups.io <wb8yyy=yahoo.com@groups.io> wrote:

Vance

nice work, just built a 20m mini here. 

the 7 level on BPF peaking means little - it changes as you adjust volume control.  as long as you get a pronounced peak, all is well. 

the other nulling calibration is more important, and sounds like that went well.  note it takes several iterations and each menu setting and knob to get it optimized. 

20m propagation has not been great recently.  i managed a few WAE contacts this past weekend, but not much since.  so hang in there. 

5 volts on coax center pin has me spooked - check it again with no antenna attached.  I confess I have not looked into that, check the schematic and see who else comments.  there should be a blocking cap that isolates the center pin from the circuitry - and with EFHW likely introducing a DC short - I am saying how can this be if the blocking cap is shorted - check it again. 

73 Curt wb8yyy
fellow QCX traveller


Ronald Taylor
 

Sorry, I finally looked at the schematic a bit closer .... 5 volts is coming from pin 8 of IC3. This is a NAND gate so it is high unless both inputs are high. It will have 5 volts DC on it except when in test generator mode. The 120K resistor limits the current. It's a non-issue ... 73 all .. Ron


On Wed, Aug 18, 2021 at 12:27 PM Ronald Taylor via groups.io <wa7gil=gmail.com@groups.io> wrote:
Vance, you are correct, the BPF adjustment is independent of the volume control setting. The IQ Bal and Hi and Lo Phase null adjustments are the ones that are dependent on the setting of the volume control. If you are peaked at the high end of 7 on the BPF and are not at max or min setting on C1, then you are good to go at that level. 

73 .. Ron

On Wed, Aug 18, 2021 at 12:00 PM Gary Bernard via groups.io <garybernard2=aol.com@groups.io> wrote:
Sounds to me like you did a great job. 20 has been dead lately. Try late afternoon and early evening. 
GL, Gary W0CKI 


On Aug 18, 2021, at 12:06 PM, wb8yyy via groups.io <wb8yyy=yahoo.com@groups.io> wrote:

Vance

nice work, just built a 20m mini here. 

the 7 level on BPF peaking means little - it changes as you adjust volume control.  as long as you get a pronounced peak, all is well. 

the other nulling calibration is more important, and sounds like that went well.  note it takes several iterations and each menu setting and knob to get it optimized. 

20m propagation has not been great recently.  i managed a few WAE contacts this past weekend, but not much since.  so hang in there. 

5 volts on coax center pin has me spooked - check it again with no antenna attached.  I confess I have not looked into that, check the schematic and see who else comments.  there should be a blocking cap that isolates the center pin from the circuitry - and with EFHW likely introducing a DC short - I am saying how can this be if the blocking cap is shorted - check it again. 

73 Curt wb8yyy
fellow QCX traveller


wa8yan.radio
 

Hi Curt and Vance....  "QCX Traveller" ..... I love it ! !  New name for this group.
73 Phil WA8YAN



Sent from my Verizon, Samsung Galaxy smartphone


-------- Original message --------
From: "wb8yyy via groups.io" <wb8yyy@...>
Date: 8/18/21 2:06 PM (GMT-05:00)
To: QRPLabs@groups.io
Subject: Re: [QRPLabs] #qcxmini Ideal BPF Peak and 5v on coax center pin?

Vance

nice work, just built a 20m mini here. 

the 7 level on BPF peaking means little - it changes as you adjust volume control.  as long as you get a pronounced peak, all is well. 

the other nulling calibration is more important, and sounds like that went well.  note it takes several iterations and each menu setting and knob to get it optimized. 

20m propagation has not been great recently.  i managed a few WAE contacts this past weekend, but not much since.  so hang in there. 

5 volts on coax center pin has me spooked - check it again with no antenna attached.  I confess I have not looked into that, check the schematic and see who else comments.  there should be a blocking cap that isolates the center pin from the circuitry - and with EFHW likely introducing a DC short - I am saying how can this be if the blocking cap is shorted - check it again. 

73 Curt wb8yyy
fellow QCX traveller


 

You guys are breakin' my heart! You didn't read my notes on how to bring up a new QCX Mini!!!  :-(

5V appears on the center pin of the BNC connector because it is put there courtesy of the output of IC3C via a 120k ohm resistor, which is used to inject an RF signal for receiver calibration.

I mentioned it because it's a quick and dirty way of checking you have 5V supplied to the logic chip IC3.

Tsk tsk ;-)

--
Julian, N4JO.


 

Jeff just asked me to post a link to my notes on bringing up a QCX for the first time.
Here it is:
https://groups.io/g/QRPLabs/message/70211
I'll try to refine it over the weekend.
--
Julian, N4JO.