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QCX 40 - Need help with BPF alignment

[email protected]
 

Hi,

I just finished building my QCX 40 and I’m having problems with the BPF alignment. I found that it peaks with a maximum amplitude value of 4 with the bars almost to the right of the display. At that point the C1 plates are fully open.

I read a recommendation that it’s best to do this step with a dummy load attached, so I used the QRP Labs dummy load.? This time I found the maximum amplitude value was only 3, with the bars again all the way to the right and the C1 plates still fully open at that point.

Per the instruction manual, I removed 5 turns from the secondary 3 winding of T1. However, doing that hasn’t changed anything, as I’m still getting the same results as above, with the peaking occurring when the C1 plates are fully open.

As I live in an apartment, I tried connecting the QCX to a magnetic loop antenna outdoors both before and after the change to T1, but no signals were received. I don’t know if that’s just due to my location, so I intend trying again in a more open location over the weekend.

However, I’d like to try to see if there’s anything else I need to due to sort out the BPF alignment before then. Any help or advice would be appreciated.

Thanks.

John

W6KJL
 

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If it were me, rather than the order perhaps it should be.....

1/.??? Continuity check the windings of all the toroids.
??? ??? ??? (The LPF is not only for the TX it is also in series with T1 BPF on receive. And needs to pass the alignment signal (and off air signals) builders not having stripped the enamel sufficiently has caught people out in the past)
??? ??? ???
2/.??? If that tests OK, carry on to the Phase adjustments do they show? good response?

3/.??? Is TX Power OK?

4/.??? Finally, Voltage checks, to see if they give a pointer to a stage that requires further investigation.

Alan




On 09/08/2018 17:10, jmcf2005@... wrote:

Hi,

I just finished building my QCX 40 and I’m having problems with the BPF alignment. I found that it peaks with a maximum amplitude value of 4 with the bars almost to the right of the display. At that point the C1 plates are fully open.


I read a recommendation that it’s best to do this step with a dummy load attached, so I used the QRP Labs dummy load.? This time I found the maximum amplitude value was only 3, with the bars again all the way to the right and the C1 plates still fully open at that point.

Per the instruction manual, I removed 5 turns from the secondary 3 winding of T1. However, doing that hasn’t changed anything, as I’m still getting the same results as above, with the peaking occurring when the C1 plates are fully open.

As I live in an apartment, I tried connecting the QCX to a magnetic loop antenna outdoors both before and after the change to T1, but no signals were received. I don’t know if that’s just due to my location, so I intend trying again in a more open location over the weekend.

However, I’d like to try to see if there’s anything else I need to due to sort out the BPF alignment before then. Any help or advice would be appreciated.

Thanks.

John

W6KJL
 

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One other thing,

Whilst adjusting C1 can you see it approaching a peak, from fully meshed Or is it a sudden "all or nothing" peak?

(The trimer is polythene construction, and thus requires careful application of heat during soldering otherwise it's possible to melt the dielectric.)


Alan


On 09/08/2018 17:45, Alan de G1FXB via Groups.Io wrote:

If it were me, rather than the order perhaps it should be.....

1/.??? Continuity check the windings of all the toroids.
??? ??? ??? (The LPF is not only for the TX it is also in series with T1 BPF on receive. And needs to pass the alignment signal (and off air signals) builders not having stripped the enamel sufficiently has caught people out in the past)
??? ??? ???
2/.??? If that tests OK, carry on to the Phase adjustments do they show? good response?

3/.??? Is TX Power OK?

4/.??? Finally, Voltage checks, to see if they give a pointer to a stage that requires further investigation.

Alan




On 09/08/2018 17:10, jmcf2005@... wrote:

Hi,

I just finished building my QCX 40 and I’m having problems with the BPF alignment. I found that it peaks with a maximum amplitude value of 4 with the bars almost to the right of the display. At that point the C1 plates are fully open.


I read a recommendation that it’s best to do this step with a dummy load attached, so I used the QRP Labs dummy load.? This time I found the maximum amplitude value was only 3, with the bars again all the way to the right and the C1 plates still fully open at that point.

Per the instruction manual, I removed 5 turns from the secondary 3 winding of T1. However, doing that hasn’t changed anything, as I’m still getting the same results as above, with the peaking occurring when the C1 plates are fully open.

As I live in an apartment, I tried connecting the QCX to a magnetic loop antenna outdoors both before and after the change to T1, but no signals were received. I don’t know if that’s just due to my location, so I intend trying again in a more open location over the weekend.

However, I’d like to try to see if there’s anything else I need to due to sort out the BPF alignment before then. Any help or advice would be appreciated.

Thanks.

John

W6KJL
[email protected]
 

Hi Alan,

Thank you for your help. Following your suggestions, here’s what I found;

  1. I checked the continuity of all the windings on T1, as well as L1, L2, L3 and L4. All were good.

  2. I checked the Peak BPF and while adjusting it noted that with C1 fully meshed, the maximum amplitude was 4, with 6 large segment bars on the scale. As I rotated C1 to fully open, the number of bars increased from 6 to 13. It didn’t look like a sudden jump.

  3. I then checked the phase adjustments with the volume set at about 50%.

I-Q Balance was an amplitude of 6 with 50% of the bars

Phase Lo was 5 with 50% of the bars.

Phase Hi was 6 with 50% of the bars.

I checked the transmit power with a meter into the dummy load and it was exactly 4w.

You also mentioned doing voltage checks. Are there any instructions somewhere on the procedure for that?

Thanks again.

John

?

?
 

The voltage check procedure is in the manual. The given voltages assume you are using the built in meter. If you use an external DVM you will find that some of the readings are much higher. This is because the internal meter has quite a low impedance and will pull some of the voltages down.

Are you sure all the windings on T1 are OK? Firstly, are they all in the same direction? If you followed the procedure in the manual then they should be OK. Then you need to be sure that the wires are correctly soldered. I found that although I got correct continuity by probing the ends of the wires, they weren't actually soldered properly to the board. I had to apply more heat to burn off enough enamel. Even if the windings aren't quite right you should be able to hear something.
 

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Hi John,

Something to bear in-mind I'm doing this blind as I have not built a QCX,
people who have, need to correct my statements if wrong!

H'Mmm
C1 seems OK.
TX Power OK.

Overall the peaks seem low, past discussions have indicated people report scaling factors of 8/9, but as you say its dependant on AF gain position for all but C1 alignment.
There is a voltage table at the end of the Assembly Manual, section 6.4
The measurements were made using the QCX inbuilt DVM which has a lower I/P impedance than most stand alone meters.
At first sight the table is a little strange because it doesn't start at IC1 but the order it is made, sets the basic requirements and then follows the signal path through the different circuits.
For sake of convenience it would be best to use the inbuilt, unless you have any concerns to its accuracy.
Oscilloscope trace & levels can be found on the troubleshooting web page. If you have access to a scope, through a friend if necessary.

A quick bit of background,
What version kit have you, Firmware revision?
The kit otherwise went together OK and raised no queries?
If possible a couple of photo's of top & bottom of the PCB just incase it's possible to spot something.

Alan


On 09/08/2018 21:26, jmcf2005@... wrote:

Hi Alan,

Thank you for your help. Following your suggestions, here’s what I found;

  1. I checked the continuity of all the windings on T1, as well as L1, L2, L3 and L4. All were good.

  2. I checked the Peak BPF and while adjusting it noted that with C1 fully meshed, the maximum amplitude was 4, with 6 large segment bars on the scale. As I rotated C1 to fully open, the number of bars increased from 6 to 13. It didn’t look like a sudden jump.

  3. I then checked the phase adjustments with the volume set at about 50%.

I-Q Balance was an amplitude of 6 with 50% of the bars

Phase Lo was 5 with 50% of the bars.

Phase Hi was 6 with 50% of the bars.

I checked the transmit power with a meter into the dummy load and it was exactly 4w.

You also mentioned doing voltage checks. Are there any instructions somewhere on the procedure for that?

Thanks again.

John

?

?
[email protected]
 

Hi Richard,

Yes, the windings on T1 are all in the same direction, as I followed the procedure in the manual. I scraped off the enamel and checked continuity before soldering. After soldering I checked it again by touching the probes onto the solder blobs.

Thanks,

John
Al Szymanski
 

A sidebar comment.? It's one thing to check continuity of windings by making contact with the exposed ends of the windings as they lay exposed on the circuit board after soldering. It's an entirely different animal to check the continuity by applying the test leads to the "next contact pad" at each end of the wiring.?
In the first case, all you will find out is if you broke a wire in the process of winding.
In the second case, you determine if the winding -> solder -> circuit board trace -> next component has a connection.

This has caused me grief in the past and is now part of my testing phase.?
Best of luck
Al
 

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Correct Al,
from the schematic the quick & dirty way to discount the windings in the LPF chain in one step for others:-
test prod on BNC Ant socket centre, and other prod to make sure you have continuity to one of the pads C27 junc C29

Having said that, the original poster states he has good (4W) TX power output.
Which infers the LPF has continuity, and not the problem in this case.

Alan

On 10/08/2018 18:25, Al Szymanski via Groups.Io wrote:

A sidebar comment.? It's one thing to check continuity of windings by making contact with the exposed ends of the windings as they lay exposed on the circuit board after soldering. It's an entirely different animal to check the continuity by applying the test leads to the "next contact pad" at each end of the wiring.?
In the first case, all you will find out is if you broke a wire in the process of winding.
In the second case, you determine if the winding -> solder -> circuit board trace -> next component has a connection.

This has caused me grief in the past and is now part of my testing phase.?
Best of luck
Al

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