#qcx #problem Radio newbie and QCX large audio hum... #qcx #problem


KB3USG
 

[Apologies if this appears twice--I've waited many hours for this second
post to show...]

Hello everyone,

Previously I posted to the list how I had had no audio output due to my
potentiometer being tightened to much--that was hard for me to debug,
and I'm grateful to the excellent manual for making the warning. I
don't think that I would have thought about it as I was trying to debug
the radio,

But now, when I touch the volume knob, (or get near the radio on the
left side) there is substantial hum in the audio channel (I assume from
my capacitance on the circuit).

So, I'll admit, I'm a complete newbie and I don't know whether this
is something I should expect, or whether I managed to damage something
in my checking solder connections when I was trying to debug my (almost
nothing) audio levels. I will also remark that I need to re-adjust my
T1 coil, but I wouldn't see how that would make a difference.

Does anyone with more experience with radio and these circuits generally
comment? Is this sensitivity to an operator being in the vicinity of
the volume pot normal, or should I be doing more debugging with the
circuit to find what I may have messed up?

I'm trying to learn, so any help or constructive criticisms would be
greatly appreciated.

Thanks!
Paul
KB3USG


N3MNT
 

What do you have connected to the antenna connector when this happens?


KB3USG
 

On 2018-12-03 14:22-0800, Chopper wrote:

But now, when I touch the volume knob, (or get near the radio on the
left side) there is substantial hum in the audio channel (I assume
from my capacitance on the circuit).
What do you have connected to the antenna connector when this happens?
Thanks, Chopper :)

It happens when I have either the QRPlabs 20W dummy load (which I have
fit to an Altoids tin) or my homebrew magnetic loop.

It does seem quite localized to IC9 and the (formerly mashed)
potentiometer.

Really my (newbie) question is "is this right or wrong for this kit?"

My current plan is to:
* Recheck the board voltages and see whether anything has changed
radically from before.
* (As part of my work peaking the BPF), take a couple of winds off
the T1 coil.

If anyone has a definintive answer or suggestions on what I should look
at next, I'm all ears.

Thanks,
Paul
KB3USG


Ronald Taylor
 

Hi Paul. I was watching this thread thinking about your issue. 

First, No it is not right for this kit to have huge hum by touching the pot and most stuff around it. Normally I would think you could get some hum by touching the high side or the center pin (wiper) of the pot or by touching the input pin 6 on IC10 But touching the pot shaft or housing or other spots should not necessarily create lots of hum. This is grounded to the circuit board.

 I saw that you possibly damaged the pot by tightening it too much. In the absence of any other symptoms, I think you should do a good checkout of the volume pot. 

 Check the pot mechanically. Does the shaft wobble? Is there anything loose floating around inside it? Etc. then test it with an ohm meter, first across the outside pins for something close to the rated value, then from the center pin to one end looking for any discontinuities as you turn it up and down, then center pin to the other side same test. Also wiggle the shaft with the ohmmeter from the center pin to either side pin and see if it looks intermittent. While you’re at it recheck (again) the solder connections to the board. They are just barely long enough to contact the board and very close inspection could show one of them not quite making contact. 

If the pot shows any irregularities it will need to be replaced. Hope this helps. Good luck 

Ron

On Mon, Dec 3, 2018 at 17:37 KB3USG <kb3usg@...> wrote:

> On 2018-12-03 14:22-0800, Chopper wrote:
>
> > But now, when I touch the volume knob, (or get near the radio on the
> > left side) there is substantial hum in the audio channel (I assume
> > from my capacitance on the circuit).
>
> What do you have connected to the antenna connector when this happens?

Thanks, Chopper :)

It happens when I have either the QRPlabs 20W dummy load (which I have
fit to an Altoids tin) or my homebrew magnetic loop.

It does seem quite localized to IC9 and the (formerly mashed)
potentiometer.

Really my (newbie) question is "is this right or wrong for this kit?"

My current plan is to:
  *  Recheck the board voltages and see whether anything has changed
     radically from before.
  *  (As part of my work peaking the BPF), take a couple of winds off
     the T1 coil.

If anyone has a definintive answer or suggestions on what I should look
at next, I'm all ears.

Thanks,
Paul
KB3USG




KB3USG
 

Hey everyone,

I appreciate everyone's feedback and suggestions.

On 2018-12-03 18:09-0700, Ronald Taylor wrote:
First, No it is not right for this kit to have huge hum by touching the pot
and most stuff around it. Normally I would think you could get some hum by
touching the high side or the center pin (wiper) of the pot or by touching
the input pin 6 on IC10 But touching the pot shaft or housing or other
spots should not necessarily create lots of hum. This is grounded to the
circuit board.
Thanks for this, Ron. I did check out the pot as you suggested, and I
couldn't determine anything really physically amiss. However ...


My current plan is to:
* Recheck the board voltages and see whether anything has changed
radically from before.
I did this, and nothing was really very different from the readings in
the manual (or from my first check-out of the voltages across the
board).

However... when I was holding the board on one edge and my fingers
touched the case of the 7805 regulator, the hum went away. Really
sounding like a grounding issue.

I'm thinking that I've done some irreversible damage to the pot that was
not recoverable when I backed off the tension on the pot (I wouldn't
think this possible, so I'm grateful for some of the remarks in the
manual).

I'll hunt down a pot to put in place for now and see whether that makes
everything better.

And then get to this:

* (As part of my work peaking the BPF), take a couple of winds off
the T1 coil.
I'll keep updating the thread as I make progress.

Again, thanks to everyone who's reading these messages and musing to
themselves, and to those who have spoken up with some advice for me.

Thanks,
Paul
KB3USG


KB3USG
 

Hey everyone,

Quick recap--smooshed the volume pot by overtightening it, got "zero"
output, and when tried to unsmoosh the pot, had lots of audio hum. My
BPF was not peaked, and I was going to get back to that as well as check
voltages around the board and compare them to values in the (EXCELLENT)
manual. This radio is being hooked up to a makeshift magnetic loop
antenna (based on G4ILO's (SK) Wonder loop).

Ok, so still could use some help, and thank you again to everyone who's
been reading my messages and to those offering some help and
encouragement.

First things first--replaced the pot with a 10k log-taper pot with a 10k
fixed resistor across the outer pins. I think that I may have wired it
in backwards as it operates in the other direction... livable for now.

The audio hum is a lot better, but not absent. As this really is my
first real radio, I'm still trying to figure out what "normal" is. It
does seem better, at least.

Second thing, peaking the BPF--In the midst of all this, I seem to have
one from the "add more turns" to "remove more turns", but I _think_ that
I made a mistake and didn't have a dummy load connected at one point
(but had my makeshift loop connected). In any case, I have been extra
careful to also try to peak into the dummy load, and have gone from 5
extra turns to removing 6 turns from the starting configuration.

Now, moving the capcitor doesn't seem to have any effect.

I _believe_ that I have made electrical connections, and I can hear some
locally strong signals, so at least the "you need to burn the insulation
off" part seems to have been done (right?). Here's where I could use
some help diagnosing this--why would the capacitor no longer effect the
peaking operation? I'm getting concerned that I'm damaging the board.

Relatedly, I did have some weird behavious with the other peaking
operations where the low audio setting would suddently jump to a very
high value and then jump back. Is it possible to damange the multi-turn
pots?

One last question (well, for this message, at least): I realize now that
my meansurements of T1, pin 5, 6, 7, 8 measure 1.8x volts (x=1,3)
instead of the 2.36 listed in the manual. I measure using the onboard
DVM. It's possible that instead of the five turns for the rest of
windings on T1 that I may have, in fact, six--will this cause a lot of
problems? Would this difference help explain anything else described in
here?

So, I'm really confused that I'm not having any effect on peaking the
BPF (and the other symptoms). I'm a real newbie on all this (first real
radio) so things that you might think "obvious" might not be for me, so
I would appreciate any elmering that anyone is willing to give :) I'm
afraid that I'm ruining my radio instead of making it better. I'm dying
to get this one working (and just might have to build another!)

Thanks for any help,
Paul
KB3USG


Alan de G1FXB
 

Paul,
I wouldn't try to attack multiple issues all at once.
It sounds from your description the build used to work better than it does now, did you ever get it fully aligned as per instructions?
If not, perhaps spend some time looking over the soldering and making sure the component leadouts are not touching something adjacent, the body of he volume pot has being reported before....
Perhaps spend some time checking the  components are in the right places, miss reading the colour code on resistors easy.


If nothing is apparent concentrate one at a time, start at the beginning of the circuit and work forward.
(If you can make no progress after giving it "a good go", then fair comment go to the next issue.
(Sometime a fault interacts and you are convinced a fault is in one particular area, when it's actually elsewhere...))

Can you take some good photo's of the PCB board, both sides & if you have doubts over  5 or 6 turns on T1 a good enough photo so others can count the number of at least the "small windings"?
I think Hans commented he erred on specifying perhaps too many turns, as its easier to remove rather than add wire.
Also in the front end, C1 can be damaged by the application of excessive heat while soldering. On close inspection does the body or looking through the opaque top does anything look melted?.
Does the adjuster feel smooth when turned all the way around?
(the fixed / movable vanes are separated by polythene plastic dielectric that keeps them separate.)


Alan

On 13/12/2018 12:54, KB3USG wrote:
Hey everyone,

Quick recap--smooshed the volume pot by overtightening it, got "zero"
output, and when tried to unsmoosh the pot, had lots of audio hum. My
BPF was not peaked, and I was going to get back to that as well as check
voltages around the board and compare them to values in the (EXCELLENT)
manual. This radio is being hooked up to a makeshift magnetic loop
antenna (based on G4ILO's (SK) Wonder loop).

Ok, so still could use some help, and thank you again to everyone who's
been reading my messages and to those offering some help and
encouragement.

First things first--replaced the pot with a 10k log-taper pot with a 10k
fixed resistor across the outer pins. I think that I may have wired it
in backwards as it operates in the other direction... livable for now.

The audio hum is a lot better, but not absent. As this really is my
first real radio, I'm still trying to figure out what "normal" is. It
does seem better, at least.

Second thing, peaking the BPF--In the midst of all this, I seem to have
one from the "add more turns" to "remove more turns", but I _think_ that
I made a mistake and didn't have a dummy load connected at one point
(but had my makeshift loop connected). In any case, I have been extra
careful to also try to peak into the dummy load, and have gone from 5
extra turns to removing 6 turns from the starting configuration.

Now, moving the capcitor doesn't seem to have any effect.

I _believe_ that I have made electrical connections, and I can hear some
locally strong signals, so at least the "you need to burn the insulation
off" part seems to have been done (right?). Here's where I could use
some help diagnosing this--why would the capacitor no longer effect the
peaking operation? I'm getting concerned that I'm damaging the board.

Relatedly, I did have some weird behavious with the other peaking
operations where the low audio setting would suddently jump to a very
high value and then jump back. Is it possible to damange the multi-turn
pots?

One last question (well, for this message, at least): I realize now that
my meansurements of T1, pin 5, 6, 7, 8 measure 1.8x volts (x=1,3)
instead of the 2.36 listed in the manual. I measure using the onboard
DVM. It's possible that instead of the five turns for the rest of
windings on T1 that I may have, in fact, six--will this cause a lot of
problems? Would this difference help explain anything else described in
here?

So, I'm really confused that I'm not having any effect on peaking the
BPF (and the other symptoms). I'm a real newbie on all this (first real
radio) so things that you might think "obvious" might not be for me, so
I would appreciate any elmering that anyone is willing to give :) I'm
afraid that I'm ruining my radio instead of making it better. I'm dying
to get this one working (and just might have to build another!)

Thanks for any help,
Paul
KB3USG


Dave Redfearn
 

Paul,
When you are describing "peaking the BPF", do you mean the receiver alignment where you adjust C1, R27, R17, & R24 or are you troubleshooting low transmit output power by adding/removing turns from the low pass filter coils?

For the receiver alignment where you adjust C1, R27, R17, & R24 (8. Alignment Menu), you need to follow the instructions in the manual and not have anything connected to antenna jack.

From the Rev 3 manual: 
"In summary: the alignment tools built into the radio consist of a signal generator which injects a signal into the RF front end, and digital signal processing which adds a 250-Hz digital filter to the existing 200Hz analogue filter, and calculates the amplitude of the signal detected in that bandwidth. During alignment, the amplitude is displayed on-screen as an intuitive bar across the bottom row of the display. Using a screwdriver, you adjust the trimmer component in order to maximise or minimise the displayed amplitude.
Unplug the antenna during alignment of the radio!"

Also, since the QCX has no built-in protection for the finals, you do not want to transmit into the Loop antenna unless it has been resonated on the frequency you are using (or any other antenna for that matter).
A safe way to do this would be to use a resistive VSWR bridge in-line which can provide a 50 ohm load to the transmitter while the antenna is being tuned.
Here is an example of a resistive SWR bridge kit: https://www.qrpkits.com/swrind_case.html and an example of this type of circuit included in an antenna tuner: https://qrpguys.com/multi-tuner

73 - Dave, N4ELM
 




N3MNT
 

Since you said that you used a little too much heat on the audio pot, it is possible that you overheated the cap C1 as that is more delicate than the large pot.