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Fix For QCX+ volume pot?

 

I have to have the control almost in the full counter clockwise position, just about off to lower the volume so it's not ear splitting.
I did a search but couldn't find where someone had a mod where a resistor was added to spread out the volume range on the pot so it's not all the way at the bottom of the range.

Can someone clue me in?
 

Hi Glenn. That¡¯s not the problem. The design is good with a wide range of audio adjustment possible. You have something else causing that. The most common issue is an open ground connection somewhere between the low side of the pot and the ground on the main board. You can check for that with a jumper wire between those two points. If that fixes it, you need to work your way back between the pot and main board ground and reheat solder joints. Ground joints take the most heat and are usually the first ones to check.?

Good luck ¡­. Ron

On Thu, Jul 20, 2023 at 14:45 Glenn W2BX <fintronicsusa@...> wrote:
I have to have the control almost in the full counter clockwise position, just about off to lower the volume so it's not ear splitting.
I did a search but couldn't find where someone had a mod where a resistor was added to spread out the volume range on the pot so it's not all the way at the bottom of the range.

Can someone clue me in?

 

Hi Ron and thanks for the reply;

Nope, the low side of the pot is at zero ohms at the main PCB ground. Maybe I'm not describing this correctly... There is very little range at the bottom of the rotation of the pot. There isn't enough attenuation at the low end of the pot if that makes sense.

I think I remember now.. there was discussion or post regarding the difference between Linear and Audio taper pots... I'm pretty sure this is related to what I'm talking about. I just can't seem to find that post / info.?
 

I just discovered something.. the headphones I've been using are AKG K55 phones... 32 ohm impedance.. the perfect match for the QCX.

I also have a pair of AKG k240 phones... 600 ohms,,? Plugged these in to the QCX and the volume range was different/better BUT the max volume was WAY lower that the 32 ohm phones. So obviously load impedance has a lot to do with this...?
 

Glenn

My 80m qcx plus indeed was ear splitting at min volume. I can't recall exact solution but I may have added some resistance in series feeding to the pot. Try some easy things first. On other bands I did not experience this.?

Curt
 

I found the same with my 20m and 40m QCX-Mini.? I found the AGC module to be helpful.
 

I have the AGC module installed. Unfortunately I've found that if i adjust the pot to quiet the strong signals, i can no longer hear the weak ones. On my next build, i may get the dev board and experiment with some limiter circuits. Right now I'll experiment with some resistors on the pot.
 

Glenn, another thing to check that?isn't as common as the open pot ground connection, is a problem with too much gain in one of the op amp stages. What you are seeing is not normal operation?for this design.?

I found this issue in one QCX?+ one time. It sounds like one of the op amp stages may have an open in its feedback circuit, causing it to go to maximum gain. It's easy to check. Just look for the proper resistance from the output to the inverting input on each stage. Looking at ICs 8 and 9 as an example, you should see zero ohms from pin 1 to pin 2 and from pin 7 to pin 6 on both of those. The same is true for pin 1 to pin 2 of IC10, and you should see 120K ohms from pin 7 to pin 6 on IC10. I'm assuming here that you didn't have any trouble with receiver alignment, so that pretty much confines the problem to the gain in the last three op amp chips.

I think it's a good idea to see if you can find the excess gain problem before modifying the volume control to live with it.?

Good luck ... 73 ... Ron

On Fri, Jul 21, 2023 at 5:57?AM Glenn W2BX <fintronicsusa@...> wrote:
I have the AGC module installed. Unfortunately I've found that if i adjust the pot to quiet the strong signals, i can no longer hear the weak ones. On my next build, i may get the dev board and experiment with some limiter circuits. Right now I'll experiment with some resistors on the pot.
 

All that checks out good Ron.... In the meantime, while tagging some resistors around the pot, and finally removing them and putting it back to normal,? I now have no audio... ugh!! probably blew an opamp.. Crap! Time to trouble shoot...... I should have left well enough alone..
 

I'm looking at the schematic, at the audio section, I see the pot is fed on either side with coupling caps, so I'm wondering how tagging resistors around the pot at various points could have blown an op-amp. I have 5532's so I could just shotgun it and replace the 3 audio opamps.

There is also a BS170 transistor connected there, looks like it's for the mute.....

How disappointing... rig was working great...
 

False alarm.... I feel like an idiot.. Cold solder joint on the pot... fixed, working....
 

I have 6 QCXs with the same linear pot and none of them act like that. I read some comments about the taper in some very old posts on the group site. You might want to do some searching on that. Some just wanted more of an audio?taper and Hans mentioned that a resistor could be parallel?with the pot to make it taper more log than lin. But I can't remember the details. Good luck. Glad you got it working again... 73 ... Ron


On Fri, Jul 21, 2023 at 2:25?PM Glenn W2BX <fintronicsusa@...> wrote:
False alarm.... I feel like an idiot.. Cold solder joint on the pot... fixed, working....

 

Found it.....

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