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While building my QCX mini, I damaged my R47 variable resistor while trying to remove it, because I forgot to cut off the feet before soldering it in. I finished the rest of the build a few days ago, and discovered when I powered it on that the display contrast was set well enough that I could use it before the potentiometer came in.

However, while I was unplugging a paddle, the screen suddenly went blank and hasn't displayed anything since, even after installing the replacement potentiometer today. I suppose it's possible I shorted something with my finger? I have an enclosure on the way to prevent further incidents, if so. After reading through the manual, I decided to take a reading on the pins of the display module, and they don't match very well with the expected values. With R47 turned 50%, the voltages I read on pins 1-16, respectively, were: 0, 4.7, 1.5, 0.1, 0, 0, 4.7, 4.7, 4.7, 4.7, 2.6, 1.8, 4.7, 4.7, 4.4, 0. I'm not sure if that's useful information or enough to help you debug it, but I'm happy to provide further detail or information you might need to help me get this fully working.

While the screen was working, I was able to get it set up and tuned, and listened in on a few frequencies and it seemed to be working fine.

TU es 73 de W5JIA

Hi Rob,?

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Thanks so much for the reply! I have verified full continuity from LCD pins 11 and 12 to IC2 pins 2 and 3. I also reflowed a few suspect joints on both sides of the 2x5 header. Voltage readings at pins 11 and 12 remain the same.

Also, on mine when I set the pot to 50% there is no displayed data. For reference, you may need to adjust the pot counterclockwise almost to the stop to get normal display and contrast

When turning the pot through it's full range of motion, I get a barely visible fully blocked out display when turned fully counterclockwise, and even an eighth of a turn after that, the screen goes completely blank and remains so until the clockwise stop, photos below. This sounds consistent with what you describe, however I've seen it displaying fully formed letters with decent contrast, and it's not doing so now.

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Fully counterclockwise:

Eighth turn clockwise:

Fully clockwise:

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I'm not sure if this is relevant but I think it is relatively unique to my build: I am still waiting for my DC power plug to arrive, so I have soldered two offcuts into the GND and +12V vias at the top of the main board. I have been connecting my DC power supply to these offcuts using alligator clips. I hope this does not adversely affect the performance or break it altogether. A physical inspection of the main board reveals that something near the DC jack is generating heat. Perhaps D3?

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Thanks again for your help,

73 de W5JIA

Matt. I noticed all of your 5 volt readings were actually 4.7. The 5v regulator is next to the power jack and it does get warm in normal use. But you may have something giving it extra loading. Please turn it off and measure resistance to ground from the 5 volt line. Let me know what you see. Thanks … Ron

On Thu, May 15, 2025 at 17:11 Matt via <matt.purnell98=[email protected]> wrote:

Hi Rob,?

?

Thanks so much for the reply! I have verified full continuity from LCD pins 11 and 12 to IC2 pins 2 and 3. I also reflowed a few suspect joints on both sides of the 2x5 header. Voltage readings at pins 11 and 12 remain the same.

Also, on mine when I set the pot to 50% there is no displayed data. For reference, you may need to adjust the pot counterclockwise almost to the stop to get normal display and contrast

When turning the pot through it's full range of motion, I get a barely visible fully blocked out display when turned fully counterclockwise, and even an eighth of a turn after that, the screen goes completely blank and remains so until the clockwise stop, photos below. This sounds consistent with what you describe, however I've seen it displaying fully formed letters with decent contrast, and it's not doing so now.

?

Fully counterclockwise:

Eighth turn clockwise:

Fully clockwise:

?

I'm not sure if this is relevant but I think it is relatively unique to my build: I am still waiting for my DC power plug to arrive, so I have soldered two offcuts into the GND and +12V vias at the top of the main board. I have been connecting my DC power supply to these offcuts using alligator clips. I hope this does not adversely affect the performance or break it altogether. A physical inspection of the main board reveals that something near the DC jack is generating heat. Perhaps D3?

?

Thanks again for your help,

73 de W5JIA

Please turn it off and measure resistance to ground from the 5 volt line. Let me know what you see.

Resistance to ground from the +5V via next to the paddle audio jack is 120 ohms (reading from the multimeter is 0.120 kOhms, in case I'm converting incorrectly). I get the same reading to the right pin of IC11, which I believe is what you were indicating as the 5V regulator.

Yes, unfortunately, with IC2 out, I'm getting about 6.2 Kohms. What is the process for replacing it?