QCX Mini: Winding and Installing T1

QCX: Tips for Winding T1

Critical QCX Mini Update and some Mini Tips

On 12/3/2020, Hans Summers of QRP Labs released the QCX Mini- a truly miniature 5W CW transceiver. By the December 7, all 997 units were sold! It was definitely exciting to see the response to this new product, and after having built most of my own QCX Mini, I can confirm- it’s an incredible little radio! I’ve got some QCX Mini construction tips to share with you. But before we get to that:

Critical QCX Mini Modification Advisory

On 12/14/20, Hans posted on the QRP Labs Groups.io page a post with the following subject:

QCX-mini critical modification advisory

Four QCX Mini builders had their QCX Mini’s go up in smoke upon power up after having rapidly cycled the power, causing Magic Smoke to be released and leaving them with non-functioning QCX Mini’s. The problem was found to be that the SMD capacitor C38 was insufficient. You see, during testing, Hans did not rapidly power cycle the radio, as that’s not a typical mode of operation. But, he has since decided to add rapid power cycles to his litany of tests that he runs on new products.

The fix is very very simple: Solder in a 10uf capacitor between a leg on the voltage regulator and to a leg of diode D33. The manual outlines the fix and future QCX Mini’s will include the fix on the PCB.

What you Need to do

Perhaps nothing! If your QCX Mini hasn’t shipped yet, then you’ll be getting a 10uf capacitor with your kit, and you’ll just install it as part of the construction.

If you have already received your QCX Mini, do not fret! You need to add any capacitor greater than 10uf and rated for at least 16V as shown on Page 68 of the updated manual. As you can see below, it goes behind the power connector. If you don’t have a such a capacitor, then ask around. These are very easy to come by and can even be salvaged from old portable radios, TV’s, and other analog electronics. And, some hams have even offered to ship the needed capacitors to those in need. Check out the QRP Labs Groups.io page to find such posts. Kudos to them for stepping up!

If it’s not possible for you to source one yourself, contact QRP Labs and Hans will make sure you get one. HOWEVER before you do that, Read This Post and consider ordering a capacitor on Mouser or Arrow.com. You got a world class radio quite inexpensively, after all.

When installing it, make sure not to melt the 8 pin header like I did. It still works though!

QCX Mini “Mini Tips” on YouTube

While constructing my own QCX Mini, I decided to take a few minutes and share some of my own consstruction tips, shortcuts, and general ideas. Each video is only 2-4 minutes long, and the hope is that you will be able to have an easier time in building your own QCX Mini. Be sure to subscribe to the YouTube channel if you haven’t already. Here’s the first video, and each video has a link to the next at the end:

If you have any of your own tips, please leave them in the comments either below or on YouTube. Thanks!


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    • Fred W0FMS on December 16, 2020 at 9:37 PM
    • Reply

    This is my opinion on this and not official QRP Labs information:

    So…… the regulator that comes on the mini is only rated for maximum 15V input versus 35V on the other versions. I strongly recommend using a 25V rated cap instead of 16V on the input if you can find one to fit and possibly putting one on the output too (10V or more here is fine IMHO) . (I can’t figure out if it can fit until I get my kit.)

    And never experiment with voltages above 13.8V for increased power output. 12V as an upper limit would even be better.

    The AMS1117-5 regulators are used on Arduino Nanos and the general consensus on these are to not use the regulator above 9V if you do internet searches. There are several reports of the same issue happening with the Nanos at 12V. The regulators fail shorted input to output. There are some reports that the TI LM1117 and the AD LT1117 have protection circuitry to avoid this (they fail open) but I am not 100% sure of that yet. But it might be another improvement to go to one of those regulators instead.

    Bigger is theoretically better on the filter capacitors, so if all you can find in the junquebox is 22uF that is actually better if you look up the stability graphs in the datasheet.

    This was an easy mistake to make on the design and I hope the one extra input cap alone fixes it for Hans. I do worry a bit though. I think it will be fine if care is used when handling the mini. It’s still going to be an awesome fun little field radio no matter what happens with the 5V supply.

    Absolute worst case is the five volt regulation can be moved outside of the radio case into the line cord to the radio.

    1. Hey Fred, thanks for the comment. In the thread linked above, there was quite a lot of discussion about whether this is the “right fix” or not. But, the proof is in the pudding. After extensive testing, the failure could not be replicated when the fix was in place. The very moment he removed the added capacitor, the failure occurred. I think I’d feel differently if the fix was “magical” in the sense that he wasn’t sure why it worked- but that’s not how Hans works. He takes a methodical approach to things, and that’s what he did here.

    • Richard on December 18, 2020 at 10:10 AM
    • Reply

    Wind L1-4 clockwise. Much easier to mount. Look at the PCB solder pads and you’ll see why.

    Also, if you build the 17m version you will most likely need to play with L1-3 by expanding/compressing the turns and possibly removing some turns.

    1. Great tip! I wound mine counter clockwise (anti-clockwise for the commonwealth!) and ran into that issue. The same is true on 20 meters about the expanding/compressing/removing turns.

    • Daniel Conklin on January 9, 2021 at 5:10 PM
    • Reply

    I have ordered one of the pocket sized oscilloscopes, but since I already had received two new OPA2277s, I decided on the shotgun approach and replaced IC6 and IC7. Still no joy. In the process I managed to melt R17 and R24. I’ve ordered replacements for those. It’s a learning process for sure, but I have to admit, I’m frustrated.
    Dan, W2DLC

    1. Indeed it can be frustrating. Kit building in general can be. But, you’ll get it and it’ll be a sweet victory 🙂

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