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QDX Down! Understanding and Preventing QDX Revision 1 Failure

The QRP Labs QDX is an incredible piece of radio engineering, but the first revision had a flaw that ultimately destroyed a some of the Revision 1 radios. To be clear: Unless you bought out of the first batch of 500, this does not apply to you. The second and upcoming third batches do not have the problem.

What is the problem?

The problem is actually the same that plagued the first revisions of the QCX Mini: The AMS1117 voltage regulators. There are two of these in the QDX, because the QDX was actually designed before the QCX Mini. So the AMS1117 lesson had not yet been learned.

The AMS1117 voltage regulators are prone to failure, and when they fail, they don’t fail in an open circuit- they fail in a short circuit, sending the input voltage straight to the output!

What Happened?

The QDX has two regulators. There is an 5v AMS1117 and a 3.3v version. Both of them take the input voltage and regulate it down to their respective value.

In my radio, the 3.3v regulator failed and sent 14v (my input voltage) all the way down the 3.3v bus, and it appears that the board actually caught fire for a moment! You can see the carnage (and the preventative fix for those of you with working QDX rev 1’s) in the video below.

Preventing QDX Rev 1 Failure

There is a VERY easy fix, and it involves getting a 78m05 regulator, installing it where the AMS1117 5v regulator is, and then cutting the input power to the 3.3v regulator, and running a jumper from the 5v regulator’s output to the 3.3v regulator’s input. That way the 3.3v regulator doesn’t have to shed all the input voltage as heat, but rather it only has to deal with 5v. This is much safer.

Here is a picture of the mods, and the explanatory video below. You’ll notice that the 78m05 has a different pinout, and so it is offset by one pin and a short jumper solves the problem with the floating pin.

What’s Next?

Next will be another QDX coming in Batch 3 which will be released soon. Hans and the crew at QRP Labs stand behind the product. I’ll happily pay for a new QDX though, because I paid to be an early adopter, and that comes at a cost. Besides- it’s just a hobby, right? I can hardly wait!


  1. I’m surprised he ran the 3v reg. off of the 12v. I would have done just what ya’ll have done to fix it and power it with 5v. Being a linear regulator, I’m sure it will run cooler now too.

    • J.P. McGinley on March 19, 2022 at 5:50 PM
    • Reply

    Thank you

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