I've been having an annoying behavior with my QDX. It worked, but occasionally would flake out, especially just when I started to respond to a CQ.
I've just made a surprising discovery. (For me, anyway.) I've been using the wrong plug to power my QDX (and now QMX).
The correct size has a 5.5 mm OD and is compatible with a pin that is 2.1 mm diameter. All the 12 V cords in my house have the other size, which is 5.5 mm OD and compatible with a 2.5 mm diameter pin. It mostly works, but it doesn't make good contact, which correlates with the flaky behavior.
I've been on a real rabbit hole with DC barrel connectors lately. I need something to power my Maestro Evolve on 12 V. I found a 5.5 to 3.5 adapter. It didn't work. It ALSO has the 2.1 diameter pin on the 5.5 mm side, and it doesn't make contact at all with my 2.5 mm plugs. So now I have to go find new power cords, new adapters, or retrofit the cords I have.
My advice is add a couple of the right plugs to your order. They're only $1. I ordered one with my first QDX, but never used it because the power cords I already had seemed work OK. Turns out they didn't really.
You can tell the plugs apart with the right drill bit. For those of us whose drill bits only come in barbarian units,
A 5/64in. (1.98mm) bit will slide into a 2.1mm connector but a 3/32in. (2.38mm) bit will not.
A 3/32in. (2.38mm) bit will slide into a 2.5mm connector but a 7/64in. (2.78mm) bit will not.
Or you could get a STARRETT S4009-750 Pin Gauge Set,Dimension Type " 4CEV2, only $617.50 at Grainger. (Free shipping!)
Maybe everyone but me already knew that, but I'll go ahead and risk looking like an idiot to save someone else from the same mistake.
