VFR check - transponder not locking onto squawk codes question

[pazmanyflyer]

Hello all.
Prepping for my AWC and first flight I had my VFR transponder check on Friday. I had the plane (RV-7) plugged into the power supply so there wouldn't be any power issues while testing. Once test started, the test equipment was showing transponder not locking on the squawk codes. Alt was fine, but codes were jumping all over. I was in the test mode as instructed in the Dynon manual and was out of auto mode. After a few attempts we decided to shut down and I turned off the power supply before restarting the next test. Ran the test again this time only on ship battery. Everything worked as it should. Solid locks and signal strengths on all codes tested. Hmmm. Is there anywhere mentioned in the manual as to be on ship power only and not on supplied power that I missed? Signal intereference? Anyone else see something similar?

 

[GKC Aviation]

I reckon you have an electrically noisy power supply.
Voltage won't be an issue, as modern avionics (Dynon included) will run no problem between 11-33 volts.
Weird things happening in a digital system is indicative of noise on the power supply. The fact it went away when you stopped using the power supply reinforces this. You didn't mention if you had the ships battery still connected while you had the power supply on, as this will usually sink some of the noise.
Was it a battery charger you were using as a power supply? These will, depending on function, pulse voltage. This is good for batteries, but not so much for avionics.

 

[Rhino]

Trickle chargers/maintainers are notorious for this too.

 

[pazmanyflyer]

I reckon you have an electrically noisy power supply.
Voltage won't be an issue, as modern avionics (Dynon included) will run no problem between 11-33 volts.
Weird things happening in a digital system is indicative of noise on the power supply. The fact it went away when you stopped using the power supply reinforces this. You didn't mention if you had the ships battery still connected while you had the power supply on, as this will usually sink some of the noise.
Was it a battery charger you were using as a power supply? These will, depending on function, pulse voltage. This is good for batteries, but not so much for avionics.

It's an MFJ 30A power supply. I did have it connected to the battery. I'm sure ATC will tell me if I have any further trouble when I start flying.

 

[pazmanyflyer]

Trickle chargers/maintainers are notorious for this too.

Good to know. Thank you.

 

[Rhino]

MFJ makes very good power supplies. But switching power supplies, though filtered at the output, have occasionally been known to cause significant noise. Might be an outside shot, but if you have access to an oscilloscope, or even a spectrum analyzer, looking at the output may be helpful. Just be aware there will always be some noise present. It just shouldn't be excessive. The twist here is that power supply noise is almost always coupled noise that usually affects analog systems long before digital, so your problem as reported is fairly interesting. Probably the easiest thing to do is just try a different power supply. If that doesn't help, and you want to play process of elimination, you might try connecting to your avionics bus as opposed to the battery, and possibly even disconnecting the battery entirely. As GKC noted, a battery can sometimes sink noise. But some batteries, or their associated connections, react badly to having a power supply attached.