RFI Problem related by routing audio through K3 (uBitx)?

Björn Pasteuning

Hi all,

I have just received my uBitx,

From start I have noticed RFI problems while TX'ing and coming audio from the speakers.

I have shorted all wireing to a minimum.

I looked at the schematic, and see audio is routed through relais K3.
Although audio is switched "loose" while in TX, could this be a culprit?

Anyone else experienced this problem?

Bjorn de PD5DJ

73' Björn de PD5DJ


ABSOLUTELY! This is a no-no and needs to be corrected on all v3 PWBs. I noticed in the new design (v4) he does not use this T/R relay audio switching scheme.I suspect it was done in part to deal with the speaker thumping everyone has complained about. There are other ways to handle that issue and many are discussed at length in other posts. You need to cut the two traces that run that audio connection near the relay. One trace can be "cut" by simply removing R70. The other trace is the one just over that trace when the PWB is positioned with R70 down at the bottom. Then, remove C51 and tombstone it on the upper pad where it formerly was. Add a jumper wire between the top exposed end of C51 and the left pad where R70 was located. Now, that will completely eliminate the RFI in your audio on your PWB. The speaker thumping may in fact become worse after you make this mod. Farhan has made an easy improvement to switch off the audio on the newest version of the PWB. I hope this helps!

ajparent1/KB1GMX <kb1gmx@...>

The combined effect of the rely mod and  the audio switch mod do help greatly.

If the DC switching scheme also used a pair of contacts to pull the floating TX+/RX+
to ground it would help as well.  It can be done with the N3GO two transistor mod.



“PWB” means?

Mark M

Printed Wiring Board,  a.k.a. circuit board.

Gwen Patton

From what I've been able to find, if it has silkscreened component locations, it's not a PWB, it's a PCB.

Most people use PCB nowadays.