Awhile back I built the (tr)uSDX multiband QRP transceiver kits, one for the low bands and one for the high bands.

One of the glitches with these kits was the “howling” from the speaker in some circumstances. I observed this side effect or glitch if either there was not enough input signal entering the radio via the antenna (antenna disconnected or connected to a dummy load), or, if the volume was turned up beyond a certain point. This occurred with both the internal speaker and with a speaker mic that I connected via an adapter that I built.

Shortly after I assembled my second kit, the mainboard was revised to version 1.2. Both of my kits I had assembled had the revision 1.0 mainboard.

Revision 1.2 has reverse polarity protection and over current protection using a polyfuse:

This revision also adds two capacitors to block DC buildup to the inputs of the op amps U9A and U9B:

My rev 1.2 board came with firmware version 2.00i installed which at the time of assembly was the most recent release version. After assembly, I upgraded the firmware to the latest beta version available at the time, version 2.00x.

I adjusted my reference frequency to 26,997,500. This time when calibrating the reference frequency, I started on CW mode and then fine tuned it for both SSB and FM modes.

I set the PA Bias max to 160 since the high bands version is what I am using this main board with, and it has the FDT86256 FET instead of BS170 FETs.

Initial Testing

After adjusting the reference frequency, I noticed that there was some modulation using FM mode and AM mode, which I had trouble with before. Although the modulation was a bit low, it did work. Perhaps a software update fixed this – the hardware changes would not account for the issue with FM modulation that I have had in the past.

Note that SSB, AM, and FM modulation modes are not the primary modes of operation for this transceiver, CW works best. The other modes need some improvement to be really useful.

I had similar results to DL2MAN with the “howling”. This version allows for a slightly higher volume level before the point where the “howling” begins when compared to the previous revision. However, the difference is not significant.

Final Thoughts

I was not expecting a complete fix for the “howling” with this board revision, but there is an improvement. Since it was possible to purchase just the main board without the RF board, the cost to try out this revision was less than buying a full kit. In addition, it took only about 30 minutes to assemble the main board, as this was my third round at assembling one of these.

I would not upgrade the main board on my other kit, nor would I suggest that this would be a significant improvement if you happen to have any kits with the previous revision. The reverse polarity protection and poly fuse for overcurrent were more significant updates, and by now, all new kits would either be shipping with this version or a newer one.