Transmitter Mods

Howard Fidel
 

I got my uBitx last week. I was surprised to get it, since it was sent 3 days before and I paid for India Post shipment, but it came by DHL, which was a nice surprise. I have assembled it, and started testing and am doing a number of modifications in my lab before moving it to its home in the shack.

Today I modified the transmitter to improve the output power at the higher frequencies. The power out in the original state is about 3 watts at 28 MHz. My modification brings the power at 28 MHz up to 7 watts on both the 15 and 10 meter bands. On my board at least, there is not enough range in VR1 to increase the power to anywhere near this level without this modification. The other bands remain unchanged in output, which I measured at 11 watts.  The output stage could be modified as others have done by changing the FETs and transformer to improve it more, but I am happy with this for now. I plan on adding a power amplifier later, something between 50 and 100 watts.

My fix is simply to add 3 passive parts. I added:
33 uH inductor in series with R86. I lifted the resistor mounting it on one pad standing up, and teepeeing the inductor between the top of the resistor and the other pad.

Add a 270 pF capacitor across both R87 and R88.

The values are not very critical, I calculated that I needed 27 uH and 220 pF, but the values I tried were in the lab.

If anyone wants to try the fix and needs the parts, I have a full reel of each, so just send me a stamped self addressed envelope, and I will send it to you by return mail. The offer is good until I run out of parts.

Next up is adding an AGC and S meter.

I also measured the receiver sensitivity and can confirm that it meets specifications. 

Howard

WB2VXW


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