BITX20 Update #23

After my first BITX20 contact today I set about finding out why my TX output was so low, 500 mW where I had previously had 4 Watts.

A small amount of testing showed that I had connected the two RG174 coax cables the wrong way around. What this meant was the receiver was working through the bias circuit of an off transistor and the transmitter was running without the pre-driver stage on the BITX main board.

After correcting this error I now had about 4 Watts output on whistle but I also had the self oscillating problem back. Grrr.

Leonard, KC0WOX, had kindly mailed me some ferrite cores and some winding wire to help me isolate my problem. I wound new transformers T1 and T2 and coils L3 and L4 for the linear amplifier using the instructions in Leonard’s┬álinear amplifier assembly┬ámanual. I also wound a new T3 for the main board using the instructions in Leonard’s BITX V3 assembly manual.

Once I fitted these new transformers and coils the self oscillating problem was gone. The transmitter output however is only 2 Watts on full whistle but it is stable on either my inverted Vee antenna or my 1/4 wave ground plane.

I had previously connected the normally open side of the K1 relay to ground thinking this would prevent the transmitted signal feeding back into the receiver. This proved to be a bad thing as the transmit signal also travels along this path and I was effectively grounding the transmitter signal form the BITX20 before the linear amplifier. I have now removed this connection.

After all these changes I was keen to try the BITX20 out on air again. With the aid of my external DDS VFO I tuned around and found a couple of local hams who were just finishing up.

After making some adjustments to my DDS VFO frequency and the BFO oscillator I was able to have a good QSO with Glen, VK3PE, and Mike, VK3XL, who were only too happy to help me out.

That makes three QSO in one day and good comments from Glen and Mike after I made some adjustments.

The next challenge is to get my 3 pin speaker/microphone to work with the BITX20 and work out how I am going to mark the VFO dial. I also need to fit NPO capacitors to the VFO to limit the temperature drift that it has at present.


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