Watts on the bench #3 – BITX20 Update # 35

This is a new section to my blog where I will detail what is currently on the work bench. It may be a kit under construction, a radio under repair or some dust from inactivity. Whatever it is, if it’s on the bench and worthy of publishing, this is the title under which it will be found.

Having been home from my trip to ‘the Prom’ for almost three weeks I have now started looking at what is wrong with my BITX20. As mentioned in my trip report I had reports of very poor transmitted audio from my BITX20. So I have put it back on the bench and I determined to get it to work properly this time around.
So far I have done the following things

  • Checked the VFO and BFO for stability during modulation – no problems here
  • Added 0.1 uF capacitors to the DC power rail to the microphone amplifier and the RX audio amplifier
  • Added a choke to the AF side of T2 to block some stray RF that was making it through the mixer
  • Re-aligned the band pass filter L1, L2 and L3 – this got about 2 dB more output.
  • Re-aligned the BFO – This highlighted that the display readout on my Huff & Puff display is wrong by 700 Hz
  • Recompiled the source code for the PIC in the Huff & Puff display to correct the 700 Hz error and passed it off to VK3AJ to be programmed to the PIC chip

After all this I think the audio is sounding a whole lot better.

Once I get this audio problem sorted I will go the next step and replace the tuning cap with a varactor diode and a ten turn pot thanks to Gary, NI9V.

I also have a JMS-1 SMD mixer that I am going to use to replace T1, T1A and the associated four diodes.

Along the way I have also been taking some waveform measurements for Toni, G6XMO, who asked asked me for some help with finishing another BITX20 V3.

Hopefully by this time next week I will have the BITX 20 finished (again).


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