Watts on the bench #5 – BITX20 Update # 36

Back in BTX20 update # 35 I outlined some things I still wanted to do to get my BITX20 back to a usable condition. Every time I have used it recently I got complaints of poor audio quality. During the past week I have been working quite hard at clearing this up.
I have added a choke and two more capacitors to the DC power that feeds the microphone amplifier as I found the RF was getting in here. I have also added two copper brackets to the back panel of the rig to provide a low resistance path between the linear amplifier board and the exciter board. This seemed to help the microphone amplifier more than adding the choke and capacitors did.
I have also replaced the varicap on the BFO oscillator as I found it was shifting too easily. When I removed it I saw that it was damaged and is now missing the bottom plate. This meant that it would turn freely and offered no stability at all. This explained why whenever I got the BFO set and the audio sounding good it would be short lived.
Once I changed the varicap I was able to set the BFO. I then corrected the offset value in the PI4RAZ display so now the rig shows the correct operating frequency.
I replaced the mixer transformers and diodes with a Minicircuits JMS-1 mixer but I found the loss using this device was about 6 dB worse than using the discrete components. I then re-instated the transformers and diodes.
I also replaced the carrier balance trim pot from a 200 ohm to a 100 ohm as required in the original design.
Since making these changes I have spoken with a couple of stations and had good reports of my audio. I even had a short QSO with Rudi, DK4KL, who was running 700 Watts into a mono band Yagi and could hear my 4 Watts coming from 16,500 km away. QRP can be a real surprise sometimes. My next efforts are to fit the 10 turn pot/varactor tuning modification, add an AGC and S meter circuit and get the output power meter to show on the LCD display.
I also want to change the chassis and shielding material from steel to aluminium to reduce the overall weight. At present the rig weighs 1.2 kg and I’d really like to get it under 1 kg. I know 2oo grams isn’t much but when hiking every little bit helps. I knew a guy who drilled holes in his tooth brush handle to reduce weight. Now that is extreme.
On a slightly different topic I had a look at what would be needed to add a back lit display to my MKARS80. The new display is about 5 mm thicker than the current display so everything in the box needs to be shifted back by the same amount. This is not a simple task and it may be easier to look for a new enclosure altogether.
bitxv3_vk3zpf_mods + pa April 2011






This entry was posted in BITX20, MKARS80. Bookmark the permalink.

Leave a Reply

Your email address will not be published. Required fields are marked *