One of the challenges with the µBITx transceiver is that there is a very active community of fixers and modders. A new constructor is immediately overwhelmed by the number of postings on the BITX20 l/O Groups List or, for that matter, the number of potential fixes and mods that can be found on this website.
This article is designed to communicate the fixes and mods that are likely to really matter most for the majority of constructors.
The first thing you should do is to identify which version of main board was supplied in your kit. You will see either v3 or v4 marked on your board. You will need this as there are some differences in fixes required.
Essential fix #1 – Reverse polarity fix
All constructors should provide their board with reverse voltage polarity protection and current limiter protection. The solution is simply to install a fuse (or better – two fuses, one for each of the power connections on the board). The kit is supplied with a diode for reverse voltage protection, but this will only work if the power supply is fused, and is likely to be inadequate if your power supply offers up 20A! See more details on the recommended fix here.
Essential fix #2 – Make your rig compliant by removing unwanted spurs
Your µBITx is almost certainly not compliant on bands above 18MHz as supplied. It has been shown that most boards radiate spurs that may only only be 25dB down on your intended frequency of operation. These spurs appear to be generated in the 45MHz mixer stage.
A simple fix has been found to these issues. This involves replacing L5 and L7 with 680nH1210 size surface mount EPCOS TDK MURATA inductors as illustrated in the photo above. It is likely that later v4 boards will have this modification incorporated.
Essential fix #3 – Make your rig compliant by removing unwanted harmonics
While the Low Pass Filters in the output stage of your µBITx work fine at suppressing harmonics, issues with the layout of this part of the circuit board and choice of switching relays will have reduced their filtering effectiveness. For most operation on the low bands, your output is likely to be non-compliant with US and international spectral purity requirements.
The good news is that a fix has been uncovered (by accident) that is reasonably straight forward. This involves replacing the existing four relays near the antenna connection with Axicom D2n 12v relays ((model V23105-A5403-A201). Removing these relays involves heating each relay lead with a soldering iron and using soldering wick to remove the solder. Use of solder flux can help with removing the solder. When all of the pins are free of solder you can replace the relay. Take your time and be careful not to damage your board.
Possible fix #4 – Audio stage issues (depending on board version and symptoms)
v3 board with WX version of TDA2822 chip
If you have a v3 board and an audio output chip marked WX, then you should read up on options for a fix. This is likely to include either replacing the WX chip with a reliable TDA2822, installing a 6v regulator on board, or using an alternative amplifier
v4 board with distorted audio
Some, but not most, v4 boards can have distorted audio. Distortion may be dependent on voltage supplied to the board. See this posting about how to fix this problem. Note that v4 boards have lower audio output that may not be suitable for driving a speaker. If you want to use a speaker chose one with a higher impedance (e.g. 32 ohms) rather than use a 4 or 8 ohm unit.
Essential fix #5 – Upgrade firmware
Essential fix #6 – Pop fix for constructors with v3 board
Recommended mod #7 – AGC mod from ND6T
Recommended mod #8 – Flatten power output across bands
Recommended mod #9 – Upgrade your display to a colour touch screen
Modern commercial transceivers are moving to colour touchscreens. You can have a touch screen too on your uBITx. Several options exist, but the easiest to install to date is the Nextion. This requires firmware provided by KD8CEC.
Change out your standard 16×2 display for a Nextion colour touch screen using KD8CEC firmware for both the display and the Raduino.