Recommended fixes and mods

Introduction

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, v4 or v5 is 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

Unless you have a v5 board, your µBITx as supplied, will almost certainly not be compliant on bands above 18MHz.  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.  Note that v5 boards have an alternative modification incorporated, which is not quite so easy to install on v3/v4.

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, the output of your v3 or v4 board is unlikely to be fully compliant with US and international spectral purity requirements.  Your v5 board may already be compliant.

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.

If you have a v5 board, then you will have quite a different relay switching arrangement, that may reduce harmonics below the legal limit.

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.  Choose a speaker with a higher impedance (e.g. 32 ohms) rather than using a 4 or 8 ohm unit.  This will help with the output level.

Essential fix #5  – Upgrade firmware

Essential fix #6 – Pop fix for constructors with v3 board

Recommended mod #7 –  AGC mod from ND6T