Getting the VFO in the band when turning on the uBITx

With the manufacturer’s firmware that the uBITX ships with, you need to switch from VFO A to B in order to save the current frequency in EEPROM memory so that next time you power up it will use that last saved value. The manufacturer’s software only saves things if there is a specific event requiring something to be saved (such as changing VFOs).


Variable IF bandwidth from W1EAT

Tom W1EAT provides a  schematic for his mod of a variable IF bandwidth circuit for the µBITx.  The RX and TX voltages are about 12V,  so he used a resistor network to keep the applied voltage to about 1 to 9V because that is the usable range for the varicap diodes he uses.  As the voltage on the caps is reduced the higher audio frequencies are reduced until the bandwidth is probably 100Hz or so at 1 volt. The filter centre frequency is very low, 200 Hz or so with Tom’s BFO setting.

The BB112 has about 500pf capacity at 1 volt applied.

The 4.7K resistors that feed the voltage to the VVC diodes could be 10 or 20 K, or what have you, as there is very little current needed.


Fake PA FETs

Watch out for fake RD16HHF1 devices.   Buying on Alibaba and Ebay is a risk some are willing to take, but there are plenty of fake devices for sale through these channels.


See your uBITx sketch in Assembly or Hex

Jack W8TEE notes how easy it is to find where Hex files and Assembly files for a compile of an Arduino sketch are placed on your computer, along with some other compile-time information.

First, go to your Preferences dialog (File –> Preferences) and check the compilation check box for the “Show verbose output during:”

Second, compile your program. Do not click the compile/upload icon, only the check mark for compile only. You will see a lot of stuff scroll by just below the IDE’s Source Code Window.

Third, look towards the end of that list of output for YourProgramName.hex. For example, Jack’s test program was:

Fourth, go to the directory and look at the output files. You will see file types:

elf — executable-linkable file, used for a debugger
hex — the flash output file
eep — th eEEPROM outout file
sym — symbol table information
lst — assembler output

The first file is very interesting to look at, as it shows the C code mixed in with its associated assembler output. (You can load the file into any text editor.) For example, which is more efficient: a cascading series of if statements or a switch/case? Write a short test program using both constructs and then look at the assembler output. You find that the switch/case produces a jump table, which is very efficient in terms of speed, but may be less efficient in terms of memory. Even if you don’t know assembler, you’ll get a feel for the answer. Use whichever fits the situation.

If you want to directly load a hex file into an Arduino, you could use an AVR programmer, or XLoader (

Fitting a pull-up resistor when you don’t want to use a key

If you don’t use a key at all (i.e. you want a phone only µBITx) then this hint from Doug AC9RZ might be useful.

Doug folded over about 5mm of the bottom of each leg of the 4.7k resistor to bulk them up. Then he crimped on molex kk terminals on each leg. He put heat shrink on each leg and then popped the resistor in the digital connector where the wires for pins 2 and 3 previously resided. After double checking with his DVM, the rig was fired up and all was good.

Justin N2TOH suggested that just tack soldering the resistor to the back of the connector was easier, so the wires could be completely removed from the harness.


How will you know you need to calibrate your BFO?

  1. Tune to 7074 using LSB. You should hear nothing.
  2. Change to USB and you should hear all kinds of PSK and other digital modes. If you hear these signals in LSB then your BFO frequency is not correct, and may even be placed on the wrong side of the passband.

My BFO was way off after I first tried to calibrate it. I then started over with mine at 996.4 and ended up at 996.7. If yours is a long way off from that then I’ll bet you hear psk at 7074 when in LSB.


Pesky volume pot

Geoff, G8BMI is cross-fertilising ideas across hobbies…

He has been assembling a uBITX, and found that the volume control has a 5mm shaft rather than the usual 1/4 inch. This means that none of his spare knobs fit.   We’ve all been there.  Most of us threw the supplied pot into the junk box and moved on with another standard potentiometer.

However, a waking inspiration suggested a very short length of 1/4 inch OD copper pipe, as used in model steam engines, could be used to ‘sleeve down’ a 1/4 inch bore knob. And it does!.

Geoff turned down the copper tube in the lathe, using a file just to ease the fit [It could be mounted in a power drill chuck to get the same result].  It only needs a skim. His tubing was 0.210 inches bore, which is just over 5 mm (5.18)


Shock-proofing your uBITx

John VK2ETA asks what he should do to protect his µBITx from the rigours of daily use when taken portable.

1. Are they other components you should be concerned about?

2. What method is recommended: hot melt glue, epoxy glue, superglue or other method?

3. What about cables

4. Any other actions that should be taken?

The recommendations from IO Group members seem to be:

  • Hot glue is your best bet. Poor man’s potting to the rescue. It holds very well and removes easy enough for repairs and upgrades. (Doug Wilner)
  • Make sure your cable bundles aren’t so tight that they are putting stress on the outer cables in the connectors as they bend into the bundle. Hot glue the connectors in place and wires coming out for strain relief. (Doug Wilner)
  • To mitigate shock and vibration getting to the PCB consider something like LORD Micro-Mounts and see the distributor page.  (Arvo KD9HLC)
  • Scotch weld (Andrew W6AVC)

Digikey Kikad library

A lot of constructors are using the Kikad free tool for drawing circuits and PCBs.   The folks at Digi-Key (a major parts supplier) have a collection of components you can drop into Kikad if you want to.  Download from here.


BCI mod from ZL1AXG

Mike ZL1AXG (editor of has taken a different approach to mounting a BCI filter.  The high pass filter design and mounting point are the same as that documented here.  However, the mounting system has been modified to make the filter easily removable. 

Mike has other filters that plug into the QRP-Labs LPF filter board so his filter takes the same dimensions (1.5 x 0.5 inches or 38.1 x 12.7mm) and uses the same mounting system (two four pin male dupont header connectors.) The protruding centre pins of the DuPont connector were cut off, and the outside pins  bent over (in opposite directions) to improve stability of the mount.  The pins were soldered to the empty pads to ground on C216 and C210 on one end (RHS in the photo below)  and to the RX signal rail on the other end (LHS below).  A cut was made to the track mid-way between the dupont connectors (not shown here).

The BCI filter as mounted can be seen below.  It makes a tidy looking unit that can be substituted or bypassed later.