Some mods to improve RX sensitivity

George UR4CRG/RX3ARG  suggests some mods to improve RX sensitivity:

  1. Add a parallel capacitor to R12.  Try C=470 pF.  George found in his signal analyser that this gave a 3 dB rise in gain at 30 MHz.  He placed an 0805 SMD (or your could use a 1208 SMD part) over R12.
  2. Add two 100 Ohm resistors to  the collectors of Q11 and Q12.  You will need to cut the PCB tracks to add in the resistors.   The emitter-followers do not like HF signals and a small collector resistor gives light feed back

After these modifications, reception was noticeably more sensitive/



Crystal filter experimentation

Rahul VU3WJM has been scratch building a µBitx and experimenting with the 12 MHz QER filter.  He notes the following:

1) With the set of xtals that I have with 100pF caps and 200Z I/O filter the bandwidth is around 1.85Khz for HC49S crystals and 3.3Khz for HC49U crystals.

2) With a filter based on HC49U crystals, a lower bandwidth is achieved using 150pF caps but impedance drops down to 150-160 ohms. Lowering the
bandwidth also degrades the shape factor to around 1:2. This can be well understood considering that HC49U crystals have a lower motional inductance Lm.

3) As suggested by Allison KB1GMX, 82pF is working just fine in the filter and achieves a bandwidth of around 2.2Khz. Capacitor value is bit touchy between 82 to 100pf.

4) The Q of capacitors used has a major impact on the filter response. Parallelling up two values to arrive at a desired overall capacitance value results in a better response.

To see sweeps of a few different filter configurations select the reference on the list.   Rahul would love to hear from others of their experiences on the 12MHz crystal filter.



Sudden loss of power


Jim Sheldon, W0EB, while prepping a V3 uBITX, he had already built up to give to a friend, all of a sudden observed the RF Power output went to ZERO.

He thought for sure that he’d somehow blown one or both of the IRF510’s, but  nothing is always as it seems.

He checked voltage to the drains of the IRF510’s — ZERO (yes it was hooked up and turned on). Tracing the voltage back, he had nothing on the supply ends of L8 OR L9.  He checked continuity between those same points of L8 and L9 and had good connections there.

He found that he had +12V on the PA Power pin of P1 but nothing on the supply end of L8 which gets connected by a short trace to a very small VIA through the board and a short trace to L8 on the underside.  Looking at the underside, the trace was not burned but it turns out that tiny little VIA was open — possibly from poor plating through the hole.

He put a piece of 30 gauge wire wrap wire through the via and soldered it on both sides. This brought the +12v back to the supply side of L8, L9 and subsequently the drains of the IRF-510’s . After putting it all back together again he now had full power restored.

A few people have observed similar problems with loss of power. This might be the cause in these cases as well, and is definitely worth checking.  There will be no visible indication of anything bad between the PA Power pin on P1 and the rest of the PA circuit.


LED power out indicator

Lee  “I Void Warranties” N9LO ran across this circuit when looking for a transmit indicator for his Small Wonder Lab PSK31.

[Note the circuit was originally drawn incorrectly, but has now been updated]

Lee threw it together (there are not many parts) and it lights up bright and solid for CW and you an watch your modulation on SSB.   I can really see the difference when I switch in my SSM2167 module.

Jerry KE7ER recommends placing the circuit before the transmit LPF, not at the antenna port.   The diodes potentially create a bit of harmonic content that the LPF’s would remove.


Using the CW keyer as a morse practice oscillator

A member of the BITX20 list asks, “How can I use the keyer in my µBITX for CW exercises with a paddle?

The answers include:

  1.  Remove the power to the finals (this is the centre pin of the power connector)
  2. Use the KD8CEC software, which has a menu setting to turn off TX
  3. Modify the supplied manufacturer’s software to disable  TX


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.


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.