Belt and braces filtering of a BITX40 power supply

Akira JJ1EPE had spuries on his BITX40 at 75kHz.

The modification for suppress the spurious signals provided by a friend involved the following steps:

1) Add 3.3uF electrolytic capacitor to VCC line of Arduino board


2) Add 0.1uF x 2 bypass capacitors to the VCC line (fuse point)

3) Insert high frequency choke of about 100 uH in the power line of Arduino

4) Connect the minus line of power supply line to the ground

5) Connect the minus line of BITX40 main board to the chassis ground

6) Add 0.1 uF × 2 bypass capacitor to VDD of BITX 40 board

7) To prevent common mode noise, add 0.01uF x 2 bypass capacitor to the power connector

This series of steps could equally well be applied to the µBITx if you have noise from your power supply and want to get rid of it with a very thorough strategy.


Fixing low SSB drive (updated)

Anders SM5NNO has drawn attention to a published fix for low levels of drive on SSB with BITx transceivers from Mr K P S Kang  (VU2KR / VU2OWF) on this blog.   While the mic gain fix relates to the BITx40, it translates readily to the uBITx.


An update

André PA3EIV confirms that the above fix (by Mr. K P S Kang VU2KR works 🙂

He changed the value of R65 to 4K7 and the value of R63 to 10 Ohm using 1206 surface mount components (desoldered from scrap prints).

He now has, on normal voice volume, 10 watt’s out on 20m.  RV1 is turned counter clockwise for about 75%.   André uses a Baofeng microphone.


An RF based AGC design from K P S Kang

K P S Kang VU2KR / VU2OWF has an item on his blog showing an RF AGC for BITx designs.  The AGC is apparently very effective at calming those 9+20dB signals that on the uBITx will damage your hearing!

The design is for the BITx40, but can be readily adapted for the uBITx – probably not at the antenna (because it is best to avoid diodes at the front end of the uBITx), but at the 45MHz IF stage.  Are there any takers to adapt this design for the uBITx?

MAX9812 Mic Pre-Amp conclusions

David N8DAH has been testing the MAX9812L Mic Pre-amp module on his BITx40.  In theory this should improve the gain and signal quality.

David says “So far its working ok at best I sound a bit robotish.”

“I am TXing at around 20w with my amp. I took the audio out through a 10uf dc blocking cap to the bitx40 mic in.  I powered the board from a 9v just for testing.  R136 is about 1/4. If you use a pre-amp you should adjust this lower or you will cause one heck of a noise on tx.  I am not yelling or shouting to get audio out now but not sure I like the audio in any case.

This is without the pre-amp … with the audio files from Michigan to Milford PA websdr

This is with the pre-amp …

David has decided he “might just stick with the slight yell to get the audio out. I like the idea of not having to shout but do not like the audio from this version of preamp”.

Others may think differently.  Mike ZL1AXG thought his “more robot-like” voice was more intelligible because it was more “punchy”.

Jeff AD6RH says:

“I used another mic housing with a DPDT switch and wired it so voltage is supplied only when PTT is engaged. I am using a CR2032 3v button cell. It seems to work fine, but I have not compared the stock vs. preamp mic with anyone on the air yet. It definitely has more average power on the watt meter. I can hear some peaks come thru the speaker when transmitting. I may try installing a pot to dial back the gain.”


BITx40 Software

For those wanting to upgrade the stock BITx40 the favoured option is to use the software developed by Allard PE1NW.  This is a well worked over piece of software and is used by 100’s if not 1000’s of BITx40 owners.

Allard’s firmware options can be found here:


You need to choose between his Bitx40 and Bitx40-raduino-v2 branches. Both take some minor hardware mods which Allard has documented extremely well.   His version 2 drives the BFO from the si5351 on the Raduino
instead of using the stock analog crystal oscillator for the BFO.


BITx40 – Essential Mods

Fantastic BITx40 build from Glenn VK3YY

Jerry KE7ER notes that the schematics on the website for the Bitx40 are final but not necessarily correct.  He then goes on to talk about some of the variances and essential hacks.

This website is dedicated to the uBITx.   However, so that everybody who also has a BITx40 doesn’t miss out entirely on the occasional “gold nugget”,  some interesting articles about the BITx40 may appear from time to time on this website.   Jerry’s comments on the variation between the published circuit diagram and the actual board currently in production meets the “gold nugget” standard.

Differences between the BITx40 Board as manufactured  and the manufacturer’s Circuit Diagram

  • The two modulator diodes D15,D16 plus balancing pot R106 got replaced by a BAT54s dual schottky:
  • All those BC849’s?  Have always been MMBT3904’s.
  • L5 and C103 of the BFO are missing, the BFO is at just about the right frequency if the crystals are matched without any adjustment.  Usually.
  • L4 in the VFO is not stuffed on the board, meaning the analog VFO doesn’t oscillate.  The VFO comes instead from the si5351 PLL chip on the Raduino through the Bitx40 connector labeled “DDS1” in the schematic.
  • T6-4 is tied to C142 and R144, U3-3 is tied only to the 12v rail labeled “TX”, schematic does not show that clearly.
  • RV1-2 is not connected to U3, the schematic drawing is just a little tight there.Otherwise, Jerry thinks that the schematics on HF Signals are correct, reflecting what is currently being shipped.   Nothing has changed in Bitx40v3 boards shipped since Dec of 2016.

Changes that should ideally be made to the BITx40 Board

He then goes on to note that he thinks there are a few things that perhaps should change:

  • D7 and C130 should be across the coil of relay K1,  D8 and C164 are correctly across the coil of K2
  • R141 is currently a 1/4 Watt 10 ohm resistor, should be 1/2 Watt as have been reports of it burning out.
  • The surface mount MMBT3904 at Q13 could be replaced with a through-hole version of the same part, the 2n3904, as it must dissipate a fair bit of power during transmit.  Even with the radio off but hooked up to an antenna, a nearby QRO transmitter or electrical storm can blow Q13.

Essential mods

And finally Jerry suggests the following as essential mods for all purchasers:

Note that he suggested adding an incandescent lamp, which can be quite effective: