Sunday, April 16, 2017

Minimum BITX40v3 Hacks

The two mods that everyone should be adding as soon as they get their BITX40v3 are the Q13 protection diodes at K1 and the 2nd harmonic reduction mod (100 pF NPo 100V capacitor across L7 in the output filter). 

The Q13 protection mod is extremely important as there have been hams who have blown Q13 within hours (or even minutes) of getting their new BITX40v3 working and hooked up to an antenna. The cause is because they transmitted on a nearby antenna with their 100W or higher station rig while the BITX40 was receiving on its antenna.

The symptom of a damaged Q13 is that receive still works fine but there is no transmit. Note that although Q13 is a driver for the transmitter circuit, its base is connected to the antenna during receive.

Q13 can be replaced by a 2N3904 transistor (there is an SMD version if desired). 73, AL VE3RRD

The 2nd harmonic is a little too high for North American standards. Add a 100 pF 100V NP0 capacitor across L7 in the output filter. This will reduce the 2nd harmonic by about 20 dB. If you are planning on increasing the output power above the normal 5 to 7W, then you should use a 200V capacitor.


  1. IS there a proper circuit for the BITX40v3 AS my board does not have the Balance pot or diodes D15 & D16.
    I wanted to unbalance the BM for a tune up signal

    1. Hi Bryan, on all the newer boards D15 & D16 have been replaced with a matched pair of diodes in a SMD package. Therefore the balance pot is no longer required since the diodes are matched. Look at ; in this improved Bitx40 sketch PE1NWL sends CW by unbalancing the mixer with 5V from the Raduino (see his CW-Carrier Wiring.png). If the 10K resistor is upped to about 47K you will get reduced CW output which would work well for tuning. AL, VE3RRD