Some great mods from PH2LB


Lex PH2LB  wrote to to tell us about a page on his website where he describes his uBitx (V3) mods.   This is a very nice build, and he has some good ideas.   Check his page out here

In particular Lex has developed some custom firmware that firmware geeks may be quite interested in …

“Second mod : custom firmware”

Originally based on the v2 software but merged to v4.3 and updated to code to have a lower RAM footprint (usage of F(…) macro and strcpy_P) with about 50%.

Source files can be found here :

Over the last few months there have been a range of ideas to boost mic drive output or to add compression.  Here’s a mod designed to work with a dynamic microphone …

“Fourth mod : dynamic microphone amplifier.”

Because I like to work with dynamic microphones, I added a dynamic microphone amplifier based on the microphone preamp designed by Javier Solans Badia, EA3GCY for his ILER transceivers.

There are a whole bunch of ways to add buttons. KD8CEC does this through paralleling up buttons with different series resistor values on the encoder analogue port).   Lex has taken a different approach that will be of interest to some constructors.   He uses a PCF8574 I2C encoder (like the backpacks for a 16×2 or 20×4 LCD display) and uses the existing I2C bus…

“Fifth mod : again adding extra buttons.”

Using a PCF8574AP I2C IO Extender and hooked it up to the all-ready existing I2C bus on the Raduino for more direct menu buttons. Needs the custom firmware to direct switch between bands with a PA bandplan limitation (also has FULL option) and Step size up and down.


Something that a number of constructors have done is to remove the 7805  and supply 5v to the Raduio using a separate 5v supply.  Most are using buck or buck boost modules, but Lex has used a P-MOSFET.   There’s a good description of his approach on his website …

“Sixth mod : removing 7805 from Raduino and reverse power protection.”

Relocating the 7805 is a good idea, but adding a reversed voltage polarity to a uBitx is a must. I used a P-MOSFETs for that (also link to good video about using P-MOSFETS for reverse power protection).

Finally, you may be interested in Lex’s use of the Manhattan style technique for PCB layout.  It can look very professional as per this example:

Microphone idea

Dave KD0UYH has made a microphone from an old desk lamp purchased from a “resell it” shop.  He says, “Not much to look at, but it serves my purpose. Used some vacuum tubing to attach the mic element and hot glued a handful of nuts into the base to give it some weight.”

Microphones galore

Constructors have been uploading their microphones.  These are included in a single gallery called “microphones” and will be separated out into their own group with a separate link shortly.


Baofeng Speaker/Mics and uBITx

There has been a bit of discussion on the BITX20 Groups IO list about Baofeng mics, mods required to work with the µBITx, and what to do about a mic jack. This article tries to sum up thoughts on using the Baofeng mic.


The Baofeng Speaker/Mic can be obtained very cheaply, either direct from China (e.g. on or, or or via third parties in your country.   Be warned that almost none of these will be a genuine Baofeng mic.  There are many different varieties of “knock-off”, and they are of very varying quality.   However, most are adequate for µBITx use.

The wiring in the microphone is not standardised (i.e the colour of the wires varies).  In some cases the wiring simply doesn’t work.  In this instance, you should throw away the Speaker-Mic as the wiring is non-repairable.   Buy several to safeguard against the odd one that has wiring issues.

The microphone element is so-so, you can replace the one in the Speaker/Mic with the element that comes with the kit as required.  You may also need to drill out the tiny hole to be a bit bigger to make a reasonable air passage to the element.   The speaker is not high quality and will not give much volume.   When using the speaker-mic in the µBITx, most of us don’t use the speaker at all.

Standard wiring will work on the BITx40, and the LED in the Speaker/Mic will even light up.  It won’t on the µBITx, because the PTT line works differently.  This is connected to +5v from an arduino line (messing up the bias on the microphone).

Disassemble the mic, and rewire (using a multimeter to test connections) so that the PTT switch is wired separately from the microphone element.  You can use a common ground return for the mic, speaker and PTT.  See the original circuit diagram for the speaker/mic here.

Panel jack mounting

A key question that those with a Baofeng Speaker/Mic, is whether you should retain the 3.5mm and 2.5mm plug, or cut it off.

John KG5WJQ observes that they sell a combined jack on Alibaba and the price is fine.  The problem is that it is a PCB mount jack so that can result in some difficulties with mounting the jack in a case.

Craig KM4YEC uses panel mount mono jacks, one 3.5″ and one 2.5″, sourced from Radio Shack in store stock (but they could probably be ordered online).  He says,  “If you turn these back to back, and butt them against each other, with the ears turned out, it is a perfect fit for spacing.”   

Craig uses only the PTT, and the Mic element in the enclosure.  He enlarges the hole in front of the element, removes the speaker, and makes sure the only two circuits are PTT and Mic.  He went as far as removing all the SMD components and unnecessary trace runs using exacto knife cuts.

Glenn VK3PE fitted standard 3.5 and 2.5mm jacks at the rear of his uBITX build. NOTE the 3.5mm jack needs to be insulated from the chassis to work as the PTT is connected to what is normally ‘ground’ on a stereo socket. Glenn used some plastic washers to insulate it.

The challenge with this approach is that the spacing between the two jacks needs to be reasonably precise.

There is nothing to stop you from cutting off the plug and wiring it directly into the circuit.  This is a cheap option (no plug and socket required), but is a bit inconvenient when it comes to moving the rig, as the microphone is permanently attached.

The other option is to remove the plug altogether and use different connectors.   Many constructors like to use a standard 4 pin mic jack or similar style 8 pin jack used by the big three Japanese amateur rig manufacturers as illustrated below:

Mike ZL1AXG uses Kenwood wiring on a standard 8 pin mic connector as shown below.

Is there a better choice of microphone?

Dave K8WPE felt his Baofeng speaker mike was of such poor quality that he went ahead and bought an almost identical microphone from Btech.  This is a QHM22, a much better product, for US$23.00.  The speaker is top notch and reports of his voice quality are also very good. So the Btech mike might be a better choice.


SSM2167 Mic Compressor: Avoiding feedback

John VK2ETA notes that Simon, VK3ELH, pointed out  an issue that when inserting an SSM2167 mic compressor circuit between the microphone and the uBitx mic-preamp, it can create feedback when the microphone was placed near the speaker while in RX.  This is because the SSM2167 module is always on.

The solution John has applied is to connect the shutdown pin of the SSM2167 (pin 3) to the Raduino T/R digital output (D7) through a 2.7K ohm resistor. This disables the chip while in RX and removed the mic feedback issue.

Pictured above is an indication of where he picked up pin 3 on the SSM2167 on his module. The purple wire is connected to what is the right hand side of resistor R4. The 4.7K resistor on the RHS is for the mic-bias and the 51K resistor on the top-left is for bringing the compression ratio towards 4.

John feeds the Vcc pin on the board from the regulated 5V of his Raduino. Measured consumption at 2mA is a very small extra load on the Raduino regulator.

There is a DC blocking cap on the input and output circuits of the board already, so no external blocking capacitors are needed.  However, a bias resistor does need to be added for the microphone.

The 2.7K resistor is not mounted on the module, so is not shown in the picture.

Also not shown on the picture are an axial choke of 100uH between the “in” connection and the Mic, plus a 1nF capacitor between the “in” connection and ground to block RF feedback when Txing on higher frequencies. For John, RF feedback was noticeable from 15m through 10m. Others may not have this issue.

John also has a 10K adjustable potentiometer between the “out” connection on the module and the original Mic input to the uBitx.  His is turned to about 80% through its range.

John mounted his board on header pins so he can remove it as required.  He extended the header pin on the “out” side (bottom LHS on picture) past the board to provide an extra connection for the shutdown wire.

John finds the compression and noise gate work quite well on the module. When he is silent the background noise does not trigger any movement of the power needle, but it goes up as soon as he speaks into the Mic. Also despite showing quite an increase in average power, he hasn’t had any negative comments on his  audio. I was told that it was noticeable, but not unpleasant, “good for DX”.   And this was with a change in the standard resistor value for compression to give around 4: 1 compression.



Mic Compression and Noise gate with SSM2167 module

John, VK2ETA, has  used the small circuit board “SSM2167 Microphone Preamplifier Board Preamp COMP Compression Module DC 3V-5V”available on eBay or Aliexpress as a compression and mic pre-amplifier.

He simply connected the input to the mic, added a 4.7K ohm resistor between the mic input and the 5VDC (taken from the Raduino) for biasing the electret and put a 10K ohms potentiometer in the output to adjust the power level to the mic preamp stage.

He didn’t modify his uBitx board,  but simply inserted the board prior to the mic input.  The gain of 20dB is reduced back with the output potentiometer. John removed the “R1” resistor and replaced it with a 51K Ohms resistor to get a 4:1 compression factor, up from the 2:1 as delivered, but this change has yet to be tested “on air”.

John hasn’t received any negative feedback about the compressor except when I pushed the output potentiometer too high.



Simon VK3ELH used the same board and a similar scheme for powering the module from the regulated 5v line on the Raduino.  It is also installed separate to the main board and inline with the mic input.

Simon used a 75k ohm resistor for compression and 1k ohm resistor for the noise gate and a 100k pot on output. At full output, his audio was readable but distorted based on an audio check QSO, so the output has been turned down.

He put a larger heatsink on the IRF510 to cater for the higher average output, as the stock one was getting warm!

A side effect of the mic being on all the time is that there is leakage through to the speaker and it causes some feedback if the mic is within 2 inches or so of the speaker.

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.


Using a dynamic microphone

Dennis Yancey asked the group “Has anyone used a dynamic mic on the uBITx?”    Dave WI6R responded with a guide for modifying the uBITx to use a Dynamic Mic:

Referring to the uBITX Schematic.   The Gain of the Mic PreAmp is controlled by R63 in the Emitter of Q6 to C62/R64.  It basically decouples the audio bypass of R64 by C62 to limit gain for the high output Electret Mics.

Substituting a Dynamic Mic

Replacing R63 with a Zero Ohm resistor and putting a 10K potentiometer at the Mic input should allow a Dynamic Mic to work now with a Mic Gain Control.  Also, R60 that supplies Mic BIAS to the Electret-Condenser Mic needs to be removed.  If there is not enough gain after this mod, you might have to reduce the value of R64 to maybe 470 Ohms or so. 

If you cannot look at your transmitted signal on a scope at least listen to your audio on another receiver to verify you are not causing distortion.

More Gain for an Electret Mic
To simply get more gain with an Electret Mic you might try dropping another 47 Ohm resistor on top of the existing R63.  You can solder another chip resistor on top by soldering one end at a time.
[EDITOR:  you can also bridge the surface mount resistor with a standard through hole resistor.  Shorten the leads and bend over about 1/8″ or 3mm for soldering to the ends of the surface mount component]

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.”