SSM2167 board – improving compression control

Brad N8YG has been reading over posts concerning the ssm2167 compression pre-amplifier with quite a bit of interest. So he finally integrated the board into his ubitx v5.

He put a trimmer on the output of the board, and potentiometers on the compression, and noise gates. The problem that he ran into was that on the air, rotating the 100K compression potentiometer had little to no effect.  However he was getting compression that was clearly audible on the air.

He re-read the data sheet for the ssm2167 more thoroughly and found that the input voltage need to be quite low at -20 to -30 dBV.  A quick check on the electret microphone output using the oscilloscope, showed a much higher output from his microphone.

This led to him adding another pot to trim the input voltage.  He still needed the 5V to get to the electret so he wanted the trimmer after the 0.1 uF surface mount capacitor on the board.

Brad modded the board by removingthe 0.1 uf capacitor, and he made connections for the trimmer after the 0.1 uF capacitor.

The net result is a board with three additional potentiometers and one trimmer.  The trimmer adjusts the output voltage, and potentiometers control the input voltage to the board, compression and noise gate.

On the air.. with the compression potentiometer turned down the rig sounds like it has no compression… while turning up the potentiometer makes a profound difference in on the air sound.  The rig now has a LOT of punch and the power meter goes way up.   

Brad believes that what was happening was that the input level was too high forcing the chip into the limiting portion of the transfer curve.  Now that the chip is operating back in the linear portion, all is well!  


Temperature measurements on the fan shroud

Mark AJ6CU released a fan shroud for the “tall” Sunil case a short while back. See

He was curious to check out its effectiveness, against  just using the supplied heat sinks or using the heat sinks with a fan.  So here are the results….

Test Setup

  • ubitx v4 with stock PA transistors and the new larger heat sinks being shipped with the case
  • Noctua 1800RPM 80mm 12V fan (Blowing in)
  • Little Volt/Temp gauge that was mounted on the front panel (measurements in Fahrenheit)

Search for: ” DROK Car Voltage Temperature Display” on amazon if you want one!

Unfortunately Mark took the “shroud measurements” on a different day than the”no shroud measurements”.  So focus on the temperature gain over “ambient temperature, for a true comparison.

The first set of tests were for 5 minutes on 40m into a dummy load.
The second set was on FT8 CQ into a dummy load, until the FT8 watchdog timer terminated transmissions.

The Results

No Shroud            Ambient               After 5 Min Xmit       Temp Gain
Fan Off                   81F                          128F                               +47
Fan On                   76F                           94F                                +18

With Shroud       Ambient                After 5 Min Xmit       Temp Gain
Fan Off                   83F                          128F                               +45
Fan On                   81F                           98F                                +17

Operating for 5 minutes into a dummy load suggest that a fan is probably as good as a fan and a shroud.

On FT8 CQ (into Dummy load, until the watchdog kicked TX off):

WITH Shroud     Ambient                 After 5 Min Xmit      Temp Gain
Fan Off                   84F                          105F                               +21
Fan On                   82F                           88F                                +6

Of course the missing measurement is the FT8 CQ with just a Fan. Mark thought of this test *after* he had put everything back together. 🙁 .


TX IMD measurements for a v5 uBITx

Doug K4DSP just finished measuring the two-tone TX IMD on his v5 µBITx. These are 3rd order products, and they were measured relative to either tone. Since the ARRL states their measurements relative to PEP I have included that as well, for those who sleep better at night secure in the belief that their radio is clean 🙂

This is at 10W PEP using 700 and 1900 Hz tones. Doug’s radio puts out 10W from 80 through 20 meters, and falls off to 5W on 10M:

80m -25.5 dBc (-31.5 dB PEP)
60m -22.5 dBc (-28.5 dB PEP)
40m -22.0 dBc (-28.0 dB PEP)
20m -24.5 dBc (-30.5 dB PEP)
17m -21.0 dBc (-27.0 dB PEP)
12m -25.5 dBc (-31.5 dB PEP)
10m -22.0 dBc (-28.0 dB PEP)

As Doug’s flight instructor used to say after one of his landings, “I’ve seen worse, but I’ve seen a lot better.” This is probably not atypical of IRF510s.


Fan cowling

Mark AJ6CU has made a 3D template for a fan cowling for your µBITx. The 3D template for the fan shroud can be found on thingiverse here:

It basically bolts onto the 80mm fan at the rear of the case (you might recognize the “tan” of a Noctua fan — much quieter than the one supplied). It sits on the uBitx motherboard with two slots straddling the heat sinks.

Marks notes that he has not powered on his µBITx yet, but suspects it will keep things nice and cool even in digital modes. This was designed with Fusion 360. There have been several other suggestions for adding ducts for cooling some of the hotter spots in our radios.

I was inspired in this effort by the work by another cooling shroud for the “shorter” of Sunil’s cases. See:


Removing the detent in a 100PPR optical encoder

This optical encoder (widely available on eBay or Aliexpress in black or silver) has been used by some constructors as a replacement to the standard mechanical encoder supplied with the µBITx kit.   It is a 100ppr encoder (whereas the one supplied with the µBITx is 24PPR) so may not work perfectly with either the default firmware supplied by HF Signals or Ian Lee KD8CEC firmware (i.e. you may need to experiment with the code for generating the VFO).

With the detent mechanism removed to allow free turning the encoder gives a very smooth tuning action.   With the encoder able to spin freely the protruding crank handle will always come to rest in the down position so it too has to be removed.

The main complaint with these encoders is the detent feature.  To remove the detent, here are the instructions from Robert GM4CID:

Remove the knob handle, then, using a heat gun or hair dryer heat the silver knob insert to soften the adhesive and carefully pry off that silver insert to reveal retaining screws that can be removed. The mechanism will now come apart and you can remove the detent. Replace everything but not the handle.


3D printers and the uBITx go together

Doug K4DSP hesitates to say that that is µBITx is actually finished, as he keeps thinking of stuff to add to the hardware and firmware! But the cover is on and he’s making QSOs.

Previously Doug had only used his rig on CW, but he used a 3D printer to make a mic case and got it wired up.   His first SSB QSO was with S51DX.

Doug is pleased with how well the little radio “hears”.   While drafting his update for the BITX20 IOGroups list he was listening to G0EVY on 40M SSB who was booming in.

Doug has yet to install the AGC board so he finds the static crashes are “pretty harsh”.   He says it reminds him of his old Heath HW-16 in that regard.

The mic case  design was sourced from Thingiverse:

It uses the mic element and microswitch that were included in the uBITX kit. He printed it with black filament and then rubbed some white paint into the front to give it some “character”.

Doug printed the uBITX case from here:

He only used the front and rear panels.  He designed his own middle section to add mounting holes and a baffle for an internal speaker.   He used FreeCAD to do the design.

Doug calibrated the frequency against a 10 MHz GPSDO. Then, based on advice of others, he used a PC audio spectrum analyzer program (Spectrum Lab by DL4YHF) to adjust the BFO frequency. That worked a treat, and he’s really happy with the way the audio sounds now.

He concludes, “It’s just a very pleasing little radio”


Reducing the audio gain on a v5 uBITx

Roman provides a no-soldering hack to reduce the audio-stage gain on a v5 (current model) µBITx.

Power off the BitX before starting…

1. Observe which end the notch on the body of the 386 is facing
2. Remove 386 carefully from socket (i.e. use tiny screwdriver pull up chip body incrementally back and forth from each side)
3. Gently lift either pin 1 or 8 90 degrees so it is horizontal. These are the pins closest to the notch, that is, in each line of four pins, the pin at the end of each line closes to the notch.
4. Reinsert the 386 into the socket in a straight down manner until firmly in place, ensuring that all remaining 7 pins go into their proper slot (and don’t bend accidentally due to misalignment). Make sure that the notch on the 386 is facing the same direction that it was when you removed it.

Try it out by turning on the BitX – there will be less hiss added to the audio (this is especially noticeable in headphones)


Add S-meter to VK3YE Audio AGC

Curt WB8YYY has been pleased with the  somewhat unusual VK3YE AGC circuit, that uses a LDR and LED pair,  as it nicely removes the top of large signals.

VK3YE has suggested measuring current in parallel with the LED that drives the AGC action, but he found this gave little indication of relative signal strength.  In fact, it works much better measuring current in series with the LED.

Curt is using a small meter movement supplied by Sunil, about 250 uV peak current.  A shunt resistor across the meter is necessary since the LED current at peak is at least 20 mA.

The approximate value of the shunt resistor can be found using the formula Rsh = ( Im x Rm ) / Ish.  Rm was unknown but Curt was able to measure it with his DVM at around 500 ohms.  Inserting the two known values gives a shunt resistance value of 6.25 ohms.

Curt found a resistor of around 5 ohms and it working nicely.

He says “Its not a real S-meter response – let’s call it a signal strength meter.”  It can discern signals from approximately S5 to S9.  for signals that do not result in meter movement, the LED itself could be observed to sense signal strength – but the meter represents a nice touch.


An interesting µBITx

Daniel Conklin says “I guess I’ve had enough fun with this radio and now it’s time to move on.”

He is selling his UBITX v3 which is built into an Apache case.  It might inspire others to build their rigs into these clamshell like cases.

There is space for the mic and a battery pack to sit when the case is closed. The mic and PTT switch are mounted in an old BaoFeng mic housing and it works well.