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.


Cheval Case with JackAl board installed

Rick KN4AIE installed his uBitx V3 in a Cheval case (see previous post) and later added the JackAl mod.

Everything fit nicely.  The case is all aluminium and easy to work with.  It also provides very good access with both top and bottom covers and front and back  panels removable.  The case is a little pricey, but very nicely constructed.

Rick ordered his case from an eBay seller.


Some interesting cases

Finding a case for your µBITx is easy … finding a good case is more tricky.  Many use one of the many µBITx cases sold by Sunil.  Others have their own favourites

Two new suggestions from Jack KZ5A are worthy of a second look.

The Cheval case illustrated below can be obtained in both steel and aluminium versions (we would strongly recommend the aluminium version).  This case will have to be imported from Thailand.  It is also available on eBay.  It is a very nice looking enclosure, reminiscent of Heathkit enclosures from the 1970s.

The Circuit Specialists aluminium instrument enclosure is a not quite as good looking enclosure, but at a very good price ($US21 excl shipping).

Mike ZL1AXG purchased several aluminium cases from that are intended to house stereo amplifiers.  This particular model (Breeze 2207) is 228mm x 70mm x 215mm.  A good size for incorporating a few additional circuit boards alongside the µBITx main board. These cases are precision made and look really nice.  They can be acquired for around US$15 each (plus shipping – that will be more than the cost of the enclosure).  Mike has yet to transplant his µBITx into the new case.


Switching AGC delay settings with ND6T mod

Derek G4VWI has used a sub-miniature MIL spec rotary switch to switch between common, off/slow/fast as per  the ND6T wiring diagram for the Kit Projects board.

A screened pre-made wiring loom helps ensure the exposed sections are kept nice and short. Derek suggests choosing good quality connecting wire and pre-soldering the pads before tacking on the leads for best results.  He believes that this is by far the easiest AGC board design to fit.


ALC and alternatives for controlling your linear output stage

Ashhar Farhan VU2ESE observes that ALC is just one way of controlling output power from a linear amplifier stage and that an easier approach is to do it in software.

This needs software that can control the ‘mic volume’.  You could set the value differently for each band.  There is another pay-off with software mic gain, it can make a major difference to the transmit IMD.  At voice peaks, the tx linear chain compresses. The gain is not constant between low and high levels of modulation. This is the cause of in-channel IMD.   Now, if we have a look up table that amplifies the peaks more than the lows, we can ‘correct’ the gain back to being linear. This simple concept goes by the name of ‘pre-distortion’ in the SDR world.


Control CW TX power by unbalancing the second mixer

John (VK2ETA)  has been working on a successful modification to get the clock #1 mixer to unbalance on CW key down.  This involves the following steps:

1. Re-wire the T4 transformer input and output as per T2. That means as Jerry said to “… cut traces to T4 pins 3,5 and 6.  Swap them so T4 pins 3,5 are in from R47, pin 6 is out to C211.”

2. Disconnect (cut trace) from R105 to the common connection of C10, R27 and T2 (3,5).

3. Connect via a wire the disconnected side of R105 to the common connection of T4 (3,5), C211.

John used an audio shielded cable, and passed the wire under the board by drilling two small holes next the two connection points to ensure minimal pick-up of RF.

Pictures of R105 trace cut and wire to T4. (partially obscured by the hot melt glue on the toroids as I use my rig /P and /PM).

Picture of T4 traces swapped and cable from R105. (The line from the yellow toroid is a hot melt glue filament, not a wandering wire..hihi)

4. Change the software.  When in TX CW mode, disable clock #0, generate a clock #1 at “SECOND_OSC_USB” – “usbCarrier” = 45Mhz (+ or -) and clock #2 at (that same 45Mhz signal + target Tx frequency).

The difference in signal strength between key-up and key-down as seen on a control receiver is from way below S0 to S9+20dB or so, giving a dynamic range of over 70dB (indicative value in light of the basic test method).

The output power in CW mode can now be controlled finely by shifting the 45MHz clock slightly along the slope of the Xtal filter. For example I go from 14W max to between 1.5 and 2 watts by shifting the clock #1 frequency by 30Khz on the 20M band. On the 10M band, I need a shift of about 10Khz to reduce the 8 watts out to the same level.

Now, thanks to a bit of programming, John has full control of SSB as well as CW power across all bands. Great for his built-in ATU.

If someone with a spectrum analyzer could check the implications for the harmonics and spurs that would be a plus. John would expect in CW mode that the harmonic for 80, 40 and 30M bands would reduce, but some spurs to appear since we have the beat of two clocks now.  If there is interest John could modify Farhan’s code to match that modification (or publish some code snippets).

By the way, before he  did the mod above, he also tried to put a trim-pot between R105 / Ground / slider to T4 (3,5) and even when turned all the way to zero (slider to ground) it would still constantly unbalance the clock #1 mixer. So the change in topology as described above is the only way he could get it to work as intended.


Adding SOTABEAMS variable bandwidth filter to your uBITX

SOTABEAMS make a  Digital Variable Audio Filter Module  that could usefully be added to your uBITx.

Dennis KG4RUL has developed an Arduino sketch for the SOTAEAMS Vari-filter which includes a nice display.  It utilizes a USB Nano V3.0 ATmega328 16M 5V Micro-controller CH340G board, or similar, available from Amazon and other sources and an OLED display module such as the HiLetgo 1.3″ IIC I2C Serial 128×64 SSH1106 OLED LCD Display LCD Module for Arduino AVR PIC, or similar.
Dennis is happy to share his firmware, but cannot offer any support – so please do not contact him for support.
You can download the sketch and a schematic, in PDF, from the SOTABEAMS website:
An additional document supplies information for linking a microcontroller with a SOTABEAMS, VariBeam (VB) module for use with a BITX/uBITX.  You can access this file at the following URL:


LM386 mods (v5 uBITx)

The v5 uBITx has an LM386 for the audio amplifier stage. Philip G7JUR suggests connecting a 10 microFarad capacitor to the LM386. The +ve end should go to pin1 and the -ve end of the capacitor to pin 8. The easiest place to access is to solder to C75. This will give a bit more gain, and bass.

Ashhar Farhan VU2ESE commented that the LM386 distortion really increases with increased gain.  He also notes that more gain is not increased sensitivity.  He thinks a better idea might be to bypass pin 7 to ground with the 10 uf.  Either way, he suggests that Philip keep experimenting!

Jerry KE7ER suggests for uniform gain from 300 to 2300 Hz
with no capacitor gives at least 26dB  but under 46dB with a 10uF capacitor.
Adding a resistor in series with the 10uF cap would reduce the gain.