The All-band HF Amateur Radio Transceiver: Just waiting to be modified
All hardware posts
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.
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 aliexpress.com 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.
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.
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.
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.
SOTABEAMS make a Digital Variable Audio Filter Module that could usefully be added to your 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.
Jerry KE7ER, provides summary details on the two know spurs mods:
Change out inductors at L5 and L7 with 1210 shielded surface mount inductors.
Here’s the post from Raj giving results for various inductor types at L5,L7:
https://groups.io/g/BITX20/message/60946
And another post from Raj, affirming that he was using a yellow S+M brand 1210 inductor.
https://groups.io/g/BITX20/message/60678
Here, Raj says that TDK-EPCOS parts work as well (EPCOS was bought out by TDK):
https://groups.io/g/BITX20/message/61261
Unfortunately, I don’t see any complete part numbers in any forum posts
from somebody who has verified with a spectrum analyzer that spurs were reduced.
I bought a similar assortment of TDK-EPCOS-Murata parts from ebay,
have not verified results with a spectrum analyzer.
The parts they actually ship may vary with the parts they can find.
But here’s what they shipped to me
The v5 µBITx provides a different method of fixing spurs on the v5 uBitx,
primarily the low pass filter at L31, L32, C205 near the first mixer.
That filter removes the 90mhz second harmonic coming out of the 45mhz IF amp. Other possibly significant changes on v5 with regard to spurs would be the BFR93W’s at Q20,21,22 (to replace the MMBT3904’s) and the move to 220 ohms at R26 (was 470).
David N8DAH suggests you can use your µBITx stock switch and microphone element with this nifty mini-microphone case from Kit Projects. They cost US$10 plus shipping. These are 3d printed with all hardware included (Mic’s use the stock element and tac button that came with your bitx40 or ubitx).
Kit Projects has red and black microphones right now and grey is coming soon!
The also have 3D printed mini paddles available to purchase, that may be of interest to µBITx constructors.
For those looking for a simple MIC to get you going try this. A simple piece of project strip will do it. Mount the supplied microphone on one end of the strip board, mount the switch on one side of the strip board, and cover with protective insulating tape as illustrated.
