Barry K3BO found that C50, C51 in the AF preamp and C63 of the mic output were causing an issue with microphonics. He replaced those three with 1uf electrolytics.
Spurs fix (See update below)
Ashhar Farhan VU2ESE, the designer of the µBITx has been experimenting with solutions to the spurs on SSB mode above 18MHz. He has posted to the list that one spur fix that is now consistently working for him, and that is easy to apply, is to fix the distortion in the 45 MHz amp.
He inserts a 0.3µH inductor in series with a 10 pf capacitor across TP13 on the v4 board. This is on the output of the 45 MHz IF amp going to the front end of the mixer. The inductor comprises 8 turns on a T30-6 toroid (to give 0.3µH or 300nH).
UPDATE But wait … he proposes a better spurs fix a day or two later
1. Change the resistors R26 and R46 to 220 ohms (from 470 ohms). this increases the darlington pair’s standing current and decreases the Harmonic distortions.
2. The 90 MHz trap was difficult to tune unless you had a spectrum analyzer. Instead, we can use a Low Pass Filter instead. A ‘T’, rather than a Pi is used. This helps improve isolation on both the RX/TX path.
3. The LPF consists of an L-C-L of 300nH, 47pf, 300nH.
Removing the BFO leak into the local oscillator
Ashhar also found that there is cross talk inside the Si5351, whereby the 12 MHz output from the BFO leaks into the local oscillator.
Ashhar used a 5µH inductor in series with a 47 pF capacitor placed across C202 of the LPF suppresses this. The inductor comprises 35 turns on a T30-6 toroid (to give 5µH). However, another constructor found this blanked out the 30m band, and calculations reveal that there may be an error with the calculation of inductance or capacitance that is waiting to be confirmed. The filter should be tuned to 12MHz suggesting a slightly small capacitance value of around 35pF
uBITx.net will be looking for verification from constructors that these mods represent a definitive fix for the spurs and carrier leak problems.
Nigel G4ZAL has tweaked his ubitx so that he has an ‘all in one’ rig for FT8 and other digimodes, including CAT control (FT-817 emulation).
To achieve this, he added an un-powered 4 port USB hub inside the rig and used a cheap USB sound card (discarded casing shown to right side of the pics below). He cut the USB cable and hard wired the USB hub from the back panel and into the Raduino via the USB hub. He didn’t connect the USB 5v power into the Raduino). He also hacked the USB sound card so he could wire it directly to the corresponding MIC and SPKR wiring (used an old PC CD-ROM audio cable).
He is using the latest firmware KD8CEC in the ubitx and now has CAT control and sound over a single USB cable to his PC/laptop.
Using the hub, he can also upload new firmware without opening the case.
In response to a question as to whether isolating transformers are required, Nigel noted that he had built a couple of devices like this and never had any issues requiring isolation transformers (He has some, but he never fitted them as the mod worked fine as is).
He used a USB cable “Mini USB 5pin male to female with screw panel mount extension cable” from eBay. Nigel’s was wired incorrectly as the colours did not match the USB standard. He had to wire according to how the manufacturer had made it. He cut the cable to suit and hard wired it to the USB hub removing the original connection.
The USB hub was made by Startech, but any hub that can fit inside your enclosure should work. Nigel removed 2 of the USB sockets on the hub and hard wired the stripped down USB sound card. He removed the USB connector and 3.5mm audio/mic sockets and soldered these connections direct to the hub. He also added connector pins for the audio/mic (CD-ROM) cable. He went to these extremes so that the hub and sound card would all fit inside his enclosure. If your enclosure is larger they can all be simply plugged together.
The USB sound card can be found on the famous auction web site. Nigel’s was an “External Virtual USB 3D Sound Audio Card Adaptor Converter Mic/Speaker PC Laptop“. Nigel hard wired the Raduino USB side of the connector to the hub as well, but he didn’t connect the 5v power line (the Raduino is powered as normal).
- the spur moves down as you tune up.
- when the radio is tuned to 28 MHz, the spur is at 17 MHz. When the uBITx is tuned to 28.150 MHz, the spur is on 16.850 MHz.
- At 28 MHz, the local oscillator is at 73 MHz.
- There is a signal X such that :
X – local oscillator = 17 mhz
Local oscillator = 73Mhz.
X – 73 = 17, so X must be 90 MHz.
Ghericoan N5WLF has developed material from the HF Signals website into a mini-manual for constructors. Very useful.
Pratap (Pop) VU2POP has provided a series of pictures for a µBITX build that may be helpful for some constructors.
This transceiver was built by Bob (WB8BEL) and his grandson Blake Fischer in 2018.
The “version 3” board and bag of parts were purchased directly from HF Signals.
Construction & Modification Notes:
Together Bob and his grandson fabricated the chassis by cutting, then soldering together, five individual panels of 0.070-inch-thick, double-clad printed circuit board material (pictured above).
They cut the PCB with tin snips and soldered the panels together with a 260W soldering gun. A drill, ½” reamer, Dremel tool and small flat file were used to shape the larger holes. #6 brass nuts were soldered in the top four corners to secure the sixth piece, a removable top.
Construction Tip: Solder the brass nuts in the corners (see photo below) and use them to mark locations of mounting holes in the top cover – BEFORE soldering the sides to the bottom of the chassis.
The incoming “negative” power conductor is routed through an LED ammeter added to monitor transmit current. A SPST toggle switch is used to turn the meter “off” when not needed because the meter does generate some RF noise. The switch also saves 15 mA of meter current.
The short incoming power line is fused at 4 Amps and the supply end is terminated with 30 Amp “power pole” connectors. This facilitates powering the rig from an AC-DC converter, a cigarette lighter adapter, a solar panel or directly from a battery with alligator clips.
A Push-To-Talk microphone was built using the supplied electret element and PTT switch. Both components were super glued into one of the plastic solder dispensers that vendors sell at hamfests. The fragile pocket clip on the dispenser only lasted a couple of days but it is not needed for operation.
Initial RF power outputs into a Waters 334A Dummy Load/Wattmeter with RV-1 fully CCW and using a 12.5 Volt supply were found to be:
- 80M (3.80MHz)=6W
- 40M (7.18MHz)=6W
- 20M (14.25MHz)=4.5W
- 15M 21.30MHz)=2.5W
- 10M (28.50MHz)=1.5W
These are “average” – not “PEP” outputs. Driven by “Helloooo – Helloooo” into the supplied microphone.
Bob and his grandson chose to install an SO-239 antenna connector since most of their coax cables have PL-259 connectors and SO-239s are used on all their Watt meters, SWR meters and antenna tuners.
After taking the initial series of transmitter output readings, they decided to make the WB2VXW (3-Component) modification and then shorted R83 (per VU2ESE) to increase output power above 10 MHz.
These mods nearly doubled transmit power on all bands. 28.5 MHz output increased 133% over the factory build. Thanks for the parts goes to Howard Fidel. New outputs were: 80M=11W 40M=11W 20M=8W 15M=5W 10M=3.5W.
The values of Q6 bias resistors were then changed to increase gain of the microphone amplifier. R63 went from 470 to 10 Ohms. R65 went from 1k to 4.7k Ohms. Bob did not notice any appreciable increase in RF output following this mod. He, and his grandson, tend to speak closer to the mic to get the desired power output, as evidenced on the Wattmeter.
Bob also mounted a 40mm x 40mm 12 VDC fan behind the final transistor heatsinks. It is controlled by a 35-degree C (95 F) temperature switch. The fan is powered through the rig’s “on-off” switch to conserve energy. It only runs when the transceiver is both “hot” and turned “on.”
They purchased an “RV24YN-20S-B101” 100 Ohm potentiometer from eBay and installed it on the front panel to replace RV-1. This was because other builders had reported the need to vary the transmitter drive level when moving from band-to-band.
The uBITX board received for the build allowed the team to find a “sweet spot” setting on the original RV-1 that provided acceptable drive across the whole 3 -30 MHz range of frequencies. They elected not to replace the original RV-1, so the “XMIT POWER” knob on the front panel is still not wired up. They may need to wire it in when experimenting with increased voltage to the finals.
Since they expect to be occasionally powering the transceiver from a solar panel with an open-circuit voltage rating of 17 VDC, Bob added 10 Ohms @ 6 Watts of resistors and 10W of 12 Volt Zener diodes in the “red” receiver power supply wire. At 17 Volt input, the Zener sucked a full half Amp to drop 5 V across the resistor. The circuit produced objectionable heat. This was subsequently replaced with one of the DC-DC buck converters listed on eBay for around $2.
The pair spent a week trying to find someone to custom print “Dry Transfer” lettering for the front and back panels. No luck. So, they just ordered some Black-on-Clear tape for the Dymo tape printer.
ADDITIONAL COMMENTS FROM BOB WB8BEL
- Our spray paint does not adhere to the bright copper surfaces as well as we would like. If we were to fabricate another chassis from PCB material, we would use single-clad board and paint the fiberglass side.
- We would be remiss not to acknowledge all the “high quality” technical support received from Mike Woods ZL1AXG. He answered every question we asked – with replies much quicker than those usually received from OEMs. Please note: Mike freely VOLUNTEERS this service for all of us BITX users. The Ham Radio community is certainly fortunate to have saints like Mike taking care of us less knowledgeable builders.
- My very first on-the-air call with this uBITX was immediately answered by W5WAZ. David was calling the INTERCON net on 14.300 MHz. He gave me an S-7 signal report from Magnolia, Texas. I was in Huntington, West Virginia using an inverted vee dipole antenna in my attic. Later the same evening, I checked into the Maritime Mobile Service Net being called by Judy, N6LSO, in San Diego, CA. My signal report was “just above her S-5 noise floor.” In a couple of days operating time, I made contacts all over the U.S., three in Canada and one in The Cayman Islands.
- I am truly impressed with Ashhar Farhan’s $100 transceiver.
John KZ1G notes a recent post by Fred (K3TXW):
“…I have problems making a rectangular hole in a piece of aluminium of exactly the right size, with the edges exactly parallel to the edges of the case. I inevitably make the opening too big or slightly wavy. The result screams “homebrew carelessness” though I’m not careless. Maybe there is a rectangular punch or something to do this right, but I don’t have such a tool; I use a set of files.”
In addition to files for work on front panel fabrication, I recommend an electronics shop include:
1. A drill press with a chuck that will accept 0.50-in.-diameter bits. Harbor Freight sells a couple of bench-top units for under $100. Put a magnet on the base as a place to keep the chuck wrench. (It’s usually best to clamp work to the drill press table.) Once you have a drill press you’ll find many other jobs for it.
2. A good set of sharp drill bits. Bits with a titanium nitride coating remain sharp for a long time.
3. A step drill bit. I use an Irwin Tools Unibit 3/16-Inch to 7/8-Inch Step-Drill Bit with a 3/8-Inch shank. Great tool when you need to make larger holes for controls or to start a rectangular cutout.
4. An Adel-brand metal nibbler. They show up on Ebay. Or buy a new one at https://www.adelnibbler.com. I’ve used one since I was a teenager and couldn’t work on chassis or panels without it.
Lay out your hole with masking tape around the outside. To make a rectangular hole (see attached image) I use a step bit and smaller bits to make round holes that remove a lot of metal. Just don’t get too close to the rectangle’s edges. Next I use the nibbler to remove remaining metal close to, but not at, the rectangle edges. Finally I clamp the panel or chassis in a bench vise so an edge of the hole aligns with the top of the vice jaws. File away any remaining metal until the edge is parallel with the vice jaws.
Kevin KU8H hides imperfections around a rectangular hole in the front panel with a bezel. A bezel will hide a lot of sins. They are also easier to fabricate with straight, clean edges. Rough edges that might show a little on internal panels of chassis – who cares.
Your use of the bench vice to keep the edge straight during filing is good but will eventually damage the vice. I use a sacrificial piece of steel like a piece of angle iron clamped up along with the workpiece. In woodworking we call that a “fence”.
Kevin VK3DAP / ZL2DAP clamps a short length of angle iron along the line of the opening, and files to the edge of angle iron. This gives a nice straight line. Rectangular chassis punches are available but are costly, and the larger ones require lots of force, and may need an hydraulic press.