Important  Information

Ubitx.net provides help for constructors:  Preventing catastrophes and providing guides, fixes and modifications for your µBITx.  We are an information site only and not associated with HF Signals.


WARNING:  As shipped, your µBITx  probably will not be compliant with emission requirements.  It has been found to generate spurs and harmonics.  There are now simple fixes available to address these issues.  Make sure you apply them prior to operating the transceiver on an antenna.

CAUTION : If you power up the µBITX without the pull-up resistor the µBITX is likely to go into CW transmit. Make sure you install the 4.7k pull up resistor on the CW key pin!

v3 Board?  – There are known issues with WX brand TDA2822 chips (U1): Read this.  Earlier UCI and later socketed TDA2822 are not affected.   You will probably want to do the audio pop mod at least.

v4 Board? – If your audio is distorted see this article.  Note that audio output is reduced over the v3 board.  If you use a low impedance speaker you may need an additional amplifier module (LM386, TDA2822, etc.) to get adequate volume. The audio pop mod is not required on v4 boards.


SHIPPING:  µBitx orders have been shipping within one or two working days of being ordered. You should receive a shipping notification through PayPal once your product has been shipped. Estimated delivery times to different countries:

EU:         IndiaPost: 2-5 days  DHL: 2-5 days
US:         IndiaPost: 10 days     DHL: 2-5 days
Asia:      IndiaPost: 2-4 days   DHL: 2-4 days
AU/NZ: IndiaPost:10 days      DHL: 5-7 days

BITX QSO DAY:   Every Sunday – 3PM & 7PM Local Time – 7277 kHz in North America, 7177 kHz elsewhere.

A spur fix and BFO leak fix from Ashhar Farhan

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.

Reference

Using a USB hub for a single USB cable to your PC

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).

Reference

The cause of spurs

Ashhar VU2ESE and Raj VU2ZAP have nailed the cause of the spurs.  The 45 MHz  IF amplifier is distorting and producing a second harmonic at 90 MHz,  which mixes with the local oscillator to produce a spur at 90 MHz.
Here is an example of how it happens :
  • 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.

After checking the spur on a number of frequencies between 15 MHz and 30 MHz, it was confirmed that the above formula works consistently to predict the spur.
Now to confirm a simple solution …  if the problem is in the 45MHz IF, then it may also be possible to address the IMD at the same time.
Reference

A grandfather and grandson team build – the Fischer uBITx

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.

Modifications:

On 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

  1. 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.
  2. 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.
  3. My very first on-the-air call with this uBITX was immediately answered by WA5WAZ. 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.
  4. I am truly impressed with Ashhar Farhan’s $100 transceiver.

Higher current Axicom relays

Jim W0EB has finally received his >500 mW Axicom relays, after they got misrouted in the mail.

H plugged these into his V4 test bed built up on an aluminium open chassis. Sockets are mica filled 2mm thick with silver plated flat contacts. This leaves the relays 2mm above the PC board.

With the >500mW coils (i.e. with coils that have fewer turns on them) the hope was that this would lessen  the extent of inductive coupling between the switched portion of the circuit and the relay coils.  Unfortunately this does not appear to be the case.  Jim got almost exactly the same results as with the 400 mW relays mounted the same way.

40 meters is still just barely in spec with the 3rd harmonic being -43.8 dBC. The others are also in spec but not much better with 3rd harmonic varying from -46 dBC on 10 meters, -44.5 dBC on 12 & 15 meters , -44.7 dBC on 20 meters and -57.3 dBC on 80 meters.  60 meters wasn’t tested.

Raj VU2ZAP notes that it makes quite a difference between socketed an unsocketed replacements.  It is best not to use sockets for the relays, in order to get the relays as close as possible to the PCB.

Before and after shots of relay replacement as observed by Raj follow:

Before replacement
After replacement

Raj and Jim both note that there is variation between boards in terms of improvement in performance.  The reasons for this are not known.

The conclusions are:

  • relays help, and ubitx.net recommends them, but replacing relays with AXICOM relays may not always be sufficient to reduce “blow by” on some bands or modes
  • there is no point in spending more on Axicom relays with a higher coil current
  • the best option would be to replace the filter section altogether, and there are a number of ways of doing this.
Reference

Relay board from VK4PP given thumbs up in testing for isolation

Nick VK4PP sent one of his LPFx4  test boards to Warren WA8TOD for blow-by testing.   It looks like the bare PCB has excellent isolation between its input and output, of around 90db.

Matched with RF rated and separate in/out relays this board will perform very well.   The updated pic above shows a few tweaks and more ground stitching.

The only downside is the US$1.50 each relays (x8).    Relay K3 can potentially be reused, or you can source AXICOM relays for a bit more.

Nick is interested in feedback from constructors on the level of demand for these boards.

Reference

Variable bandwidth filter mod

Broadening the crystal filter

Michael N2ZDB notes that the BITX40 he is building receives somewhat OK but the transmit audio is terrible with distortion and/ narrow bandwidth.

He figured he may have one or two bad 12.0 MHz crystals, as the bandpass is only around 1,700 Hz at best (11.998800 to 11.997100)!  The crystals are from MEC and the letters / numbers on them are: 12.000 G   MEC AH6L1 .  He asked whether anybody knew of where to obtain suitable replacements.

Well, one supplier was disclosed by Lawrence W8LM:

SSSSSSShhhhh!!!!!!  THIS IS MY **SECRET** U.S.A. SOURCE…

between YOU and I ONLY!!!!!

12Mhz Crystals 5 for a $1.00  or 15 cents each in lot 100 quanities…

Right! 5 for a DOLLAR.!!!…. over $75 free shipping, under $75 flat $7 shipping.. They have 1/8″ uBITX volume knobs too..

https://www.allelectronics.com/item/cry-12/12-mhz-crystal-hc18/u/1.html

Also read my review on 50ft. RG58/u cables with BNC’s both ends (GO KIT SPECIAL) … and adjustable 6.8uH inductors..
I usually get my order Calif. to Kansas in 2 days…

Variable bandwidth filter design

Meanwhile, Jim AB7VF, posted a circuit for a variable bandwidth filter for the BITx transceivers.  This is based on a circuit patented by TenTec (the Jones Filter design).  Some constructors may want to try this one out, as it involves only a few additional parts and replacement of C217 -C221.

Reference