NOVEMBER 2, 2018 BY ADMIN 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.