The All-band HF Amateur Radio Transceiver: Just waiting to be modified
A member of the BITX20 list asks, “How can I use the keyer in my µBITX for CW exercises with a paddle?
The answers include:
With the manufacturer’s firmware that the uBITX ships with, you need to switch from VFO A to B in order to save the current frequency in EEPROM memory so that next time you power up it will use that last saved value. The manufacturer’s software only saves things if there is a specific event requiring something to be saved (such as changing VFOs).
Tom W1EAT provides a schematic for his mod of a variable IF bandwidth circuit for the µBITx. The RX and TX voltages are about 12V, so he used a resistor network to keep the applied voltage to about 1 to 9V because that is the usable range for the varicap diodes he uses. As the voltage on the caps is reduced the higher audio frequencies are reduced until the bandwidth is probably 100Hz or so at 1 volt. The filter centre frequency is very low, 200 Hz or so with Tom’s BFO setting.
The BB112 has about 500pf capacity at 1 volt applied.
The 4.7K resistors that feed the voltage to the VVC diodes could be 10 or 20 K, or what have you, as there is very little current needed.
If you don’t use a key at all (i.e. you want a phone only µBITx) then this hint from Doug AC9RZ might be useful.
Doug folded over about 5mm of the bottom of each leg of the 4.7k resistor to bulk them up. Then he crimped on molex kk terminals on each leg. He put heat shrink on each leg and then popped the resistor in the digital connector where the wires for pins 2 and 3 previously resided. After double checking with his DVM, the rig was fired up and all was good.
Justin N2TOH suggested that just tack soldering the resistor to the back of the connector was easier, so the wires could be completely removed from the harness.
My BFO was way off after I first tried to calibrate it. I then started over with mine at 996.4 and ended up at 996.7. If yours is a long way off from that then I’ll bet you hear psk at 7074 when in LSB.
Geoff, G8BMI is cross-fertilising ideas across hobbies…
He has been assembling a uBITX, and found that the volume control has a 5mm shaft rather than the usual 1/4 inch. This means that none of his spare knobs fit. We’ve all been there. Most of us threw the supplied pot into the junk box and moved on with another standard potentiometer.
However, a waking inspiration suggested a very short length of 1/4 inch OD copper pipe, as used in model steam engines, could be used to ‘sleeve down’ a 1/4 inch bore knob. And it does!.
Geoff turned down the copper tube in the lathe, using a file just to ease the fit [It could be mounted in a power drill chuck to get the same result]. It only needs a skim. His tubing was 0.210 inches bore, which is just over 5 mm (5.18)
John VK2ETA asks what he should do to protect his µBITx from the rigours of daily use when taken portable.
1. Are they other components you should be concerned about?
2. What method is recommended: hot melt glue, epoxy glue, superglue or other method?
3. What about cables
4. Any other actions that should be taken?
The recommendations from IO Group members seem to be:
Mike ZL1AXG (editor of ubitx.net) has taken a different approach to mounting a BCI filter. The high pass filter design and mounting point are the same as that documented here. However, the mounting system has been modified to make the filter easily removable.
Mike has other filters that plug into the QRP-Labs LPF filter board so his filter takes the same dimensions (1.5 x 0.5 inches or 38.1 x 12.7mm) and uses the same mounting system (two four pin male dupont header connectors.) The protruding centre pins of the DuPont connector were cut off, and the outside pins bent over (in opposite directions) to improve stability of the mount. The pins were soldered to the empty pads to ground on C216 and C210 on one end (RHS in the photo below) and to the RX signal rail on the other end (LHS below). A cut was made to the track mid-way between the dupont connectors (not shown here).
The BCI filter as mounted can be seen below. It makes a tidy looking unit that can be substituted or bypassed later.
Mike KU4QO suggests that how the relays select the relevant output low pass filter is done in a very clever way.
Three relays control four different filters.
The relays are only energized when transmitting (and only when needed). It is an unusual configuration, saving the expense of a 4th relay.
Mike had a problem where he had no power out on 30m or 40m. Receive was fine. It ended up that one of the legs on relay KT2 was not soldered. A quick soldering of the pin and he was back in business.
Raj VU2ZAP responded, “Farhan has his mysterious ways of being thrifty. The real reason is as I think is to save one data out control line from Raduino.”
“The relays are known to fail, so I cut the TX line to filter ralays and linked it to +12V . The relays remain on during RX and when you tune across bands you can hear them clicking! This affects power consumption but I can live with that.”
Jerry Gaffke KE7ER, provided an analysis of the RF path through the relays and suggested:
“With no relays energized as shown in the uBitx schematic, RF power from the finals goes through KT1-14 to the 30m lowpass filter then through KT1-3 out to the antenna. That’s as short as it can get.
“With KT1 energized and KT2 not, RF power goes through KT1-16 and KT2-14 to go through the 20m lowpass filter. Still pretty short.
“With KT1 and KT2 both energized, we get the 40m lowpass filter through KT3-14.
“With KT1,KT2,KT3 all energized, we get the 80m lowpass filter through KT3-16.
“That’s getting to be a pretty long path for that RF, going through all three relays. But at 3.5 – 4mhz it simply doesn’t matter.”
