In the July 2020 update, I made mention that my BITX40 BITX’d the dust, so to speak. Receive worked, but transmit failed. In Part 1 of the BITX40 Rebuild series I talked about the history of my radio, some of the modifications I’ve made, and how badly I messed it up when troubleshooting the problem! In March ’21 I posted Part 2 of the BITX40 rebuild and a link to my YouTube channel and the BITX40 Rebuild Playlist, with a link to all the videos. I’ve added Part 5.1, 6, and 6.1 since then. You can check out Part 6 here:
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BITX40 Back on the air!
After a lot of work to get the PA working, not to mention some stabbing in the dark, the BITX40 is back on the air! It’s a hot mess right now, here’s a picture of it on the workbench:
To make it easy to identify the different parts, I labeled them with numbers. In order:
- The QRP Labs 10W Linear HF PA.
- QRP Labs 40M Low Pass Filter
- 20×4 LCD
- TX control. The QRP Labs PA requires the TX pad to be grounded to put it in TX mode. This PTT control board uses a BS170 and a couple of 47K resistors to ground TX when supplied with 12V from PTT. Thanks to Ronald Taylor on the QRP Labs Groups.io forum for his help on this part.
- There’s a QRP Labs 40M Band Pass Filter tucked behind the PTT control board.
- PTT control board. The opto-isolator in the Easy-Digi board is too weak to power the relays, and so this 2n2222 and 4.7K resistor act as a switch to turn PTT on/off. I wrote about it here.
- QRP Labs Si5351a synth board on a custom board that I put together on prototype board. It ties the Nano and the Synth board together, has a 78L05 voltage regulator. There’s also header pins to provide 5V, 12V, GND to any accessories that get added later.
- Arduino Nano clone.
- The rebuilt first and second stage of the PA driver. I had to get a 2N2219A and heat sink from Kitsandparts.com to rebuild the second stage driver. The schematic calls for it to be powered through a 2.2K resistor, but this didn’t provide enough power to get 10W out of the PA. I ended up going down to about 300 ohms on that resistor, and it works great now.
- There are other hardware mods that are harder to see. The IF has been moved to 11.0592mhz, and the SI5351a outputs are terminated in 3db pads on the BITX40 board. They feed through .01uf caps into the mixers. I learned the hard way that the mixers are biased with DC voltage, and I let the magic smoke out of the first SI5351a that I connected. One DC blocking cap later…
There’s still quite a bit to do! Clearly it needs to be put in a case, and a lot of those wires need to be shortened up. I also need to put the diodes in to protect the driver transistor. Once that’s done I think I’ll be done with the hardware mods. Then comes the software!
The Arduino sketch that I’m using right now is very basic and was given to me by Bob, GM4CID. Thank you, Bob! It’s great because it allows me to independently tune the IF and the main frequency. This allowed me to discover the characteristics of the new crystal filter and determine the correct IF frequencies for USB and LSB.
One thing I’d really like to implement is an IF shift control for receive, probably 100-200hz either direction in order to change the audio qualities on the fly. I *might* implement multiple bands as well, but I haven’t decided. It really depends on how I end up using this radio in the long run. It also depends on me getting my hands on an ATU-10 kit when they come on the market, or else building a multi-band end fed antenna, which is something I’ve wanted to do for a while now.
If you’ve got this far in reading, then Thank You! Please consider subscribing to the blog way up there in the top right corner. I’ll be posting more about this project as I make more progress with the software and getting it into an enclosure. 73!