August 2020 Update: Camping, Knives, Trike, Truck. And JS8Call.

JS8Call: Another “Zombie” mode?

BITX40 Rebuild Part 1: Mistakes, Planning and Teardown

Things have been pretty busy around MiscDotGeek, and we’ve barely had time for Radio Related things. As I mentioned in the July 2020 update, my BITX40 quit transmitting while sitting stationary on my desk, dutifully doing its job as a JS8 radio, it quit transmitting. I put the uSDX in its place, only to have it fail too! In this post we’ll outline the history of my own BITX40 and the mods it has had. We’ll also talk about the problems, the proposed fixes, and how you can help!

My BITX40 History

Before I go into this too far, its good to understand how my BITX40 got where it is. My beloved wife purchased the BITX40 for me in December 2016. It was the origitnal HFSignals.com BITX40v3 with an analogue VFO- the drifty disaster that it was! My goal was digital work- JT65 or PSK31. Farhan (maker of the BITX40v3) at the time recommended the QRP Labs VFO kit to stabilize the radio, and so that’s what I did. And that’s how my love of the BITX40 and pretty much anything QRP Labs makes began!

Before long I was hacking it for Upper SideBand (USB) operation, experimenting with bypassing the Band Pass Filter to use it on 80 meters, and I also started experimenting with making the Band Pass and Low Pass filters modular, using QRP Labs filters. This worked very well, and it was in this form that I submitted my BITX40 to the 2018 Builders Fest, which landed me a spot on W5KUB’s show, which was a real hoot! A custom 3D printed case was designed and obtained, and I even called it done. And, until it broke, it was!

Troubleshooting myself in the foot

I’m going to make this fairly short: While troubleshooting problems with the PA, I zapped the custom VFO with 12v and things went downhill from there. Now there was no receive. The Si5351 had shorted internally. I replaced the QRP Labs Si5351 module with a spare, and it went to a dead short after being on for probably 15 seconds. And then the Arduino bit the dust.

Not pictured: My broken heart

Rebuild? Or… Rebuild.

At this point I had a decision to make: Should I just shelve this thing and wait until I can scratch build my own QRP SSB radio? That is a goal, but I really do love this BITX40 especially since it was a gift from my wife. I have had ideas for some “extreme” mods for the BITX40, and after some consideration, I decided it was time to start gathering parts to both upgrade the BITX40 and replace some of its most troublesome parts.

Planning the BITX40 Rebuild

If you go through my various BITX40 posts, you’ll find that I’ve burned traces off the board. I’ve hacked this thing to death and back more than once. I’ve customized it, built a VFO for it, ran the BFO from the Si5351, modified the PTT circuit, and even tried RDF15HVF1’s for PA output.

Because of the extensive modifications, mess-ups, fixes, etc, the board has seen better days physically. The IRF510 is only held in by one pin, and the other two are jumper wired, for example!

To start with planning, I made a list of things that are wrong:

  • PA not working
    • PA power drops off at 20 meters when it is working
  • Arduino VFO/BFO dead
  • Tuning clicks, poor tuning (uBITX based sketch)
  • Improper connection of the BFO/VFO
  • 12mhz SSB crystal filter interacts with 16mhz Arduino clock
  • SSB filter not symmetrical- USB/SSB sound quite different
  • Custom designed 3D printed case too small for easy servicing
  • Wiring constantly breaking due to improper construction
    • soldering directly to pin headers is bad, totally my fault

As you can see I have quite a list of things I’d like to fix. And so my plan began. Here are the fixes and further modifications I have decided on:

  • Replace PA with QRP Labs 10W HF PA
    • Push/Pull design capable of higher wattage across the HF bands
    • Better PCB layout than BITX40 or uBITX means more stable PA
  • New VFO/BFO based on Arduino Nano and QRP Labs Si5351 module
    • New, simpler, better sketch
    • Proper termination by using the resistor pads from the uBITX design
  • Replace 12mhz 4 crystal filter with homebrew SSB filter from uBITX design
    • Moved to 11.0592mhz
  • Larger Case, probably from one of the CB’s I scrapped this year
  • New wiring, possibly with a crimper this time (depending on funds) but at least with strain relief for durability
  • Review the last few years of published modifications to see what else needs to be done
    • Better audio circuit muting on transmit
    • Diode protection on Q13 (Yes, it survived all this time without it)

The first thing to do was to remove all of the parts I’m going to replace. This exposes the faults of the board as well so that I can better address them during the rebuild. So I got out the soldering iron and braid, and got to removing a ton of SMD parts as well as several transformers and inductors. The result:

Oh no… what have I done!?

While tearing down the PA circuit, I found the problem: The inductor L8 (next to the relay on the left) had a broken wire, which took out the rest of the PA. You can see that I replaced Q13 with a 2n2222 (I though it was dead- it wasn’t), but we’re not out of the woods there- Q13 will be needed to get the necessary wattage to feed the QRP Labs 10W HF PA. Once I get receive working again, I can start working on getting the PA back.

You can see I removed the VFO/BFO circuitry, the PA, and the Band Pass and Low Pass Filters. I left the crystal filter in place because I’d like to get receive working again, and the new crystals aren’t here yet. Those will need to be matched as well, and that’ll be a small sub-project.

How can YOU help?

You can help in two ways. First, if you have ideas for this rebuild, or for anything else you’d like to see on This Blog, let me know in the comments below!

Secondly, these projects cost money, and I try my best to do things in ways that anyone on this planet can do with a little bit of learning and basic tools. You can show your support by clicking the “Did you know?” in the top right corner on any page (or at the bottom, if you’re on a mobile device) of this site when you shop at Amazon. There’s also the QRP Parts Catalog for some specific recommendations from both Amazon.com and Bangood.com. Shopping through either of these links will give me a small referral fee and not cost you anything. They’ll help me to keep funding projects like this.

Thank you for your support!

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