Building a HF Signals BITX40

Building a HF Signals BITX40

by N6SX

21 Mar 18 –

Just got a HF Signals BITX40, thanks Professor πŸ™‚ and I thought I would document and share my feeble attempt to build it πŸ˜› I am usually pretty busy, but will post my progress when I some spare cycles.

This is a picture of the contents of the kit:
HF Signals BITX40 Parts

Right now I am doing my research about any concerns I should have before building it and trying to decide on an enclosure. It really doesn’t seem too bad to build … I recall “helping” my dad build a set from totally unpopulated boards from Heath Kit (hey, they’re still around). As far as the enclosure, I’d like to make an acrylic one (I know it has been done to death) but I would like students and Club Members see what it looks like inside.

23 Mar 18 –

With my spare cycles, I have been modifying an acrylic enclosure design used on a project in my research group. I’ll add a pic after I finalize the design and get it laser-cut. At the same time, I have been reading more about the BITX40. As stated by Dave Casler, KE0OG, unlike a typical American-made kit which would usually include all the parts necessary to make a particular radio, the designers of the BITX40 wanted people to experiment with it and play with it. I think the best analogue would be the original VM Bug. You can buy it and drive it around, but it was very easy for owners to modify virtually every aspect of the car from engine to body. However, at least when you bought a Bug, you would have doors, locks, and upholstery on the seats. The BITX40 kit basically has everything you need to get it up and running without those unnecessary luxuries like an enclosure, knobs, a switch … you get the idea πŸ™‚

Since this is a kit, many people seem to take a few stabs at making their radio. Their first attempt comes from impatience in wanting to get their radio up and running (I don’t blame them). If they’re up to it, their second and possibly subsequent attempts correct for the shortcomings of the kit and/or their design/layout. Normally, I would be in the first group and jump right in and build it, but I am trying to be really, really good about it and take my time.

So far, I learned I should add a speaker, fuse, and a switch. A few people mounted the PTT switch on the enclosure, but that makes it awkward to use. The kit includes a mic element but nothing to hold it which means you should consider a design that can hold the mic and PTT switch together to make it easier to use. For completeness, I will try to make a mic stick but I may ultimately end up using a spare hand mic for a Baofeng instead.

26 Mar 2018 –

Wow, there are so many mods/tweaks available for the BITX40 and other boards in the same family (the ones I am considering are listed below). Although I would like to apply the mods before assembly, I would also like to see what the differences are in performance so I may need to assemble the kit as shipped, make notes then apply the mods. In preparation, I have made a basic BoM and will be ordering the parts I don’t have … there really aren’t that many parts.

20 Apr 2018 –

Okay, its been a while (sorry about that) but it took a while to order and a bit longer to receive the parts. Yay! … well, all but one part ….

The part I am waiting for is some wire so I can repair this …

The toroid (L3) was cracked, and I just nudged it a bit so you can see it better. I thought I could find and order a replacement, but I couldn’t find one that was pre-wound which is why I am waiting on the wire.

I also have the initial design of the enclosure done, but the laser cutters have been occupied all the time. I’ll post those when it’s done and should be able to make more posts since I have much of what I need to proceed.

23 Apr 2018 – Mod

While waiting for the wire, I started applying some of the mods. The first two are the ones many community sites say are a must do. This site does the best job describing the first two mods. Here is an excerpt:

    The Q13 protection mod is extremely important as there have been hams who have blown Q13 within hours (or even minutes) of getting their new BITX40v3 working and hooked up to an antenna. The cause is because they transmitted on a nearby antenna with their 100W or higher station rig while the BITX40 was receiving on its antenna.

    The symptom of a damaged Q13 is that receive still works fine but there is no transmit. Note that although Q13 is a driver for the transmitter circuit, its base is connected to the antenna during receive.

Everyone shows you the bottom of the board but few show you what is on the other-side.
Here are pics of the K1 relay and the diodes soldered below it: 2 x 1N4148 Diodes

While I was scouring the community sites, I ran across another fix I wish I had done instead. It uses an incandescent bulb instead of the two diodes.

    At pin 12 of K1 relay solder 2 4148 diodes in parallel but in opposite polarity. This will limit the receiver from getting high RF voltage.
    Also if you cut the track between K1 pin 12 and K2 pin 14 and insert a 12v 60 or 40 ma miniature incandescent bulb it would be better. This is common protection method in commercial transceivers and amps.

The next one addresses the 2nd harmonic … should have waited on this one, but it is easy enough to remove if I really want to see if there is a difference later.

    The 2nd harmonic is a little too high for North American standards. Add a 100 pF 100V NP0 capacitor across L7 in the output filter. This will reduce the 2nd harmonic by about 20 dB. If you are planning on increasing the output power above the normal 5 to 7W, then you should use a 200V capacitor.

Here is the L7 coil and the capacitor soldered below: 100 pF 100V NP0 Capacitor (200V if higher than 5W)

Above mods from

I would still like to be able to fire it up before making performance altering mods just to see if I can notice a difference, but I figure a heat sink in preparation for the future USB mod wouldn’t hurt right now … I have seen some much, much, much bigger heat sinks on the BITX40, but I am not familiar enough with the science of heat sinks (besides reading the charts depicting performance) to determine what would work best … is bigger always better? πŸ˜‰ I noticed the fins were thicker on the new heat sink … is thicker always better? πŸ˜‰ No comments from the peanut gallery πŸ˜‰

25 Apr 2018 – Mod

Still trying to find mods to do while I wait for the wire. This one is in preparation to add USB capabilities. This is the hardware portion, but I will need to update the firmware on the raduino later to allow us to use USB.

    USB is supported in Allard’s sketch without hardware modification, but it is a good idea to remove C91 and C92 as they are unnecessary when using a Raduino and only serve to reduce sensitivity when in USB mode.

At the center of the board, we remove C91 and C92.

Above mod from and here

6 May 2018 –

Finally received the wire so I can fix the L2 and L3 coils. The HFSignals website stated it should be 36AWG but it can be a smaller-sized wire. 40-turns later, here is the first one. It was suggested to have 0.5″ of wire per turn with an extra inch on both sides. I did that but added a bit more which helped when it was time to solder the coil to the board.

The most time consuming part of replacing the L2 and L3 was the hot glue that held the coils in place. I could see bits of the broken coil in the hot glue and thought I should make sure all remnants were removed. I ended up using a solder wick to remove the glue. Since there is a chance I may need to adjust the coils, I am not going to apply hot glue until I am satisfied.

4 Nov 2018 –

Okay, it did take a little while to get back to this, but I hope to chug away at this. Besides waiting for the parts I had ordered, I was waiting … and waiting for a student to laser-cut the enclosure. He really wanted to do it for me, but it was definitely not at the top of his queue. The enclosure I designed was based on the enclosure K6TDK designed for a router used in a research project.

Router for Hamilton Project

These are the pieces with the protective sheet still attached.

Assembled, view from front

Assembled, view from rear

Its been quite a while since I looked at this so I will have to see where I left off. Also, I was hoping to get the enclosure back immediately so I could see if everything fits properly and to see if I should relocate something … I may just live with what I have for now πŸ˜›



Reference Sites:

In no particular order, some (maybe a little too ambitious) additions/mods I am considering … we’ll see how it goesΒ  πŸ™‚

Mod:   Band Scope
Desc:   Add a portable band scope
BoM:   10 pF Cap, screened lead, old andriod phone for display
Mod:   Diode Protection (transmit)
Desc:   Protect diode from being fried
BoM:   2 x 1N4148 Diodes
Mod:   2nd Harmonic Reduction
Desc:   Nuff said
BoM:   100 pF 100V NP0 Capacitor (200V if higher than 5W)
Mod:   Reduce Drift from Heat
Desc:   As set starts to get warm, some drift may be experienced
BoM:   6 x 220 Ohm 1/4W (SMD 2106 or reg)
Note:   May not be necessary. May have been for an earlier
  rev of the board.
  *** Looks like this was for the older VFO version ***
Mod:   Function Key
Desc:   Add ability to cycle through added features, like dial lock
BoM:   Momentary Switch
Mod:   Fuse
Desc:   Other builders have experienced smoking … smoking is bad, m’kay
BoM:   2A fuse and holder
Mod:   Headphone Jack
Desc:   IDK if it’s worth it πŸ˜›
BoM:   TBD
Mod:   Multi-band Modules
Desc:   Add more bands to the BITX40
BoM:   Module kits for purchase
Mod:   Multiple-Line Display
Desc:   Provide more information on set
BoM:   LCD or TFT
Mod:   On-Board Voltage Monitor
Desc:   Good for battery operation
BoM:   1.8K Ohm and 5.6K Ohm resistor
Mod:   Possibly Replace Raduino
Desc:   More hackable
BoM:   TBD … possibly a kit available
Mod:   Reduce Receiver Gain to Improve Audio
Desc:   Nuff said again
BoM:   Remove a capacitor
Mod:   Reduce Receiver Gain to Improve Audio
Desc:   Nuff said again again
BoM:   10K pot, 10k Ohm resistor
Mod:   RX/TX Indicator
Desc:   Because lights are always cool
BoM:   3mm LED, 12k Ohm resistor
Mod:   S Meter
Desc:   Not accurate but can provide a ballpark figure
BoM:   0.1 uF capacitor, 4.7M Ohm resistor, silicon switching
  diode (1N914 or 1N4148) or a better choice a 1N32
  germanium diode
Mod:   Speaker
Desc:   Because we don’t always want to wear a headset
BoM:   Speaker
Mod:   USB
Desc:   Add ability to switch to USB
BoM:   Larger heat sink necessary because of transmit times
  necessary on USB
  Remove two capacitors
  Update firmware on raduino
Mod:   10-Turn Tuning Pot
Desc:   Allows us to use the full 40m band
BoM:   10-turn pot
Primarily for CW
Mod:   Morse Key or Captive Touch
Desc:   Add ability to key
BoM:   Morse key: 1K Ohm resistor / Captive Touch: 2 x 1K Ohm
  resistor, 2 x 470K Ohm Resistor, 2 x 3-22pF Capacitors
Mod:   CW – Carrier Wiring
Desc:   Helps generate a CW carrier
BoM:   4.7K Ohm resistor
Mod:   PTT Sense Wiring
Desc:   PTT sense is required for the CW, RIT, SPLIT functionality
BoM:   10K Ohm resistor
Mod:   CW – Side Tone
Desc:   So you can hear what is going on
BoM:   1K Ohm and 4.7K Ohm resistor
Mod:   CW – TX-RX Wiring
Desc:   To bypass PTT wiring for CW operations
BoM:   4.7K Ohm resistor, 2N3904 transistor