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The success of my project to make an FM version of G3XBM’s Fredbox was always going to depend on the receiver. The Fredbox receiver is a simple three transistor super-regenerative design. This is the only practical way to make a receiver small enough to fit in a hand-held case. It is also the only cost-effective way to do it, because making a conventional double-conversion receiver with crystal control just wouldn’t be worth it for such a project.

What I didn’t know were a) would the super-regenerative receiver work with FM signals (the original Fredbox was designed for AM) and b) would it be sufficiently stable to stay on frequency without the benefit of crystal control? However, what I didn’t give too much thought to was the possibility that I wouldn’t be able to get it to work at all.

Because this is a VHF circuit I didn’t think I could try it out on the breadboard so I made it up directly on to Veroboard. The layout is pretty similar to Roger’s original layout from what I can tell from the pictures on his website, although I think he made an etched circuit board, something I’m not able to do.
I drew up the layout using a bitmap editor. This shows the components from the top view. The large outline components are the Toko S18 inductors, and the red lines are wire links below the board connecting the ground traces. The schematic for the receiver part of the Fredbox is shown below. As with the Nano-40 I used an MPS13 in the audio stage in the hope of getting a bit more audio gain.

I applied power and expected to hear a hissing sound in the earpiece which stops when a signal is tuned in. However I heard nothing at all. The audio stage appears to be working, as I can hear a faint buzz if I touch something metal to the input.

Alan, VK2ZAY had a lot of trouble getting the receiver to work as well, and he is an experienced constructor who knows what he is doing. I suppose this should have warned me off trying this project. Alan replaced the axial choke for L3 with half a dozen turns of wire on a ferrite bead, and I did the same, but to no avail. I also substituted a variable trimpot for R2 which in many other super-regenerative receiver designs is a variable regeneration control. But no matter what the setting of the pot I could not get any regeneration to occur at all.

I think my FM Fredbox is about to become yet another abandoned project. 🙁

One of the things I have long wanted to do in this hobby is build a hand-held transceiver. I tried once back in the 1970s but the receiver didn’t work and the project eventually ended up in the garbage. Many months ago while browsing Roger G3XBM’s website I came across an old project of his called the Fredbox, a small, low-power hand-held 2m transceiver. This rekindled my desire to try to make my own VHF handy transceiver so I started the process of accumulating the parts that would be needed to make my own Fredbox.

However there were two problems. Roger designed the Fredbox as an AM (amplitude modulation) transceiver, a mode that was probably still in common use back in the 1970s when he first made it. Here, the only chance of making a contact with such a radio is if it transmits on 145.500 in FM mode, so I would have to modify the transmitter to produce FM instead of AM.

The other problem is that the Fredbox transmitter is crystal controlled. The days when suppliers advertised crystals for popular FM frequencies in RadCom at reasonable cost have gone, along with the crystal controlled transceivers that used them. I didn’t know where to obtain the crystal I would need but hoped that someone might have crystals from an old 2m radio that they wanted to get rid of.

My luck seemed to be in when, a couple of months ago, someone started selling batches of 2m crystals from old Japanese radios on eBay. I bid for and obtained three batches containing crystals for 145.450, 145.500 and 145.550MHz. I don’t know what radios these crystals were originally for or what oscillator circuit they were used in but by using the XBM80 as a test oscillator it appeared that most of the transmit crystals oscillated at around 12.1MHz.

I tried breadboarding the first oscillator stage of Roger’s Fredbox circuit in order to experiment with ways of FM-ing the signal but the crystal did not seem to be oscillating. I did some searching for other 2m FM transmitter circuits in the hope of getting some inspiration and came across one using an MC2833P IC – a complete FM transmitter on a chip. This seemed like the ideal solution, especially as the crystal used to get 144MHz output was 12MHz just like the crystals I had. Unfortunately I couldn’t find a supplier of this chip until on a whim I typed the part number into eBay. There was someone selling one chip! I ordered it and it arrived in the post this morning.

I built up the circuit from the application note on my breadboard (as you can see in the photo) but to my dismay I could not detect any signal when using any of my 2m TX crystals. It was very disheartening. I didn’t know if the chip was dead, whether I’d blown it by accidentally shorting together wires from components on the breadboard, or whether it was just a very fussy circuit. I experimented with different components and coils and got nowhere. I was just going for lunch when I had the idea to try one of my QRP CW frequency crystals. I had one for 10.106MHz and another for 14.060MHz. With both of those crystals I could detect not just a strong carrier on the fundamental frequency but also plenty of output in the 140MHz region. The chip is working, the problem is my eBay crystals!

The question is, what to do now? I guess the crystals I bought, being probably 35 years old, have deteriorated with age and are reluctant to oscillate. It appears that the only way to proceed with the project is to get a brand new 12.125MHz crystal custom made, if possible.

I still haven’t made a contact with the little Nano-40 QRPP transceiver I made, but after listening on several occasions I finally heard its signal coming back from the Web SDR at the University of Twente in the Netherlands at 2100 this evening.

I sent a few V’s and clearly heard them come back with a second or so delay from the remote receiver. I then attempted to send CQ, but the distraction of the delayed signal made it difficult. The signal was lost in the noise some of the time, but at other times it was perfectly clear and readable. I calculated the distance between my QTH at locator IO84hp and the Web SDR at JO32kf to be about 750km or about 450 miles. Not bad for 150mW to an MFJ magnetic loop antenna in the attic! Isn’t QRP amazing?

I will treasure the first contact I make with this little radio, so for the time being I am not going to cheat and try to arrange a sked because it would be much more of an achievement to make a contact that happened naturally. But if you should hear me one evening calling CQ close to 7.030 please reply slowly because I can probably hear 4 or 5 different stations at the same time and my CW is not that hot so it will take a lot of concentration for me to read you. A bit more power might help too because the little guy’s receiver isn’t as sensitive as the one you’ll be using.

I didn’t plan to produce a schematic of the Nano-40, the tiny 40m CW transceiver I built based on Roger G3XBM’s 80m XBM80-2 design as modified by Alan VK2ZAY as I don’t claim any originality for the circuit. I provided the original sources and discussed the changes I made to the circuit and thought that other builders would follow the same path.

However I have been asked by a couple of people already if I could provide a schematic so I have patched a copy of the circuit diagram produced by Alan to show what I ended up with. I hope that will keep everyone happy, but please remember that without Roger G3XBM’s genius and Alan VK2ZAY’s improvements the Nano-40 would never have happened.

A few weeks ago fellow blogger Dominic M1KTA wrote that he was selling off some unwanted projects from his shack, including some unbuilt or part-completed kits. A little while ago he wrote: “I have decided to stop selling off project builds now and I am keeping hold of everything until a rally when I can sell them in person as I have had a complete nightmare after selling one of the projects over the internet to someone I believed was capable of finishing it and has demanded I rebuild and re-align it after they hacked about with it themselves to the point where it no longer functions, they melted the pcb connectors and filed away part of the pcb and at least one track in the process to attempt to squeeze it into a box that was too small and demanded a paypal refund. I am never again selling a ‘built’ project over the internet it is too risky for me.”

I have bought things before, either at rallies (hamfests) or from ads in RadCom, that were found not to work and sometimes revealed some astonishingly ham-fisted handiwork inside. The purpose of some modifications defied understanding. I either fixed them myself or wrote it off to experience.

The idea that someone could buy an unfinished kit and then try to make the seller liable for their inability to complete it just beggars belief. It seems some members of this hobby have no shame and no pride.

I told Dom he should publish the callsign of this so-called amateur as a warning to other sellers to steer clear. I know I would.

Today I finally completed the little 40m CW transceiver based on Roger G3XBM’s 80m XBM80-2 design as modified by Alan VK2ZAY and further modified by me to work on 40m. I tried to think of a name for it that would acknowledge both these sources of inspiration, but I couldn’t come up with anything snappy, so in the end I named it the Nano-40 because it is very small and works on 40 metres.

The circuit is essentially the same as the one given by Alan VK2ZAY with a few component changes determined empirically while the rig was on the breadboard to allow for the fact that it is working on 40m instead of 80m. There is only one stage of low pass filtering instead of two, as I described in an earlier post. This seems to be quite adequate. I checked the output waveform using my new oscilloscope and it looks nicely sinusoidal. I also used an MPSA13 Darlington transistor instead of a 2N3904 in the audio stage for a bit more sensitivity and gain.

With the component values I chose the little rig gives 150mW output on a 12V supply or about 75mW using a 9V PP3-style rechargeable battery. There is no sidetone, but it has full QSK. Using my Elecraft XG1 test oscillator I found that a 1uV signal is just about audible. However, bearing in mind that you can receive all activity around 7.030MHz plus and minus the limits of your hearing, a workable signal would need to be a bit stronger to stand out from the crowd!

The circuit was built on to a small piece of Veroboard and the result is shown above. In case anyone wants to try to replicate it, the reverse side of the Veroboard showing the breaks in the copper is shown below.

The rig was built into a small translucent blue project box. The box I ordered for the project turned out to be just a bit too small for comfort. It was not quite high enough for the crystal, so I had to cut away part of the circuit board so the crystal could be mounted with its base below the board itself. The space was also a bit too tight for the connectors. Consequently final assembly took much longer than it would have done using a larger enclosure.

A fair bit of “fine tuning” with a file was needed to adjust clearances which were down to the last millimetre. With the lid on there is not enough room for the spring loaded contacts of the two 3.5mm jack sockets to fully open so the key and earpiece plugs are a tight fit. There was not enough space for a power socket to be fitted internally so after a lot of thought that was eventually resolved by super-glueing a PCB-mounted power socket to the side of the case.

Although I had tested the circuit prior to assembling it into the case there was a heart sinking moment when I applied power and heard no signals at all. Close inspection showed that I had a solder bridge across the antenna socket. This was quickly removed and then the transceiver was found to perform as well as it always did.

I have put out a number of CQs with the little rig but have yet to make a contact with it. Because I don’t live in a densely populated area I don’t have any nearby hams to make a sked with to prove that it really works. You don’t build a rig like this to make a lot of contacts, of course, you do it to prove that it is possible to make a fully functional transceiver this simple and this small. Nevertheless I do hope to make a QSO with my little Nano-40 one day.

There was a knock on the door this morning and the postman asked me to sign for a parcel from overseas. The customs declaration amusingly – if appropriately – described the item as “Toy”. It was the UNI-T UT-81B Oscilloscope Digital Multimeter that I ordered last weekend from eBay seller hk360radio in Hong Kong.

My first thought on opening the box was: “Wow! All this for a hundred quid? Amazing!” Inside the zip-up fabric carry case was the scope/multimeter, test probes, a BNC oscilloscope probe adapter, an opto-isolated USB cable for connecting the instrument to a PC, a two-pin wall-wart power supply and a two-pin to UK three-pin mains adapter, manual and CD containing the PC software.

The instrument is both a multimeter and a storage oscilloscope. The multimeter is auto-ranging, so the control switch simply selects the function: voltage, current, resistance etc. It measures DC voltages to 1000V, AC to 750V, current to 10A, resistance to 10M, capacitance to 100u and frequency to 10MHz. There is also a continuity tester. It can read volts down to 100uV and current down to 0.1uA. The frequency counter accuracy isn’t good enough to calibrate your radio, but it’s still quite handy.

Many years ago, back before I had a ham radio license, I had an oscilloscope that I built from a design in Radio Constructor magazine. It only covered up to about 200kHz and wasn’t calibrated. I had to sell it when I left my parents home and had nowhere to keep so much electronic stuff, and I haven’t had one since. But there have often been occasions when I wished I had one, so that was one of the main reasons for buying the UT-81B.

The oscilloscope bandwidth of the UT-81B model is quoted as 8MHz, so I can’t use it as a monitor scope for the whole of HF, but it covers up to 40m at least. I was very keen to try this. I connected my FT-817 up to my QRP power meter via a T-piece and connected a cable between that and the oscilloscope. When I spoke into the microphone I could see the modulation envelope on the display.

I was interested to see whether the horizontal scan rate was fast enough that I could see the actual RF waveform. I increased the timebase speed until I was able to see the display above of the carrier wave from the FT-817 in CW mode. The sample rate is given as 40MHz and you can just see that the waveform is a little jagged. You can see that the scope has also displayed the frequency to within 1kHz. There is a full range of trigger functions and also a manual hold you can press to capture the display, which I used to take the photo above.

There is a Windows software application that connects to the instrument via a USB port. It can log measurements over time and also capture scope displays and save them to a bitmap (BMP) file. I was able to capture the waveform shown in the photo. However it was not able to capture an RF modulation envelope when sampling the RF at a much slower time base setting. I just got a thin wiggly line that seemed to bear no relation to the envelope displayed on the scope itself. Not a major issue, though.

I am really delighted with my new “toy”, which cost me £95.50 plus £13 for the shipping (and no tax.)