A Pretty Good Troubleshoot -- Fixing the Transmitter in my 75/20 meter Mythbuster Transceiver -- Mind the Gap!

Bidirectional Termination Insensitive Amplifier by W7ZOI and K3NHI

All of a sudden the transmitter in my 75 & 20 meter dual band homebrew Mythbuster transceiver stopped working -- there was no output at all.  I went into troubleshooting mode.

The first clue was that the receiver was working fine. This meant that many stages of the rig were taken out of suspicion:  It probably wasn't the VFO, the first mixer, the BFO/Carrier Oscillator,  the second mixer, or the bandpass filters. Nor was it any of the receive sections in the bidirectional amplifiers I was using.  

Suspicion fell on the power amplifiers and on the transmit sections of the bilateral amplifiers.  

With the output going to a dummy load,  I put the rig into transmit mode and put a bit of audio into the mic jack.  Then with the 'scope I started to work my way back from the antenna jack.  I wasn't seeing anything.  Then I got back to the transmit side of the TIA amplifier between the crystal filter and the mixer.   There was a strong signal at the input, but nothing at the output.  Bingo!  I had found the faulty stage.  But where, exactly, had this stage gone wrong? 

There are three transistors on each side of a TIA amp (see schematic above) -- I just started from the input of the first one with my scope probe and moved through the circuit.  Finally, at the output of the last of the three amplifiers, the signal stopped.  I knew I was very close to the problem.  

Looking at the components, suddenly I could see the problem:  At the output there is a 47 ohm resistor (R2 in the circuit diagram above) and a .1uF cap in series.  The cap went to a Manhattan pad.  But when I looked at it closely, the lead was kind of floating above the pad.  See it? 

Mind the Gap 

And when I moved it, the connection between the 47 ohm resistor and its pad seemed quite flimsy. 

I quickly replaced both components and was back on the air. 

I don't really know how or why the lead to that capacitor broke.  Maybe I had bent it repeatedly, to the point of weakness, and, over time, it just let go.   

Whatever the cause,  I found this to be a satisfying troubleshoot and repair.  It required me to think a bit about what could be wrong, and to use some test gear to zero on on the faulty component.  

Out with the old...

A Cuban Knack Story, and a Pandemic (SITS!) SSB version of the DSB Jaguey Rig -- Viva el Cacharreo!

 

First, the Knack Story.  Andy CO2AFV clearly has The Dilbert Disease: 

Hello my name is Andy. I had an interest in Ham radio before knowing that existed.  While I was a child my entertainment was building quartz oscillators that later I tried to receive on neighbors' and friends' radios. One day I succeeded in modulating two of them and I finally established a conversation with a friend about 200 meters from my home!!!

Andy with his FB HB rig

Here's a description of a version of the 7 MHz Jaguey transceiver that Andy built during the pandemic.  It looks to me as if he took the Jaguey DSB rig and added a 455 kHz filter with an additional mixer to turn it into an SSB rig.  So he is generating the SSB at 455 kHz, and mixing it with a VFO running at around 6.8 MHz.  The sum output would put you in the 40 meter band; the difference output at around 6.35 MHz could (mostly) be knocked down by a bandpass filter.  I think the Cuban Radio Federation Web Site gets it a bit wrong -- the purpose of the filter is probably to eliminate the unneeded sideband, not really to suppress parasitics. 

Federation of Radio Amateurs of Cuba Published: September 17, 2020 Viewed: 2352 Comments: 12 

Radio Transceiver CO9BIA 455 A construction carried out in times of Pandemic by its author, Andy Fernández Valdespino (CO2AFV). 

Cuban radio amateurs continue to accept the challenge of isolation caused by the incidence of COVID-19, but this does not mean they paralyze their activities. Such is the case of Andy Fernández Valdespino (CO2AFV), who for more than four months has been working on the development and construction of a new transceiver, the CO9BIA 455, a device that already works perfectly in the 7 MHZ Band. 

Andy, who is technical secretary of one of the Havana Radio Clubs, has to his credit the construction of two Jagüey-type radio models, as well as several types of interfaces for programming and Digital Modes; and various prototypes of antennas, among other elements that make up its constant “cacharreo” activities, as we say in our language. 

He has now completed and tested a new model that he has named with the callsign of his Radio Club, CO9BIA, and the model 455 is due to the use in this prototype of a filter of the same capacity. Asked about the details and other construction bases of this radio, whose transmission and reception tests using only outputs from the driver were carried out on September 14, Fernández Valdespino pointed out that his objective was to build a portable QRP equipment, of very large proportions. small, that it would be capable of being operated in the 40 meter band on both sides, by incorporating an improved VFO from the traditional Jagüey, but this with some modifications, and that the radio in question would work powered by a 7-inch battery. .2 volt, the same ones that come with most of the “Handy” used by radio amateurs. 

To complete the “portability” characteristics of the new radio, the possibility of exchanging antennas has been incorporated, and a very light variant of the telescopic type can also be used, just over one and a half meters long. Andy explained that for the development of the new equipment, he was based on studies that he has been doing on some of the characteristics of the Jagüey, a direct conversion radio with very good sensitivity, but that does not have good selectivity, so in the conditions of the current solar cycle, its behavior is not optimal. In Jagüey, the signals, after being modulated, do not pass through any band-pass filter, which causes many “spurious” signals to be released into the ether, which represents an obstacle to be solved in order to incorporate a linear that can increase its output power. All of this, the creator assures, was taken into account for the construction of this new design. For example, in the transmission step, in the CO9BIA 455, the microphone signal is mixed, pre-amplified, filtered and re-amplified, until it is delivered to the 455 kHz filter, to finally be mixed with the VFO signal; and as a result of these steps, the sum and subtraction of these mixtures is obtained, which are in the order of 6 and 7 MHz. As a final result, after these signals are injected into the input bandpass filter, only one output is obtained of 7 MHz, whose operating segments are carried out through the use of the improved VFO. Given these characteristics, with which spurious signal outputs are reduced or eliminated, in this new radio it is feasible to add a linear that can raise the power to approximately 7 watts, which would adjust to the power conditions described above.

This experienced “clunker” says that for the development and construction of this transceiver, three fundamental aspects were combined: the first, applying the experiences of having built other radio models, to ensure that the new prototype could be built by any radio amateur. with minimal knowledge of electronics, using recycled components and materials. Secondly, he used and adapted parts of the construction schemes of a radio project called LU3DY, from the Argentine Radio Club “Almirante Brown”; and finally, the adaptation of some parts of the traditional Jagüey, such as the VFO board and circuit. Although, as already explained, the radio works, 

Andy Fernández is immersed in the construction of a small linear amplifier similar to the ARARIHNA project, by a Brazilian radio amateur, as well as making final adjustments to what is already a reality: the conclusion and final adjustments of the new CO9BIA 455 Transceiver, a portable QRP device for the 40 meter Band, developed in these times when we must all stay at home, to protect ourselves from COVID-19. 

By Luis Enrique Estrada Hernández (CO2BK) FRC Information System Coordinator 

Circuit details.  

The VFO Board

Here is the web site of the Federation of Cuban Radio Amateurs that describes Andy's work: 

https://www.frcuba.cu/filiales-grupos/la-habana/radio-transceptor-co9bia-455.html?fbclid=IwAR1DptfJdfAS5PSBbyMMUd6zjHo2EA3SHo7hhD1hUPRdtYVF29mKPD8dgPE

And I learned a very useful Spanish word through this:  "Cacharreo" is a Spanish word that means to tinker with something in an attempt to fix, mend, or improve it. 

Thanks Andy!  And thanks to  Trevor for alerting me to this great project.  

Sunburst and Luminary -- A Poem about Transistors and ICs

 

Sunburst and Luminary by Don Eyles has a lot of the kind of color that helps the reader understand what was going on technologically during the 1960s.  For example, there is this poem about integrated circuits (you don't get to use "poem" and "integrated circuits" in the same sentence very often): 

The transistor's a marvelous invention

Replaced the tube convention

        Found its niche

        To amplify or switch

Whatever the designer's intention. 

But the breakthrough was the IC

Integrated monolithically

        It became pivotal

        As computers went digital 

With increasing complexity. 

Eyles tells us that this poem was written by hardware designer Jayne Partridge, and appears in Eldon Hall's write-up of the Apollo Guidance Computer and the decision to use ICs in it: 

https://www.amazon.com/Journey-Moon-Guidance-Computer-1996-09-03/dp/B01F9FSN7C  

Another Evaluation of the TinySA Ultra (with Teardown) (Video)

More info on using the TinySA Ultra. 

-- Makes me wonder if it makes sense for me NOT to enable Ultra mode now.  I am not going into the UHF range.

-- Waterfall!  Sig Gen! 

-- Kerry got a lot of distortion when listening to FM broadcast signals.  I wonder if this is due to the Tiny SA detector being an AM detector? 

-- Teardown was very cool.  

-- Comparison to the HP spectrum analyser was very illuminating.  Bottom line:  TinySA Ultra gives you a lot of capability for $150.  

 

"Sunburst and Luminary" author Don Eyles was a Ham, a Hacker, and a user of Plywood who Understood Juju

-- As a kid, Eyles took a summertime shop class with W4LRO.  Eyles himself went on to get his ham license -- he was K4ZHF and was active for a while on the 40 meter and 6 meter bands. 

-- He writes of how the Apollo software acquired more "juju as labor and logic were poured into them." Juju. 

-- He describes the electronics lab in the MIT Instrumentation Laboratory: "If you had a private project you could sometimes get some simple milling done for a smile, and you could scrounge the odd resistor or capacitor... On the second floor there was a small "hackers shop" with a drill press, metal shear, a bending brake, and a few hand tools which was open to anyone, including software engineers. That was the first use of the term "hack" in a technical context, that I can recall hearing. I took the term as referring to the sometimes messy process by which perforations of suitable sizes were made in the aluminum boxes, or chassis, that were used for constructing electronic devices."  Indeed.  We hack.   

-- After describing the first integrated circuits, Eyles looks back at high school and notes that he and a friend, "after learning about truth tables, James Chambers and I had experimented with similar devices composed of relays mounted on a piece of plywood."  Plywood.  

  

More to follow on this book. 

Bezos Bucks! New Amazon Link Working Well! Please use it!

 

Click on image for a better view

In the graph above you can see the Amazon Associates stats for SolderSmoke.  While obviously I'm not going to get rich on this, it is nice to see that the new Amazon Associates link is working well.  Note the difference between early September and late September. 

The link is over here >>>>>>>>>>>>>>>>>>>>>>>>>>>>>

You can use it to buy anything on Amazon (not just the item that is advertised).  Just start your search from the Amazon link on the right hand column of this blog, and buy the item within 24 hours of entry into the Amazon system and -- CHA CHING for SolderSmoke!  Thanks.  

The TinySA Ultra Spectrum Analyser (video)

I got mine this week, and I've been playing with it.  When I spoke to Dean KK4DAS, I asked what he thought the first thing I did with it was.  He guessed that I tried out the greatly improved Resolution Bandwidth.  Good guess, but not quite:  I tried out the  "listening" feature on this SA.  You will recall that the plain-vanilla, non-Ultra TinySA required a hardware mod to allow for listening.  Dean had told me that the Ultra came with a headphone jack.  Indeed.  I fired it up and was able to listen to 1220 AM and also to the FM broadcast stations in the area.  With the FM stations, I'm guessing I was using a form of slope detection (IMSAI guy says it detects AM).   I tried to see if I could see/hear stations on the ham bands -- so far, no luck.  I'm not sure why, but I will work on this.  

The IMSAI guy video (above) does a great job in comparing the TinySA Ultra to a "real" spectrum analyser.   I think it compares very well.  

One note on where I got mine:  I ended up getting it from R&L Electronics, the recommended U.S. dealer for the device.  I had tried getting it (cheap) through AliExpress.  This didn't really work out.  The tracking info from AliExpress showed that the box had made it to my local post office, but I never got it.  It may have been that they just didn't take the complete mailing address from PayPal.  In any event, I was able to get a refund from PayPal, so no loss here.  R&L turned out to be a great source. 

Ultra, of course. 

"The Art of Electronics" #8 -- Why Not a Simple Emitter Follower as The AF Output Circuit?

Click on the image for an easier read

Back when Dean KK4DAS and I were trying to find a suitable AF amplifier circuit for our High School Direct Conversion receiver project, we were debating what to use as the final.  One option was the standard NPN-PNP push-push amplifier (like in Figure 2.53 above)  -- an advantage with this one was that it would not require an AF transformer.  But we decided that this circuit would add complexity to a project that we were hoping to keep very simple. 

Another option was a simple common emitter amplifier with a transformer in the collector circuit.  This worked, and was simpler.  We ordered the transformers.  

In the midst of all this, at the local radio club hams asked us why we just didn't put a single emitter follower at the output to handle the impedance transformation to an 8 ohm speaker (sort of as in Figure 2.52 above).  They were convinced this would work.  I was not so convinced and pointed out that we had never seen such a circuit in any of the ham radio literature.  If this could be done, why hadn't the likes of Doug DeMaw and others used this circuit in their many, many rigs?  

This discussion kind of ended there (we opted for the common emitter transformer circuit), but I have thought about it from time to time.  A couple of weeks ago, when I got the second edition of The Art of Electronics, I found the above discussion of the use of this kind of emitter follower circuit.  You can see why this circuit has not been used.  Just to be sure, I built one in LTSpice.  Sure enough, it takes way too much current.   

Thank you, Horowitz and Hill! 

A Look at Old Radios in Australia (video)

I thought you guys would like Peter Parker's latest video.  And in it, Dean KK4DAS might see a clue or two for his Halli restoration/repair project.  

What really struck me was the dial markings on the Australian radios.  They seem to be mostly oriented toward the reception of Australian AM or LW broadcasters -- not many exotic DX locations are marked.  Australia is big! 

Looks like a lot of crystal detectors in glass tubes.  At first I thought they might have been coherers, but I think they are crystal detectors. 

Many variometers visible. 

I saw one Geloso-Milano Communications 8 receiver.  Va bene! 

I also saw one "Tasma 780" Superhet.  Cool name! 

What is up with the "Green Theme"? 

I have some of the headphones they showed. 

Please note in the Comments section anything special that you noted in watching Peter's FB video. 

More on the coil winding machine here: 

https://hackaday.com/2023/08/30/all-mechanical-coil-winder-is-a-scrap-bin-delight/#more-612604

Ciprian YO6DXE Puts the Michigan Mighty Mite on AM! (video)

Excellent work Ciprian!  

Here we have a Michigan Mighty Mite being modified for AM in Romania, with input from Hungary, and inspiration from Melbourne, Australia (Peter Parker VK3YE).  With a very nice shout-out to SolderSmoke.  

I really like Ciprian's emphasis on having fun with the electronics. 

TRGHS! 

F6CRP's FB Homebrew Receiver

I like F6CRP's homebrew receiver.  I like the way he presents it (block by block) and I like the inclusion of the W7ZOI IF board. The JF3HZB VFO is quite cool (even though it is digital),   We have posted about this VFO several times: 

 https://soldersmoke.blogspot.com/2022/05/who-is-jf3hzb-creator-of-that-nice.html

Denis has a very nice station and workbench: https://www.qrz.com/db/F6CRP

He also has some very nice videos on his YouTube channel: 

https://www.youtube.com/@radiorider8249/featured

Thanks Denis! 

Crushing Spurs with Better Bandpass Filters

PA3AKE's passband (on screen) an mine (on NanoVNA) 

While I was away in the Dominican Republic (3-9 August 2023),  I was thinking about spurs.  While there I watched Nick M0NTV's video about mixers.  The video was all great, but I was especially taken by the way he used a spectrum analyser to evaluate the output of various mixers.  This made me think that I should do the same thing with the output of each of my dual-band BITX rigs. 

I was especially worried about the output from my 17-12 rig.  The IF is at 21.4 MHz.  The VFO runs around 3.5 MHz.   So if you add the IF and the carrier oscillator signal you get to 12 meters.  If you subtract them you get to 17 meters.  But you need some good bandpass filtering to sufficiently knock down the unwanted output from the mixer.  And the BP filter should be sufficiently narrow to take out any remnants of the carrier oscillator signal. I had taken the easy way out and had used simple dual-tuned-circuit (DTC) filters.  I started to wonder if these simple BP filters would be enough to knock down the 12 meter signal while on 17 and the 17 meter signal while on 12.  I pulled out my NanoVNA to look at the passbands: 

Here is what the 17 meter DTC filter passband looked like. The cursor is at 29.6 MHz and you can see that near the 12 meter band it is only providing about 21 db of attenuation.  That is not enough. 

And here is what the 12 meter filter looked like. Here the cursor is at 18.150 Mhz and shows about 25 db of attenuation at this frequency.  Again, not enough.  

The results are as you would expect: I could see 24.9 MHz signals in the output when I was on 17 meters, and I could see 18 MHz signals in the output when I was on 12 meters.  The spurs weren't strong, but they were there.  I knew that more robust BP filters would help.  

At first I used the circuits prescribed by Martein PA3AKE.  He used larger toroidal cores, I used smaller T-50-6 (yellow) cores.  The results were very similar.  See the first picture on this blog post. 

The results were really good.  See pictures below.  I was using a TinySA with the signal fed through a 50 ohm 30db attenuator.  I was putting a 1 kHz signal in to the mic input.  

This picture shows the 12 meter output and the now non-existent 17 meter spur.  The cursor is at 18.142 MHz:    

This picture shows the 17 meter signal and the now non-existent 12 meter spur. The cursor is at 24.993 MHz.   We can see some second harmonic signal getting past the LP filter -- I will fix this.  

Then Farhan commented on Martien's filters, noting that they are all in the "LSB" configuration.   You can see from the charts below how they would be really good when you are trying to use the "difference" output from your mixer while knocking down the sum output,  but not vice versa.     So I built new USB filters for 12 meters,  and for 10 meters in my nee 15-10 rig.  I got better results on the two "sum" bands in my rigs (10 and 12 meters)