BITX Build Update #10 -- Darling(ton)

Having concluded that I was significantly short on overall receiver gain, I went in yesterday and changed my AF amp from a 40db direct-coupled circuit to a 100db Darlington pair.  I immediately noticed a big increase in audio output. 

I did a quick receiver alignment using my Arduino/AD9860 sig generator.  First I determined the actual bandpass of the crystal filter: 4.998170 MHz -- 5.000960 MHz.   Using a freq counter, I set the BFO at 5.00126.   I immediately started hearing 17 meter SSB signals from the West Coast.  That's always a nice moment:  first signals through a new receiver.  Kind of like "first light" in a new telescope.  Even with the filter ripple, it sounds great.

I think I'm still significantly short of gain.  Audio is still faint. I notice that in the BITX17A they have added a second transistor (Q17) in the second RX receive amp.  Maybe I should try something similar.   Or should I add some gain in the audio chain?

I'm really enjoying this BITX project. 


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Hackers' Paradise

http://hackaday.com/2013/09/23/guest-rant-ham-radio-hackers-paradise/

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BITX Build Update #9 -- Discretion

With the exception of the PA, all of the stages of my BITX 17 are built.  Over the weekend I put in the DC wiring for the receiver and the inter-stage connections (using the Belden coax with the exposed shield and Teflon di-electric).  It looks nice. 

In my experience, almost all new superhet receivers require a certain amount of debugging and coaxing before they will work.  This one is no exception.  The VFO and the BFO work fine, and all three RF and IF amp stages are also good.  The bandpass filter  that I built passes the desired band and tunes up nicely on the right frequency.  The product detector was acting weird and wasn't balancing out properly, but I got that all sorted.  

I can put an 18.110 MHz signal at the antenna connection and see the signal go through the bandpass filter (with loss), on to the RF amp stage, to the first mixer where it meets the 23 MHz energy from the VFO.  A very messy mixture goes from the mixer to the first IF amp which sends it to the 5 MHz Cohn filter.  The filter works, but it has a lot of ripple, so I need to work on the termination impedances.  Second IF works fine, then the signal goes to the product detector.  AF comes out.  

Here's where the discretion comes in.  Instead of the LM386 chip, I built a 40db two transistor direct coupled AF amplifier. 

So it all works, but the receiver is quite deaf.  I think I just don't have enough gain in the whole system.  I looked at the schematic for the BITX-17 kit.  It very helpfully has total (net) gain figures for the RX.  I can see that my current configuration comes up short.   

Here is what I'm thinking of doing:  I might replace the 40db direct-coupled AF amp with a 100 db Darlington pair.   I really like the discrete Darlington AF amp that KD1JV has in his "all discrete" transceiver:
http://kd1jv.qrpradio.com/ADC/ADC-40.htm  Nice.  3 2N3904's driving a speaker.  I may use that.  
 
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BITX Build Update #8

Big progress on the BITX:  All the bidirectional amp stages are done.   A bag of 20 5 MHz crystals arrived from Mouser today.  I put one in the BFO/Carrier oscillator and it fired right up.  I'll soon be checking frequencies on these crystals, looking for four that are closest in frequency for use in the filter. 

While waiting for the mail I built the audio amplifier for the receiver (lower left corner in the picture above).   Here I need some advice/encouragement:  In an effort to keep this rig "all discrete" I decided to dispense with the LM386, and replace it with an AF amp using individual transistors.  I found a circuit in the 1980 ARRL Handbook that I liked.   It has two direct coupled transistors, one NPN, the other PNP.  I went with a 2N3904 and a 2N3906.   The Handbook said it would yield 40 db gain.  I figured this was a close enough replacement for the 46 db gain of the LM386.  

As usual, I'm not sure of the impedance matching.  I built the first AF stage from the BITX schematic (the stage that precedes the LM386).  It goes to a 10K pot.  The wiper of the pot would normally go into pin 3 of the LM386.   I have the wiper going through a 4.7 uF electrolytic into base of the first transistor.  The Handbook says the circuit has an input impedance of 1000 ohms.  Does my arrangement sound OK? 

Output impedance from this Handbook circuit is also 1000 ohms.  I tried it with some HI-Z headphones that I have, putting AF in from my Maplin AF sig generator.   It sounds OK.   Not a LOT of audio available.  But OK.  I may need one more stage to drive a speaker.     

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Inside a Tek 465

The TEK 465 'scope is as they'd say back in the Bronx "a piece of work."  It has real character.  Perhaps a bit too much.  If you look closely, you'll see transistors in sockets.   I had kind of gotten my head around that, when I noticed that many of the transistors were not in sockets.  They looked like they had been soldered to into the holes in the PC boards.  But NOOOOO!  They are snugly in the holes, but they are NOT soldered in!  You can pull them out, then (if you are lucky!) slide them back in.  There is even quite a bit of "ugly construction." with connections floating in the air.  You can really see the influence of tube era thinking in this instrument.

I think I have narrowed the problem down to the HV supply.  Alan Wolke is lending me a high voltage probe.  I hope to be able to get it fixed this weekend.  I need to get it working because my BITX 17 is nearing completion and I'll need the 'scope for the lineup and de-bugging.

The picture above is from:  http://www.eevblog.com/forum/projects/tektronix-465-repair-and-restoration/   More pictures of the scope are available there.  Also see http://www.spurtikus.de/basteln/repair/tek465/index.html for more scenic views of this instrument.


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Battushig has The Knack (Mongolia and the MOOCs)

"How does a student from a country in which a third of the population is nomadic, living in round white felt tents called gers on the vast steppe, ace an M.I.T. course even though nothing like this is typically taught in Mongolian schools?"

"Most homes in Ulan Bator have Internet connections, and almost everyone, including nomads, has at least one cellphone. Even on the steppe, with only sheep in sight, you can get a signal."

http://www.nytimes.com/2013/09/15/magazine/the-boy-genius-of-ulan-bator.html?pagewanted=all&_r=0

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BITX Build Update # 7

As you can see (above) I've made lots of progress on the BITX-17.  Going counter-clockwise from the lower right, you can see the mic amp, the BFO/carrier oscillator, the first mixer, the first and second bi-directional amplifiers, the diode ring VXO mixer and the VXO.

I plan on building the whole rig (including the power amplifier stages) on this wonderful piece of PC board (thanks to Jim, W8NSA).

I had what I thought was the brilliant idea of using LEDs for  all the T/R switching diodes.  I thought I'd use red for receive and blue for transmit.  I consulted with the BITX yahoo group and cooler heads prevailed.

The design has an LM-386 as the speaker amp.  I'm kind of bothered by this -- I'd prefer an all-discrete component rig, so I am thinking of building an AF amp with discrete transistors instead, perhaps the circuit from Roger Hayward's RX for the Ugly Weekender.  What do you all think about this? 

I'm having a lot of fun building this rig.  If anyone out there is looking for a fall/winter project, build one of these.

I have to order some 5 MHz crystals for the filter and oscillator.  Mouser has them at about 70 cents each.   


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BITX Build Update #6: Rock Steady

So, having given up on the VFO,  this week I built a VXO.  The IF in my W4OP-built Barbones Superhet is at 5 MHz.   That receiver has a VXO running at 23.133-23.168 MHz with two crystals switchable from the front panel.  My plan is to build a 5 MHz filter for the BITX 17.  So I just plucked one of the crystals in the Barebones RX and used it to test and tweak the VXO.  Trying different values for the series inductor, I got wide variations in freq swing.  With .7uH I could only pull the rock 9 kHz (too little).  With 5.6 uH, it was pulling 434 KHz (way too much -- it was acting like a VFO). 3.213 uH was just right:  With my 19-148 pF variable cap (with 53pF in series) I got a swing of 22 kHz, with crystal-oscillator stability.

I know, I know:  I could have done this with a DDS chip, or with an SI570.  But simplicity is a virtue, and the BITX is all about simplicity, right?  Having recently built a DDS RF generator, and now this simple three transistor device, I must say that I like the simple analog circuit better.   But hey, that's just me.  I'll talk about this in the next podcast.

Today I built the mic amp circuit.  You can see it off to the right of the VXO.

One of these days I'm going to have to actually troubleshoot and fix my Tek 465.  But I'm
afraid of that thing!  The voltages in there are HIGH, and so is the circuit complexity.  

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Moon Launch from Virginia (video)

NASA launched a mission to the moon last night.  The rocket went out of Wallops Island, Virginia and was visible from Washington D.C.   I forgot about it, so we missed seeing it ourselves, but this fellow got a nice view from the balcony of his Washington apartment. 


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Burning Man: Probably more Fun than a Hamfest!

Check out this report from the Burning Man event.  Two intrepid nerds head out to the dessert  (with ham radio gear) and appear to have an amazing time. 

http://blog.thelifeofkenneth.com/2013/09/my-first-burn.html


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NASA needs Ham Help! Say HI to Juno (on its way to Jupiter) on 10 meter CW!

This is really interesting:

http://www.jpl.nasa.gov/hijuno/

Thanks to Richard, KK4JDO, for the heads-up.

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BITX Build UPDATE # 5: ADRIFT!

Sometimes it is better to have LESS test gear.  Watching my 5 MHz VFO on the frequency counter was driving me nuts.  If I hadn't had the counter, I might have been satisfied with the VFO.  Ignorance is bliss!   It did settle down a bit.  But then it would wander off.  I hated watching it drift around.  I have been spoiled by Doug DeMaw's VXOs.   So, I went for the VXO.   It will run around 23 MHz.  I'll build the filter at 5 MHz.  That's definitely the plan.  For now. 

I intend to go back and build a real VFO someday.    There seems to be a lot of magic and lore involved.  Bees Wax!  Hot Glue!  Nail Polish!  Q-Dope!  Air-wound coils!   

TEK 465 is still broke.   I think it is the high voltage supply or (less likely) the CRT. 

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Fixing TEK 465 Oscilloscopes

I'm having TeKnical difficulties.  My beloved Tek 465 'scope quit on me.  I'd been running it continuously for a day or so, watching the output on my BITX VFO.  Then, suddenly, it blew the 1.5 amp fuse.  I replaced the  fuse, but afterwards the trace was gone and the "Low Line" light was acting weird -- on when it should have been off, then flickering.   I consulted with Alan Wolke.  He happened to be out at TEX HQ in Beaverton, Ore.  He provided some good suggestions, but before I had a chance to try them, the trace came back.  I attributed this good luck to the Radio Gods being pleased with Alan's presence in Beaverton... but my reprieve was short-lived.  Trace went out again today. 

I found this video of the legendary Jim Williams (RIP) fixing a Tek 465.  (Check out Jim's junk box!)
I'll start by checking the Tantalums.  I may get lucky. 

If anyone has any other suggestions, send them this way.  The Tek scope may be old, but it is a COMPLICATED piece of gear... 

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BITX Build Updat #4 VFO Stability

I wasn't quite satisfied with the long-term stability of my VFO.  It seemed like it was drifting about 70 Hz per hour, even after the initial warm-up period.   So, all out of Q-Dope, I went to the local pharmacy in search of clear nail-polish.  (My family was making fun of me.)   I got home and applied several coats of "Sally Hansen No Chip Top Coat Vernis de Protection."   I let it dry a bit, then started watching the frequency counter.  At first results were disappointing.   Drift continued.  Now it seemed to be drifting up!   I cracked the books.  EMRFD has a good chapter on temperature compensation of VFOs, but the process seemed painful.    I wasn't looking forward to it.

I left the VFO on when I went to work, noting the freq as I departed.   Ten hours later I returned, and was delighted to find the frequency almost exactly where I left it!  It may have taken a while for the nail polish to completely dry.    I'll leave it running again today to see how it does.


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BITX Build Update #3

This weekend I built the VFO.   I used Doug DeMaw's FET Hartley, from his "QRP Notebook," page 50.

Getting a VFO into the desired frequency range always seems to involve a bit of "cut and try."  Not only do you have to get in the correct freq range, but you also have to try to get the tuning range of your variable cap (or coil) to match the band you want to cover.  In my case, thinking of 20 meter SSB, I need about 200 kHz of tuning range.  And sticking with my 9 MHz (for 20) and 13 MHz (for 17) plug-in filter plan, I need the VFO to be in the 5 MHz range. 

That main tuning cap you see above has more capacitance than necessary (19-148 pf).  One option would be to pluck out some of the rotor plates (been there, done that).  But that seems a bit barbaric, so instead I just reduced the overall capacitance by putting a smaller fixed capacitor (about 20 pf)  in series.      

Yesterday I had it percolating nicely.  Waveform looks beautiful on the Tek 465.   But it was drifting too much.  I had a cheap plastic trimmer cap in there.  This morning I replaced it with a more substantial ceramic trimmer.  This seems to have improved stability quite a bit.

I need to put some stabilizing substance on that toroid.   DeMaw prescribes Q-dope.  I'm all out.  What is the field expedient substitute?   Was it clear nail polish? 

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BITX Build: Update #2

Not much progress to report.  But I have been thinking about the filter frequency.  Here is my latest idea: 

Maybe I'll build the VFO in the 5 MHz range.  This would allow me to use the 9 MHz Yaesu filter (and associated crystals) that Steve Smith sent to me (see above).  With this I could be on 75 and 20 meters. 

I could build another filter at 13 MHz and, using the same VFO in the 5 Mhz range, get on our beloved 17 meter band.  I kind of like the idea of plug-in filters. 

You can see my ideas for the board layout.  I'm thinking of a Doug DeMaw Universal Hartley VFO inside the box shown above.   The tuning cap you see is out of an old Heath QF-1 Q Multiplier.  It has a very nice reduction drive built into the tuning shaft.  It measures 19 to 148 pf.     
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Building the BITX! Update #1

There it is guys:  A blank canvas of copper-clad board.  A clean slate ready to be filled with the components for a BITX transceiver.  As you can see, I am fighting my "build first, design/plan later" tendencies.  No real design work for me on this one (thanks Farhan!) but I am trying to plan where everything will go on the board.  (Thanks to Jim, W8NSA for the board.)   I'm going to build it Manhattan style (perhaps with an ugly dead bug or two).  I'm starting with a big board because I always seemed to end up with a shortage of space.  It looks like I can easily get all of the circuit (minus the PA) on this board.  I'll build the PA on a separate piece of copper-clad.

I'd like to build it for 17 meters, but if I stick with the 10 MHz filter that means I have to build a VFO at around 8.1 MHz.  That's not impossible, but in my experience it is easier to build simple, stable VFOs at lower frequencies. 

 

I notice that there are a lot of cheap crystals available at higher frequencies.  So, instead of keeping the filter at 10 MHz and trying to get the VFO stable at 8.1, what do you guys think about keeping the VFO in the 4 MHz range and  building the filter with crystals in the 14 MHz range? 

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Pete Juliano's KWM-4

 

Pete Juliano, N6QW, has authorized me to provide SS Daily News readers with a look at his magnificent KWM-4, a solid state version of the legendary Collins transceiver.  This project is in the Summer edition of QRP Quarterly.  Amazing job Pete! 

 

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A COOL Norwegian "S" Meter (video)

Immaterials: Light painting WiFi from Timo on Vimeo.

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Peter Parker's Video on SP5AHT's Phasing Receiver

There are so many very cool aspects of this project:  I love the phasing receiver arrangement -- it uses the same basic concept that allows my old HT-37 to generate SSB (I struggled to understand it as a kid, and finally succeeded!).   The use of the mobile phone as an audio spectrum analyzer is wonderful.  The N3ZI signal generator looks a lot like the DDS project I was working on (I should have bought N3ZI's! ).  I've been seeing these square pattern PC boards in many projects lately -- I may try this in my next project.  And of course, it is very cool to see a circuit designed in Poland being put to use by a fellow radio amateur in Australia -- a good example of the International Brotherhood of Electronic Wizards!

Here is the page for the SP5AHT rig that inspired Peter:

http://www.sp-qrp.pl/modules.php?name=Content&pa=showpage&pid=54

It is in Polish, but Google should translate it, and, in any case, we all speak Schematic!

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