BITX Build Update #15 -- Necessary Mic Amp Mod

I was having trouble with the mic amp on my new Manhattan Discrete BITX 17.  I had it wired as per the schematic, but it just wasn't working right -- I was getting very little DSB out of the balanced nodulator, but everything worked fine if I 1) unbalanced the bal mod or 2) injected audio (from a sig generator) directly into the audio in port of the balancec modulator.  Clearly something was wrong in the mic amp circuit.  I noticed the collector voltage seemed quite low.

Some quick Googling revealed that others had struggled a bit with this problem also.  Nicolae's note was especially illuminating and useful: 
http://users.tpg.com.au/nfieraru/Electronics/BitX20_Mic_Preamp.htm

As Nicolae noted, the 10K value may have worked with lower gain transistors.  We must remember that BITX was (very admirably) designed for minimal cost and maximum use of scrounged parts.

I swapped the 10K resistor for a 39K and all is right with the world.  The band is not yet open here, but I bravely called a European aeronautical mobile station, hoping that he would be my first QSO on this rig.  I think he heard me, but no QSO (yet!).

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BITX Build Update #14 -- STABILITY!

Good news on the BITX.  I think I have solved the low frequency oscillation problem in the power amplifier.  I tried running the PA off a separate power supply -- the oscillations disappeared leading me to conclude that the feedback was taking place via the power supply lines.   I put a 22 uF cap to ground from the top of the PA's RF choke.  With this cap in place I can dispense with the separate supply kludge. The amp is stable.

This morning I also resolved a different stability problem.  I'm using a relay for T/R switching.  One set of contacts controls the DC power, the other set was supposed to disconnect the receiver input from the antenna/lowpass filter when on receive.  But I had positioned the relay too far from the LP filter and RX input, so I ended up with this long piece of coax that was hanging off the PA output terminal and carrying lots of RF on transmit.  Yes, this led to oscillations.  I thought about repositioning the relay, but I think part of the problem is that even if optimally positioned, the long unshielded contact levers inside this relay would be radiating a lot of RF and causing stability problems.   So instead I put a small reed relay very close to the PA out and RX in connections.  It carries the signal to the receiver when in R mode, and disconnects the RX when in T mode, without any long coax lines or unshielded relay levers.   It works.

I still have a few things to do:
-- I need to build a proper speaker amplifier so that I can dispense with the amplified computer speakers KLUDGE.
-- I need to get a 3 pole double throw switch and some crystal sockets so that I can switch crystals from the front panel.  Three rocks will cover all of 17 meter phone.
-- I have to build a case for this rig.  I'm thinking wood, with tin or aluminum sheeting glued to the inside (for shielding).  


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From Bits to Atoms -- Software vs. Hardware

I thought this was pretty good.  On Hack-A-Day.

http://hackaday.com/2013/10/18/guest-rant-from-bits-to-atoms/#more-105097

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Feynman at G-QRP Convention!

What do Richard Feynman and the G-QRP club have in common?

The answer is an annual pilgrimage to Ripponden in Yorkshire, England. The G-QRP club have their convention in a local school and Feynman went there every summer to visit his wife's family. Here's an interview with Feynman filmed at the Old Bridge Inn which is about a mile from where the G-QRP club gather:

http://www.openculture.com/2012/11/richard_feynman_talks_physics_in_take_the_world_from_another_point_of_view.html

Is this enough to make Ripponden the Knack Centre of England?

Regards,  Alan G0RWB

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Yikes! Screeching Transistors Throwing off Blue Light!

I found a quote from Doug DeMaw that I wanted to share.  This one seems appropriate as I struggle with unwanted oscillation (and as Halloween approaches). From the January 1986 QST:

"Self -oscillation occurs not only in the low-frequency and high-frequency spectrum, but it often takes place at audio frequencies!  I have actually heard the transistors "screeching" when strong audio oscillations were taking place in a homebrew transistor power amplifier. On one occasion I could see a bluish glow coming from within the transistors (visible through the ceramic heads of the devices) during a period of instability.  Needless to say, the transistors self-destructed."

I've been having some instability problems with my BITX 17 IRF510 final.  But I think I have the problem identified.  I was getting low frequency oscillations (around 100 KHz).  Tonight  tried putting the IRF510 on a separate power supply.  The oscillations ceased and I am getting a nice clean 3 watts out.   So I'll go back and beef up the decoupling on the power supply lines.


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20% off on SolderSmoke Book

Just use the coupon code PUMPKINS at checkout:

http://www.lulu.com/spotlight/soldersmoke

(You can also use this coupon to buy Grayson's "Hollow-State Design" :
http://www.lulu.com/shop/grayson-evans/hollow-state-design/paperback/product-20987562.html

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SolderSmoke Podcast #155 BITX17, TEK465

Podcast # 155 is available for download (or you can listen by clicking on the button above):
http://soldersmoke.com/soldersmoke155.mp3

October 14, 2013  (Delayed Back-to-School edition)

Topics covered:

-- BITX Beauty -- Building a BITX17

-- VFO woes, and the wisdom of Doug DeMaw's love for crystals

-- Why does a 55 year old man NEED clear nail polish varnish?

-- How the VFO KILLED the Tek 465

-- Trying to fix the Tek 465 (not for the faint of heart)
 
-- New products:  Justin Beiber Duct Tape, bifocal safety glasses,
magnetic reading glasses, Ward Silver's new book

-- Backwards British Bike Brakes

-- MAILBAG 

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"Robert Ford -- A British Radio Operator in Free Tibet..."

http://www.economist.com/news/obituary/21587201-robert-ford-british-radio-operator-free-tibet-died-september-20th-aged-90-robert-ford

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Nice Slide Show on BITX

https://sites.google.com/site/arvidevans/BITX_QRPacificon..pdf?attredirects=0


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BITX BUILD UPDATE #13 -- VIDEO TOUR

I hope to get the next podcast out within a week or so.  The BITX has been keeping me busy!


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BITX BUILD UPDATE #12 -- Relay and PA, BITX 17/10?

As you can see, the board is starting to fill up.  I really like it.  It seems (to me) like the canvas of a painting approaching completion. This morning I put in the T/R relay.  That space in the upper left is reserved for the driver and the IRF510 Power Amplifier.

I got some help from the Chief Designer this week.  As noted in an earlier post, Farhan  advised me to check the mixers.  I knew that I had them wired correctly and that they were in fact mixing, but when Farhan advises you to check something, YOU CHECK IT.  Farhan was right (see earlier post).

It really sounds great.  I know that the filter still has significant ripple in it, but the receiver sounds so good I'm reluctant to mess with it.  Should I de-ripple it?

On the train yesterday I was thinking about this rig, and it occurred to me that my 23.1 MHz VXO/ 5  MHz IF arrangement means that this 17 meter transceiver could also generate signals on the 10 meter band.  Of course, I'd have to build a second bandpass filter, but the radio gods guided my hand and caused me to leave space on the board for just such a filter (see above).  I realize the IRF510 PA wouldn't be too good up at those frequencies,  but when the radio gods speak, you have to listen.   

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Tek Troubleshooting Triumph

Thanks to the manual provided by Jim and the HV test gear provided by Alan Wolke, I was able to finish the troubleshoot on the broken Tek 465.   It is the HV multiplier, U1432.   The cathode voltage is supposed to be -2450.   It initially tested at -1000.    The manual calls for a jumper to be removed to take the HV Multiplier out of the circuit.  As soon as I did that, the cathode voltage went to -2500.   Bingo.

 

Now I have to get a replacement for U1432.    Any carcasses laying around?

 

Thanks guys. 

 

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BITX Build Update #11 -- Peakin' and Tweakin'

Oh how I love the sound of a newly built receiver!  I'm sitting here listening to G0MJS on 17 meters.  Lots of other stations from across the pond coming in very nicely. 

Earlier in the week I had some sensitivity problems.  I could hear the noise floor, but just barely.  And the receiver just seemed to have trouble inhaling.  So I started poking around.  It seemed that each poke improved things a bit.  I had used Farhan's original schematic (mostly).   Later versions put an additional transistor in the IF amp.  So I went ahead and added that mod.  That helped a bit.  Then I noticed that BFO energy was getting into the AF amp.   So I put a .1uF cap to ground at the input to the AF preamp.  That took care of the RF and did no damage to the AF.  

But the rig still seemed a bit hard of hearing.  This morning Farhan advised me to take a look at the mixers.  I used some junk-box diodes that I didn't know too much about...  I measured the forward resistance and found it to be quite a bit higher than the usual 1N914s.  So I switched all 6 mixer diodes.  That helped noticeably.

I also checked the input bandpass filter.   It seems OK.   In the process I learned to use the EMRFD filter programs (thanks Wes) and the ELSIE filter program (also very nice).

Anyway, the rig sounds great now.   On to the transmitter.  

Farhan advises building the power amplifier on a separate copper clad board.  But I have room for it on my main board.  Should I live dangerously?   


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Occam's Microcontroller

Paul, M0XPD, has what I'm sure is a wonderful article on a simple microcontroller-based rig in the current issue of our beloved SPRAT.   I look forward to seeing it.  And I really like the logo with William of Occam (of razor fame). I have encouraged Paul to take a break from the microcontrollers and build something discrete and analog... like a BITX!  

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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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