Improving the Product Detector in the Lafayette HA-600A

Diode product detector on the left, BFO amp in the right

As noted in an earlier blog post, I didn't like the sound of SSB and CW when using the product detector in my Lafayette HA-600A.  It just did not sound right.  The receiver sounded fine on AM with the diode detector.  But when I switched in the product detector, it sounded bad.  The BFO was fine.  The problem was there even when I used an external BFO.  And SSB sounded great when I just coupled some BFO energy into the IF chain and used the diode detector to listen to SSB.  My suspicions were focusing on the very simple BJT product detector. 

Steve N8NM built the HA-600A product detector both in LTSpice and in the real world.  It worked fine in both versions.  Steve even put the product detector into his S-38 receiver -- he reported it worked well there.  

I too built the thing in LTSpice.  Then I went and rebuilt the circuit on a piece of PC board.  I connected the new circuit to the HA-600A, using my external FeelTech sig generator as the BFO.  IT STILL SOUNDED BAD ON SSB.  

At this point I started Googling through the literature.  I found a promising article by Robert Sherwood in December 1977 issue of Ham Radio magazine entitled "Present Day Receivers -- Problems and Cures." Sherwood wrote: 

"Another area that could use additional work is the product detector.  As the name implies, its output should be the product of the two input signals. If BFO injection is removed, output should go to zero. If this is not the case, as in the Heath HW series, envelope detection is also occurring, which causes audio distortion." 

I checked my circuit.  When I removed the BFO signal from the product detector, envelope detection continued.  In fact, with the receiver in SSB mode, and with the BFO disconnected, I could listen to the music of WRMI shortwave.  It seemed that Sherwood was explaining well the problem I was having: Simultaneous envelope and product detection was making SSB sound very bad in my receiver.   What I was hearing just seemed to SOUND like what you'd get with a mixture of product and envelope detection: "scratchy" sounding SSB.  This also seemed to explain why SSB would sound fine when using the diode detector with loosely coupled BFO energy -- in that case it would be envelope detection only, with no ugly mixture of both kinds of detection.

So I built a better detector.  I had had great luck with the two diode one trifilar transformer singly balanced design used by both Doug DeMaw and Ashhar Farhan. I built the circuit using one of the trifilar toroids given to me by Farhan, and connected it in place of the original BJT product detector.  With the FeelTech Sig Gen as BFO, I got good results -- most of the signal disappeared with I disconnected the BFO.  Looking at the circuit, I realized that I was balancing out not the IF signal but instead the BFO signal.  To minimize envelope detection I needed to put the IF signal on the balanced input of the product detector (to L1 in the diagram above).  When I did this, envelope detection seemed to disappear completely and the receiver went silent when I disconnected the BFO.  

Finally, I needed to find a way to use the BFO in the HA-600A with the new product detector.  Obviously I needed more BFO signal -- I needed about 7 dbm, enough to turn on the diodes.  I converted the outboard product detector board into a simple amplifier and put it between the HA-600A BFO and the BFO input port of the new product detector.   This works fine. 

A few issues remain: 

1) The output from the HA-600A BFO through the above BFO amp (and across the 50 ohm resistor) is NOT a pretty 455 kc sine wave.  But the peaks of the distorted wave appear to be enough to turn on the diodes, and when I look at the voltages across each diode (on my two channel 'scope) I see mirror images -- one is on when the other is off.   Is this good enough? 

2) Moving the BFO input from L1 to the junction of the two 50 ohm resistors (that is actually a 100 ohm pot) has big implications for how this mixer works.  With the BFO energy going through the toroid, BOTH diodes are being alternately turned on and turned off.  But both are on, and then BOTH are off.  With the BFO energy going in through the other side, one diode turns on when the other is off.  I think the mixing result is the same, with AF coming out of the output port, but the way the mixer works in this configuration is very different. Does this sound right? 

Peter Sripol's Electric Ultralight (and his workshop)

It has been a while since we looked in on Peter Sripol.  Thanks to Hack-A-Day we got to see this really nice video on his latest project -- an electric-powered ultralight.

I thought the foam wings held together by Gorilla glue were cool.  The electric motor was very nice.  But I guess the thing I liked the most about this video was the window it gave us into Peter's workshop.  That is obviously a REAL workshop where things are really built.   I like the white board -- I need one of those. The heat gun also seems very useful. 

Three cheers for Peter's very supportive dad.  it must be tough for him to watch his son take off in those foam contraptions, sometimes with water bottles taped to the tail!    

Our Editorial on the U.S. Election

In our last podcast we took a few minutes to share with our listeners our views on the U.S. election and who we think they should vote for.  Here is the text of what we said.   We stand by every word.

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Just days before a very important U.S. election we feel obligated to express our opinion and to let our listeners in the U.S. know who we think they should support.

Some of you will think this is inappropriate -- we disagree.  Several of the long-running and more recent themes of SolderSmoke are wrapped up in this election:

SolderSmoke is all about global community, the International Brotherhood of Electronic Wizards.  Trump just rejects the idea of global community. He is all about building walls, imposing travel bans, rejecting refugees, circling the wagons, and blaming our problems on foreigners.  This is one of the reasons we oppose him.

SolderSmoke is all about Science and Technology. Trump is anti-science.  He is a climate change denier. That's another reason we oppose him.

Speaking of science, since the onset of the pandemic we have been urging SolderSmoke listeners to protect themselves, their families, and their communities by following the advice of doctors and scientists.  We urge them to socially distance, and to wear masks. We even invented an acronym in support of this -- SITS – “Stay In The Shack.”  Incredibly, Trump has been pushing in the opposite direction:  He ridicules the use of masks.  He calls our leading doctors "idiots."  He stages super-spreader events at his rallies and at the White House. Look, more than 200,000 Americans have been killed by this thing.  I know five people who have buried close relatives.  Yet Trump STILL treats this virus as some sort of political hoax.  This is one of the many reasons we oppose him.

We are both military veterans.  We have both been offended by the way Trump -- who is a draft dodger himself -- has disparaged those who have gone into harm's way for the United States.  We remember what he said about John McCain.  His scorn for veterans and service members is another reason we oppose him. 

There are many other reasons to oppose Trump, but those are the ones we feel are most relevant to SolderSmoke. 

Election day is Tuesday.  PLEASE, for the good of the country and the world, get out there and vote against Trump. Wear your mask and stay safe as you do so, but get out and vote.  Especially if you are living in Florida, North Carolina, Georgia, Pennsylvania, Wisconsin, Michigan, Arizona, Texas, or Omaha, Nebraska, please help vote Trump out of office and please urge your friends and relatives to do the same.

SolderSmoke Podcast #226 The U.S. Election, Solar Cycle, uSDX, Hermes, HP8640B, SGC 600 Sig Gen, HA-600A, Mailbag

SolderSmoke Podcast # 226  

http://soldersmoke.com/soldersmoke226.mp3

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About the U.S. election

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Mars:  Setting early, will have to shift to evening observation.  Weather has been poor. 

Sunspot Cycle 25 is underway -- SFI 78, SN 32  

The Gliessberg cycle

Pete's Bench:  #49,  #50,  uSDX,  Hermes Lite

Bill's Bench: HP8640B,  Global Specialties Corp 6000 counter, Lafayette HA600A.

MAILBAG:

Peter VK2EMU Sent me copy of 1947 Handbook.  Thanks Peter

Brad W1BCC Spotted 10 S-38s for 80 bucks on Craig’s list.  What’s going on here? 

Dale K9NN sent both Pete and I care packages with very cool part, including DG Mosfets

Stuart ZL2TW sent me Les Moxon’s Antenna Book.  TRGHS. Moxon will be back! 

Alvin N5VZH got his receive converter with a little Tribal Knowledge from SS. 

GM4OOU The Bitsy DSB rig from Scotland

Peter VK3YE DSBto DC incompatibility SOLVED

Paul VK3HN's Digital SWR and Power Meter and Low band AM TX VFO/Controller FB Videos. 

VK2BLQ alerts us to article about Jac Holzman of Elektra Records. 

AA0ZZ great message on assembler language and writing software the hard way. 

1962 Ad Describes Origins of the Drake 2-B

 

VK3YE: Solving the Direct Conversion RX -- Double Sideband TX Incompatibility Problem

Peter:  

You have long been one of the leading gurus on DSB.  I remember absorbing all the info I could from your website when I was getting started in DSB back in 2001.   

It's great that you found  the article about DSB with inverted audio.   It would be very cool to build a transmitter with the inverted audio, then confirm that it could be received with a direct conversion receiver without distortion.  

The incompatibility of DSB TXs and DC RXs seems like a very cruel trick of nature.  There are only a few people in the world who think about this, and most of them are in the comments section of your YouTube video!  An elite group indeed.  

Back in 2015 your review of a DSB rig got me thinking about this incompatibility: https://soldersmoke.blogspot.com/2015/07/peter-parker-reviews-dsb-kit-and.html   

It is easy to see how a slight frequency difference between TX VFO and RX VFO would cause a lot of distortion, but similar distortion would be caused by a phase difference between the two VFOs.   AM SW Broadcast receivers try to minimize the effects of fading by using an internal oscillator to replace the wavering carrier -- but they have to have it exactly on frequency and locked in phase with the distant station's carrier. I have a little Sony portable that has this "synchronous detection" circuitry.   It is a complicated task and I don't think you could do it with the highly suppressed carriers of our rigs.  Inverted sidebands to the rescue!     

Thanks for the great video and all the tribal knowledge.  

73  Bill N2CQR

Kludges in SPAAAAACE!

On the Cover of The Rolling Stone (Almost) -- Jac Holzman, Elektra Records, and Ham Radio

 

Thanks to Stephen VK2BLQ for alerting us to this.  That is Jac Holzman of Elektra Records fame, pictured in a recent article in Rolling Stone: 

https://www.rollingstone.com/pro/features/elektra-records-the-doors-jac-holzman-1074071/

The Rolling Stone caption says he is in his "home studio," but we recognize it clearly as a ham shack. 

Here is another article about Jac: 

https://www.nytimes.com/2013/05/07/business/media/miss-the-music-of-the-doors-theres-an-ipad-app-for-that.html?ref=todayspaper&fbclid=IwAR38NEVLwrIA_2x4TQC6w6v68iE1Z51z9ZU_ty3njuSPRnWN7FESyU1H9bw

ARRL reports that his callsign was K2VEH. 

Hey, Pete plays guitar.  So does Farhan.  Should we have our people call Jac's people?  Maybe do lunch? 

Listening in on the Deep Space Network

 

Not long ago we took the DISH satellite antenna off our roof.   For a while I resisted pleas to put the big thing on the curb for pickup.  I fantasized about using it for radio astronomy.  In the end, I threw it away.  I do have VHF/UHF aspirations, but being able to use that dish just seemed to be something in my distant future (if ever!).  

But check out what David N2QG is doing with his dishes:  He is listening to very distant spacecraft normally picked up only by NASA's Deep space network.  Very cool.   Truly inspirational.  

Here are the links: 

http://www.prutchi.com/2020/10/15/recap-of-x-band-dsn-activities-and-plans-for-the-future/

http://www.prutchi.com/wp-content/uploads/2020/10/DSN_Lessons-_Learned_N2QG.pdf

Too Simple? Deficiency of the Lafayette HA-600A Product Detector?

 

I've been having a lot of fun with the Lafayette HA-600A receiver that I picked up earlier this month.  Adding to the mirth, I noticed that on SSB, the signals sound a bit scratchy, a bit distorted, not-quite-right. (I'm not being facetious;  this is an interesting problem and it might give me a chance to actually improve a piece of gear that I  -- as a teenager -- had been afraid to work on.) 

Before digging into the circuitry, I engaged in some front panel troubleshooting:  I switched to AM and tuned in a strong local AM broadcast signal.  It sounded great -- it had no sign of the distortion I was hearing on SSB.   This was an important hint -- the only difference between the circuitry used on AM and the circuitry used on SSB is the detector and the BFO.  In the AM mode a simple diode detector is used.  In SSB a product detector and BFO is used.  The BFO sounded fine and looked good on the scope. This caused me to focus on the product detector as the culprit. 

Check out the schematic above.  Tr-5 is the product detector.  It is really, really simple.  (See Einstein quote below.)  It is a single-transistor mixer with BFO energy going into the base and IF energy going into the emitter.  Output is taken from the collector and sent to the audio amplifiers. (A complete schematic for the receiver can be seen here: https://nvhrbiblio.nl/schema/Lafayette_HA600A.pdf )

I had never before seen a product detector like this.  One such detector is described in Experimental Methods for RF Design (page 5.3) but the authors devoted just one paragraph to the circuity, noting that, "We have not performed careful measurement on this mixer."  The lack of enthusiasm is palpable, and probably justified.  

A Google search shows there is not a lot of literature on single BJT product detectors.  There is a good 1968 article in Ham Radio Magazine:   http://marc.retronik.fr/AmateurRadio/SSB/Single-Sideband_Detectors_%5BHAM-Radio_1968_8p%5D.pdf      It describes a somewhat different circuit used in the Gonset Sidewinder.  The author notes that this circuit has "not been popular." 

To test my suspicion that the product detector is the problem,  I set up a little experiment.  I loosely coupled the output of a signal generator to the IF circuitry of the HA-600A.  I put the sign gen exactly on the frequency of the BFO.  Then, I switched the receiver to AM, turning off the BFO and putting the AM diode detector to work.  I was able to tune in the SSB signals without the kind of distortion I had heard when using the product detector.   

So what do you folks think?    Is the product detector the culprit?  Or could the problem be in the AGC?  Should I start plotting a change in the detector circuitry?  Might a diode ring work better?  

Quino, The Creator of Mafalda ("BASTA!") RIP

Putting "Basta" in the SolderSmoke search box yields many blog posts.  The cry of ENOUGH! from six year-old Mafalda has been part of the podcast for many years and is now part of the SolderSmoke lexicon.   

https://soldersmoke.blogspot.com/search?q=Basta

We don't do a lot of obits on this blog (we try to keep it all upbeat) but the passing of Mafalda's creator Quino is news that many of you may have missed, and that I think merits mention here.  This link has a nice 3 minute report from NPR: 

https://www.npr.org/2020/10/04/920038792/mafalda-cartoonist-quino-dies-at-88

Adios Quino. Gracias por todo. 

Ryan Flowers' Admirable Approach to the BITX40 Module

 

We must remember that Farhan designed the BITX transceivers -- and especially the BITX40 Module -- in the hope that these rigs would encourage hams to tinker, to modify, to change and to repair.   When I read Ryan Flowers' blog post, I thought that Farhan's mission has been accomplished.  

https://miscdotgeek.com/bitx40-rebuild-part-1-mistakes-planning-and-teardown/

I was also struck by how nice it is that Ryan has a sentimental attachment to this BITX40 module because it was a gift from his wife.  That's the kind of thing that gives a piece of electronic circuity soul. 

Above we see Ryan's module with many of the parts removed in the sections that he feels he messed up.  This is obviously a good approach, but it reminded me of the nightmare I've had (and I am not the only one) where, in frustration, I take ALL the parts off a recalcitrant board.  

Stick with it Ryan!  You are on the right track.  And it sounds to me like you WILL soon be homebrewing from scratch your own SSB transceiver. 

A while back we built a blog with many nice mods for the BITX40 Module: 

http://bitxhacks.blogspot.com/