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? 

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. 

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?  

HA-600A Gets a New Coat of Paint -- After Almost 50 years!

 

The HA-600A that I picked up last week was looking kind of sorry.  There was a lot of rust on the cabinet.  Below is the before picture. 

I'm not really into cabinetry or radio aesthetics, but it is amazing what a 6 dollar can of spray paint can do. Formula 409 also helps. I moved the light bulbs forward a bit to get more light on that Juliano Blue dial. 

I am really enjoying this radio.  It has brought back many memories.  I think I got one for Christmas in 1972.  I was 14.  I got my Novice ticket on April 27, 1973 and made my first contact on July 19, 1973.  For that first contact I was using an HA-600A and a Heathkit DX-40.  Later I used the Lafayette with a Heathkit DX-100.   The HA-600A was replaced by the far superior Drake 2B on April 11, 1974.  So I used this receiver for more than two years.  

Looking around inside this receiver (and following up with Google) I learned some more about it: 

-- It was made in Japan. 

-- The manual says it has a "mechanical filter" but in fact it has a Toyo ceramic filter.  This may have been just an honest mistake by the folks who wrote the manual -- maybe they didn't understand the difference between the two types of filters. 

-- There is a big difference between the HA-600 and the HA-600A, mostly in the front end circuitry.  The HA-600 has fewer amplifier circuits at the front end.  This probably explains why the HA-600 I picked up did not seem to live up to my memories of my teen-year HA-600A.  

The fellow who gave it to me tells me that it had belonged to the short-wave listener father of a friend of his. 

I know we have a lot of tube-type receivers that are much older than this thing, but I still think it is pretty amazing that this is a receiver that I used almost half a century ago.  And it is still as good as new.

TRGHS -- My First SW Receiver Offered FREE for Pickup -- The Lafayette HA-600A (Looking for Globe VFO Deluxe)

 

So on September 27,2020,  I was sitting quietly in my shack, perusing the postings on various radio-related Facebook groups, when suddenly I saw it:  my very first shortwave receiver, the magic box that had put me firmly on the path to amateur radio, the Lafayette HA-600A.  Joe, the owner,  was offering it FREE to anyone willing to pick it up at his home in Virginia's Shenandoah Valley.  Holy Cow!  I was scheduled to drive through that very valley later that week.  A message was sent and the deal was done.  CLEARLY THE RADIO GODS HAD SPOKEN (TRGHS).  

Sure, the cabinet looked a bit rough, but I had high hopes for this receiver.  A while back I had -- in a similar fit of nostalgia -- bought what had been advertised as a Lafayette HA-600A on e-bay.  But it turned out to be a Lafayette HA-600 (no A).  I immediately noticed a big difference in performance.  That was NOT the radio that I remembered, not the receiver that had carried HCJB and Radio Moscow to me. Joe was clearly offering the A model.  

A few days later I was in Joe's front yard for the hand-off, and a few days after that the HA-600A was on my bench.  

I quickly realized how little I knew about this receiver.  Mine was a Christmas gift, probably in 1973. (A few days ago I talked to my mom and thanked her for driving all the way to New Jersey to get this receiver for me.)   I was so taken with this thing that I feared doing something -- anything -- that might mess it up.  I lived in fear, for example, that some sort of freak mid-winter lightning bolt might destroy it.  I covered it with a towel each night lest dust encumber its "jeweled movements." Obviously I was just not inclined to crack open the case and have a look around. So I didn't, and the receiver remained pretty much an appliance for all the time I owned it.  (I eventually sold it on consignment at Electronics 59 in Spring Valley, New York.  The proceeds probably went toward the purchase of a much better Drake 2-B receiver.) 

I downloaded the manual and familiarized myself with the receiver:  It is a single conversion superhet with a 455 kc IF.  It is all solid state with no ICs -- all discrete transistors and diodes. The manual claims it has a mechanical filter.  I kind of hoped for something like a Kokusai mechanical filter,  but it turns out that the filter was really ceramic, not mechanical.  Bummer.  

The thing fired up right away and was inhaling on the correct frequencies.  I noticed immediately that (as Joe had indicated) some of the controls were scratchy.  I also noticed that the ganged band selection switch was intermittent and required some jiggling to get it to work properly.  A few squirts of Deoxit D5 took care of all that. There seemed to be a bit of dirt in the main tuning capacitor, but I think I managed to blow that out using a can of Dust-off.    I was quickly listening to the SW broadcast stations, and to radio amateurs on 75 and 40 meters. 

Out of curiosity, I compared schematics of the HA-600 and the HA-600A.  There was indeed a big difference -- the front end of the 600 lacks a lot of the RFA amplification circuitry of the A model.  That's probably why is seemed so deaf and so different from what I remembered of the A model. 

There is really not a lot to do on this receiver.  I'll get some paint to fix up the top cover.  I may check the alignment.   But this single conversion receiver is so simple that alignment would be quite easy.  In many ways this receiver seems like a solid state analog to the Hammarlund HQ-100, but without the clock, and without the regeneration circuitry.   The dial lacks the exotic station locations (Java!) that make many of the older receivers so much fun.  I guess this is an indication that this receiver was aimed more at amateur radio operators than at shortwave listeners ( I was both).  I wonder how the ham band-only HA-800 compares to the HA-600A? 

I could pair this receiver up with a DX-40 transmitter that I have on the shelf and I'd be most of the way toward re-creating my novice station.  Anyone have a Globe VFO Deluxe?   That would complete the setup.  

Thanks very much to shortwave listener Joe Pechie for providing what is, for me, a very meaningful piece of gear. 

Here's a short video on the receiver: