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Thursday, October 15, 2020

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?  



9 comments:

  1. Recently I have been wrestling with the question, Is it wrong to remove legacy circuits and replace them with more modern improvements. I am getting ready to start work on an SP210 receiver, it is not rare, and really has extremely low value in the marketplace. I am sure that a modern solid state audio amplifer would be "good enough" as would replacing the mixer with a passive device such as a MiniCircuits SBL. I love to tinker more and more these days and am convinced that most of what was offered in the 50s and 60s can be improved if you have the time and inclination. 73, Eric NM5M

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  2. Go for it Eric. Make it better. Have fun. Fix it up. 73 Bill

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  3. If ya haven’t already Bill, I would try varying the bfo level by using your signal generator as the bfo and see if it improves at any point. How much bfo voltage have you got?

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  4. Gary: Good idea. I tried it this morning.
    Normally the BFO puts 108 mv rms onto the base of the product detector. Without any melting of solder or removal of parts, I shut down the BFO by grounding its base with a clip lead. Then I replaced the BFO signal with a signal from my FeelTech sig generator. The same SSB distortion was there. I tried lowering the sig gen level -- no improvement. I could raise the BFO level up to about 130 mv rms -- again, no improvement. What DOES eliminate the problem is using the diode (AM) detector with BFO energy loosely coupled. What do you think? 73 Bill

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  5. Just a SWAG, but have you tried subbing capacitors C40 and C41? I'd expect boatloads of distortion if either of those is slacking.

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  6. Your Herring Aid 5 rx has a transistor mixer too, time for a comparison?? Ed KC8SBV

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  7. Hi Bill.
    I did a bit of a think about that product detector. By that diagram fragment, the (Tr-4) BFO works into the Tr-5 base: a high Z: why wouldn't it look good? Now feeding the IF into the emitter is a different kettle of fish: that's by definition Low-Z (common-base tells me that) - AND C39//R40 loads it: wouldn't that be less than optimal? I'd be putting another transistor in there, high-Z base fed by IF, to turn it with Tr-5 into a 'long-tail pair' like op-amp inputs. There would likely be change of biasing setup, but that would be trivial. What say?

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  8. Over-simplified: Einstein RULES! :)

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  9. Another thought, Bill: That extra transistor would likely self-bias from the emitter(s): long-tail pairs do that. It might provide too much gain though, swamping the BFO, needing a pad (or other) to calm it down some.
    Have fun! :)
    73 de ZL3LH.

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