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?