SSB tuning with a vintage receiver

Discussion in '"Boat Anchor" & Classic Equipment' started by KK4RUT, Nov 24, 2021.

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  1. KK4RUT

    KK4RUT Ham Member QRZ Page

    @WA1GXC and @SM0AOM
    Thank you so much for the enlightenment! I kinda assumed that is what you guys were gonna say, but you never know until you ask.

    I am so close to completing all the electrical work on the 75A-1. I bought it from a SK XYL. It is in some rough cosmetic shape, and it definitely needs a good cleaning.

    I just finished all the capacitor replacements last week. I just replaced all the tubes today, and now all I’ve really gotta do is get her cleaned up, get the cabinet sand blasted and repainted, and do an alignment. After I replaced all the tubes and turned it on today she sounded 10 times better. It was incredible!
    WA1GXC and KA0HCP like this.
  2. K6BSU

    K6BSU Ham Member QRZ Page

    With same practice, my SP-600 does pretty well tuning SSB.
  3. G3YRO

    G3YRO Ham Member QRZ Page

    I used a Marconi CR100 as my main receiver for many years when I was first licensed. Being a schoolboy, it was all I could afford!

    Although it was designed before WW2, it was excellent on SSB (and CW). It had selectable bandwidths of 6kHz, 3kHz, 1.2kHz, 300Hz and 100Hz (the last two achieved using a crystal in the IF, and then an Audio filter).

    To set it up, in the 1.2kHz bandwidth position I adjusted the core of the BFO coil so that with the BFO Capacitor midway (and the knob at 12 o'clock) the BFO was in the middle of the passband. (easy to tell in the AM position - you would swish it about until you got the lowest pitch white noise).

    For SSB I used the 3kHz bandwidth position, and found the optimum point for the BFO for LSB or USB - this was with the knob at 10 o'clock or 2 o'clock - and I then marked the panel accordingly. (this obviously put the BFO on the edge of the Filter response, just like you do with a proper SSB receiver). So from then on I just turned the knob to the LSB or USB position, and tuned the station in using the main tuning dial.

    As others have suggested, I always used the receiver with the AGC off, the AF Gain right up, and adjusted the RF gain for the desired volume. (this is mainly because the AGC really wouldn't work properly on SSB or CW - it was designed for AM use).

    In terms of signal reports, I got used to where the RF Gain was to estimate the signal coming into the receiver - it was like a calibrated attenuator.

    Hope my experiences 50+ years ago help !

    Roger G3YRO
  4. W9BRD

    W9BRD Ham Member QRZ Page

    Yes. Commonly when professional gear includes a signal meter, it's calibrated in units related to an absolute value, such as decibels relative to a milliwatt (dBm) or decibels relative to a microvolt (dBuV). Even when a well-calibrated such meter is available, antenna impedance, gain and directivity affect the meter indication as with a hammy S meter, so signal reports in terms of meter indication are pointless.

    I like that "R3" (signal report with three possible values) idea, Karl-Arne. Think I'll try it out in some CW QSOs and listen for the sound of distant heads exploding.
    K1APJ likes this.
  5. G3YRO

    G3YRO Ham Member QRZ Page

    I'm sorry, but I totally disagree!

    I have always made sure the S Meters on my equipment are properly calibrated, so when I tell someone their signal strength, that tells them how many microvolts he is actually putting into my antenna.

    This is especially useful to compare conditions from one day to the next. I also find it very useful when comparing my signals to locals with distant stations. (ie I want to know how efficient my antenna is compared to theirs)

    Also, some people have vary high noise levels these days . . . so they may have trouble copying me even though my signal is S9 with them . . . but that is THEIR problem - at least I know they are getting a strong signal from me (and that's what I want to know)

    It also makes me laugh when I'm giving somebody an S9 +20dB report, and they only give me S7. I then ask them if perhaps they have the RF Stage ("pre-amp") on their rig turned off . . . and of course they have, which makes the S meter calibration completely wrong (the same as switching in an Attenuator).

    Roger G3YRO
  6. W9BRD

    W9BRD Ham Member QRZ Page

    Good proof of why S meters are (largely) useless for signal reports.

    I've actually heard hams apologizing for the signal reports they're giving "because my beam is down for maintenance, and I'm only using a dipole." Absurd.

    "How many microvolts they're putting into my antenna" is not itself useful. How many _microvolts per meter_ a station is producing at your QTH, a measure of field strength, _would_ be useful, but that takes more than just calibration of the receiver; it requires calibration of antenna and receiver as instrument capable of measuring field strength.. (Because, as you've seen, switching in an RF amplifier or attenuator, and/or switching to a different antenna, changes the value reported. A field strength _measurement_ factors out such variations. [That aspect of testing the ICOM IC-R9500 for _QST_ was fun, BTW: Switching in attenuation or RF amplification doesn't change the signal reading on its absolutely-calibrated signal meter. Cue distant sound of heads exploding.])

    Yes, an S meter is a useful _relative_ indicator, such as a "one day to the next with the same antenna [pointed in the same direction]" comparison, or when you're adjusting stages or gear under test/alignment/development for min or max.
    WA1GXC likes this.
  7. K1APJ

    K1APJ Premium Subscriber QRZ Page

    The FlexRadio products do the same thing, turning on the pre-amp lowers the noise level on the display, but shows the correct (unchanged) signal strength. A number of people complain that it doesn't act like a "real" S meter. It also doesn't go to zero with the antenna disconnected...
  8. SM0AOM

    SM0AOM Ham Member QRZ Page

    This is how calibrated measuring receivers and spectrum analysers handle pre-amps and attenuators in their signal paths, which in turn leads to a somewhat counter-intuitive behaviour.

    Generally speaking, the "S-meter" deflection by itself is quite meaningless; as it is the S/N or S/I in a given bandwidth that actually becomes used for communications.

    Any absolute signal levels without regard to the background noise or interference levels are quite meaningless, and I suspect that many if not most of the "you are 59+ but repeat your callsign, name and QTH" reports can be attributed to this...

    K7TRF and K1APJ like this.
  9. W7IMM

    W7IMM Premium Subscriber QRZ Page

    This has always been a "Pet-Peeve" of mine when asking for a signal report. I always hear "I've got you 5-9, 4-8 etc" I guess 5-9 means "I hear every word and you're signal is S-9" etc..... But I routinely talk to a guy that has an S-9 noise level! He might still get every word but @S-9, my signal is "right at the noise"

    So, an S-meter reading is actually quite meaningless until you know where your signal is compared to the ambient noise (or other interference)!

    To that end, (if they don't give me either the noise level or how "far" I am ABOVE the noise), I ALWAYS ask for the noise "level" after I get a signal report.

    It's FAR more meaningful if you get "you're 5 S-units above the noise" or "I have an S-3 noise level and you're S-9 + 20dB" etc...... without reference to the noise, you might as well NOT have an S-meter at all (just like ALL the airborne and many many marine and military HF SSB radios)


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