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Thread: Another Heathkit SB-102 question

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  1. #1

    Default Another Heathkit SB-102 question

    So I have been slowly bringing back to life my SB-102 and with the help of the folks here on the forum is working real nice, however it does have one annoying issue, from one band to another, the dial is off by as much as 100KHz so I have to calibrate the dial every time I switch bands.
    Any clues?

  2. #2
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    Default

    Sounds like an alignment issue.

  3. #3
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    Default

    Check the frequency of the last conversion oscillator...there is a different crystal for each band...one of the rocks might be lame.
    "The more you know, the less you don't know."

  4. #4

    Default

    I'd recommend properly cleaning the bandswitch before any of the above...


    73

  5. #5

    Default

    WJW:

    I really don't think that you mean 100 kHz change in dial calibration when you change bands!

    The dial in the SB-102 is calibrated with 100 "ticks" per revolution and then there is the sliding portion of the dial which very roughly indicates every 100 kHz over the 5-revolutions of the main dial when tuning from one end of the frequency range to the other. As such, you actually have to mentally add the "ticks" from the rotary dial to the rough indication from the sliding dial and then that total to the indication on the bandswitch. For example, if you read 64 on the rotary dial, about 200 on the sliding dial you get 264. Then, this is added to the bandswitch reading. If the bandswitch is set to 7.0 then the actual frequency will be 7264 kHz.

    The heterodyne oscillator crystals in the SB-102 are almost never "exactly" on the stated frequency. Since the bandswitch indication chooses the particular heterodyne oscillator crystal the mixing in the unit produces the correct frequency. There are several things which affect the absolute frequency on which the heterodyne oscillator crystals oscillate. These include the actual tolerance of the crystal (how close the original production came to the desired frequency), temperature (changes in temperature can affect the absolute frequency of the crystal), and the actual adjustment of the coil marked as "het osc". Just by adjusting the heterodyne oscillator coil the frequency of the crystal can be pulled up to a kilohertz or more.

    The simple fact that the heterodyne oscillator crystals are almost never on the exact frequency is why there is a movable fiducial (the fiducial is the "line" indicator on the rotary dial) which compensates for this difference in crystal frequency. When the 100 kHz crystal calibrator is activated, the main tuning dial is adjusted for zero-beat and then the movable fiducial is set to the "0" point on the rotary dial. It is not unusual for the fiducial to be several kHz away from being centered in the dial window. As such, it is very uncommon for the fiducial not to have to be adjusted for each band.

    Glen, K9STH

  6. #6

    Post

    Check the frequency of the last conversion oscillator...there is a different crystal for each band...one of the rocks might be lame.
    At 40 to 50 years of age, that is a realistic possibility.
    I had a used SK estate Drake R-4C that had that exact problem (off > 50 kHz) about 7 years ago --
    I had to replace all 5 crystals -- since Drake did not provide a trim adjustment for each oscillator (original OEM crystals were Erie, used JAN for replacements).
    While this is not a cheap repair -- it did return this receiver to factory spec.
    Last edited by W9GB; 04-29-2012 at 03:43 PM.
    Nullius in verba

  7. #7

    Default

    Ooops, sorry I meant up to 10KHz. While most bands fall within 2-5KHz of each other, on the the 40M band the "fiducial" has to be adjusted way to one end of the dial which is around 10KHz. While it does not affect performance, it's something I will eventually like to bring closer to "0".
    I will eventually change the 40M crystal and see how that goes.
    Thanks again, Gil

    Quote Originally Posted by K9STH View Post
    WJW:

    I really don't think that you mean 100 kHz change in dial calibration when you change bands!

    The dial in the SB-102 is calibrated with 100 "ticks" per revolution and then there is the sliding portion of the dial which very roughly indicates every 100 kHz over the 5-revolutions of the main dial when tuning from one end of the frequency range to the other. As such, you actually have to mentally add the "ticks" from the rotary dial to the rough indication from the sliding dial and then that total to the indication on the bandswitch. For example, if you read 64 on the rotary dial, about 200 on the sliding dial you get 264. Then, this is added to the bandswitch reading. If the bandswitch is set to 7.0 then the actual frequency will be 7264 kHz.

    The heterodyne oscillator crystals in the SB-102 are almost never "exactly" on the stated frequency. Since the bandswitch indication chooses the particular heterodyne oscillator crystal the mixing in the unit produces the correct frequency. There are several things which affect the absolute frequency on which the heterodyne oscillator crystals oscillate. These include the actual tolerance of the crystal (how close the original production came to the desired frequency), temperature (changes in temperature can affect the absolute frequency of the crystal), and the actual adjustment of the coil marked as "het osc". Just by adjusting the heterodyne oscillator coil the frequency of the crystal can be pulled up to a kilohertz or more.

    The simple fact that the heterodyne oscillator crystals are almost never on the exact frequency is why there is a movable fiducial (the fiducial is the "line" indicator on the rotary dial) which compensates for this difference in crystal frequency. When the 100 kHz crystal calibrator is activated, the main tuning dial is adjusted for zero-beat and then the movable fiducial is set to the "0" point on the rotary dial. It is not unusual for the fiducial to be several kHz away from being centered in the dial window. As such, it is very uncommon for the fiducial not to have to be adjusted for each band.

    Glen, K9STH
    Last edited by N2WJW; 04-29-2012 at 08:17 PM.

  8. #8
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    Default

    Quote Originally Posted by N2WJW View Post
    Ooops, sorry I meant up to 10KHz
    That's more reasonable.
    My SB-301 which I had built had crystal aging issues once it reached the fifteen year old mark. Ten KC is not really that bad for a radio of that age.
    When they make crystals after the quartz is etched to the right frequency they are placed in the holder, tested and sealed usually in a dry nitrogen environment.
    Crystals leak usually around where the leads exit the holder, Outside air gets in and with it humidity and aging begins. In a few hundred years they will fail altogether.
    When it's time, it's time, and it maybe sooner than you think.

  9. #9

    Default

    You might try adjusting the heterodyne oscillator coil for the 40-meter band. You might be able to pull the crystal a little bit closer while still keeping it oscillating.

    For some reason, the 40-meter and the 15-meter crystals seem to "go" a bit more often than the others.

    Glen, K9STH

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