SB-101

Discussion in '"Boat Anchor" & Classic Equipment' started by WB3EIM, Nov 21, 2021.

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  1. WB3EIM

    WB3EIM Ham Member QRZ Page

    Yes, the meter goes FS when the RF gain is CCW. The meter does zero when the pot is adjusted.
    The 100k resister was replaced at V3 but I can't see where a 68 ohm resistor is. The parts list doesn't list a 68 ohm.
    Switched V2 and V4 with no improvement. Any more thoughts?
    Also noticed on the bandpass CB there is a 1000 ohm resistor and a 36uh coil #40-484 missing, looks like they were never installed. This is a SBW-101(factory wire) so it seems a little strange. Should I install them?
     
  2. W3RU

    W3RU Ham Member QRZ Page

    Sorry Stan.

    I meant the 56 ohm R104/105 resistors off the cathode of V3.

    If the meter zero circuit and ALC bias (RF Gain) actions are OK, then you probably have a sensitivity issue between the antenna terminal through V10, V11 and V3 chain or an oscillator injection level issue. The 33K plate resistors on V16 are usually high.

    The manual has a good stage by stage signal level chart you can follow.

    What does the S-meter read on Cal for 3.5 MHz band with no antenna attached? Is the 3.5 MHz driver coil peaked correctly?

    You do have the manual, correct?

    Jack W3RU
     
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  3. W3RU

    W3RU Ham Member QRZ Page

    I'll do a little research on that one - back later. Perhaps Mike can weigh in.

    Jack W3RU
     
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  4. WB3EIM

    WB3EIM Ham Member QRZ Page

    Both replaced
     
  5. WB3EIM

    WB3EIM Ham Member QRZ Page

    Will check the S meter reading. Not sure how to peak the coil.
    I have a manual.
    Been looking over the V10- V11- V3 chain none of the resistors are to out of tolerance. The 33k resistor at V16 measure a little over 34k in circuit.
    Thanks again for the help.
     
  6. WB3EIM

    WB3EIM Ham Member QRZ Page

    Got the S meter working, not sure how it got fixed but it's working. Did a lot of cleaning on all switches, pots and any moving contact. Also changed cap C928 on the audio CB. After cleaning the meter zero pot I re zeroed it and it works as it should now.
    Still wondering about the Bandpass CB , it's missing R220, L202 and C222. There is a 56 ohm resistor jumpered between where L202 and C222 are suppose to be.
     
  7. K7TRF

    K7TRF Premium Subscriber QRZ Page

    That sounds like a factory modification to the fixed crystal oscillator circuit. Unless you use a fixed crystal, for instance to serve a particular net frequency, you'll never use that portion of the circuit. It sounds like they replaced the shunt inductor from V5B's cathode to ground with the 56 ohm resistor and decided they didn't need to use a blocking capacitor to couple the signal out of that oscillator. I suspect the oscillator works just fine with that mod but unless you use fixed crystal control instead of the LMO you'll never even use that portion of the transceiver's circuit.
     
    Last edited: Nov 26, 2021
    WB3EIM likes this.
  8. W3RU

    W3RU Ham Member QRZ Page

    No coil to peak other than the IF transformers during the alignment. I was referring to the preselector capacitor peaking.

    34K is well within tolerance. But check the cathode resistor.

    If this zeroed in the mid-third of adjustment range, all is well. The closer to actual center, the better.

    What is the calibrator S-meter reading on 75 meter band with no antenna connected?

    Jack W3RU
     
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  9. WB3EIM

    WB3EIM Ham Member QRZ Page

    Walk me thru this if you could.
     
  10. W3RU

    W3RU Ham Member QRZ Page

    OK Stan..

    Turn bandswitch to 3.5; disconnect antenna from rear jack.

    Set dial to 200 (3.7 MHz receive; upper slide rule needle at "2" and rotating dial to "0" +/-)

    Turn function switch to CAL

    Tune around the 200 mark for the calibrator carrier with preselector pre-set to 12:00 o'clock position.

    When found, peak the preselector and dial for max volume and max S-meter reading attainable with meter switch set on "ALC".

    Read the S-meter strength and let us know what it is. Ideally, it should be about 75-80% full scale.

    If you move up to "3" on the slide needle, the calibrator signal would be found 100 KHz higher and the preselector control would need to be re-set a small amount clockwise to attain the same S-meter reading - perhaps at the 1:00 o'clock tick mark.

    Hopefully, your calibrator crystal is working OK. S9 +30db demonstrates a properly working RF and IF chain with unknown BFO (carrier), Het Osc and LMO oscillator injection levels, and a strong audio response would indicate the rest of the receiver stages are probably good enough for further evaluation and fine tuning..

    Jack W3RU
     
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