Heathkit SIG Gen SG-8

Discussion in 'Troubleshooting and Repair' started by nugent, Feb 15, 2016.

  1. nugent

    nugent

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    I have a SG-8 sig. gen. It uses a 6C4 Colpitts AF oscillator and a 12AU7 as a RF oscillator and cathode follower. When I change the tuning cap. I get no change in output signal . When I put my scope near the Rf Inductor coils there is plenty of RF being generated. I wonder if the B+ voltage is high enough for proper operation.
     
    nugent, Feb 15, 2016
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  2. nugent

    duke37

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    If there is oscillation, the B+ is sufficient.

    Look at the RF output and also the AF output.
    What do you mean by no change in output signal. It may be difficult to see a difference in frequency with a scope?
    Put the output close to a radio and see if the RF frequency changes.
    Put the AF signal into an amplifier and listen to it.
    Is the circuitry clean or corroded?
     
    duke37, Feb 15, 2016
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  3. nugent

    nugent

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    The circuitry is old and it is corroded . I have cleaned the step switches etc but am considering re wiring the unit as I feel this may help.
    Thanks Duke 37
     
    nugent, Feb 15, 2016
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  4. nugent

    duke37

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    Please answer my questions.

    Rewiring will do nothing, the problem is likely in the switches, use switch cleaner. I would also look at the valve pins and bases if there was no oscillation.
    No change in RF frequency may be due to poor connection in the tuning capacitor. Use switch cleaner.

    What RF output do you get?
    What AF output do you get?
     
    duke37, Feb 15, 2016
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  5. nugent

    nugent

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    The unit has 5 bands 160kc-220mc
    the RF output is in excess of 100,000 microvolts and the audio is 2-3volts
     
    nugent, Feb 15, 2016
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  6. nugent

    nugent

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    Thanks Duke I did as requested and found I had a very weak ouyput. I replaced the tubes and now all is ok

    Nugent
     
    nugent, Mar 20, 2017
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