bias circuit of Kenwood KX-1030

Discussion in 'Electronics Repair' started by mikefc55, Jan 8, 2017.

  1. mikefc55

    mikefc55

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    hello all

    I'm just finishing the resurrection of this beautiful cassette deck. the only remaining problem is that the bias adjustment circuit is not working. It records fine and the dolby circuit works as well. A feature of this deck is to imprint two frequencies to adjust bias. a 400Hz and a 10kHz. I'll be damned if I can find the oscillator on the schematics, in my deck only the 400Hz is turning on. There is NO 10kHz signal and no effect on the level of the 400Hz when adjusting the bias knob. I'm looking for a crystal oscillator, am I wrong? From there I could trace the circuit and find the problem.
     

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    mikefc55, Jan 8, 2017
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  2. mikefc55

    AnalogEL

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    Hi Mike,
    Your two oscillators are the circuits surrounding Q401 and Q402, they're RC "phase shift" oscillators. Basically in either case there is 180 degrees phase shift from the base to the collector, and then four stages of RC phase shift providing another 180 degrees from the collector back to the base, for 360 degrees total and positive feedback (the requirement for oscillation). Sorry for the wonky detail. They should probably oscillate with beautiful sinewaves.
    The lower frequency oscillator will have the larger capacitors, hence Q401 is the 400Hz one.
    Q405 and Q406 form an amp, and there's some funky feedback going from their output back to the oscillators that I haven't figured out in my cursory look.
    I've noticed several polarized capacitors (C401, C407, C414, C415 etc.). If they're solid tantalum they're probably fine, but if they're electrolytic (that go bad with time) that's the first place I'd look.

    Good luck, let us know how it goes!
     
    AnalogEL, Feb 21, 2017
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  3. mikefc55

    mikefc55

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    wonky is good...... it's about 30 years since i went looking for a phase shift oscillator... thanks for setting me on the path..... to be continued
     
    mikefc55, Feb 21, 2017
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