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Thread: Maxon AD900 no effect

  1. #1
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    Maxon AD900 no effect

    This AD900 is the mn3008 model. I've been tracing a signal through the circuit and I found it stops at IC3 and goes no further.
    Does this mean IC3 is the culprit or does it mean IC5 or IC8 are bad?
    I don't hear any signs of delay effect with the signal tracer
    Any thoughts?
    thank you in advance!
    pete
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    Last edited by pontiacpete; 08-24-2017 at 09:50 PM.

  2. #2
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    Check that you have 10-12v on IC3 pin 8 and a high frequency square wave on pins 2 and 6.
    If so, the bias trimmer might be off. Clip the scope probe to CP1 and adjust TR2. At some point
    you should see a signal. Otherwise the chip is probably dead. Swap it with one of the others.
    The balance trimmers TR3,TR5,TR8,TR10 should be centered but don't touch them yet.
    pontiacpete and Mick Bailey like this.

  3. #3
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    Thanks for help dmeek. Pin 2 and 6 don't seem to have a signal. So l clipped probe to CP1 and turned SR2, nothing there.
    Switched IC3 with IC 7 still no signal.
    There's about 11v(give or take a volt) on pin 1,2,3, 4 , 6 and 8.
    Last edited by pontiacpete; 08-25-2017 at 01:36 PM.

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    You need to have a square wave on both pins 2 and 6. Don't touch any other trimmers, the problem is with the clock oscillator.
    check for a square wave (~20-50 kHz) at IC8 pin 7. If nothing there look around the oscillator circuit and delay pot.
    C25,C26 or D4 could be shorted, or a bad delay pot, Possibly IC5 or IC8 could be bad.
    After you get the oscillator working go back and adjust TR2. Don't touch any other trimmers.
    Also, after you swap any ICs, put them back in their original positions.
    Last edited by dmeek; 08-25-2017 at 07:25 PM.

  5. #5
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    Checked C25 & 26 and D4 for shorts, all looked good, with 8v at the zener diode. No square wave on IC8 pin 7, so I changed out IC 5 & 8 and we have effect!!
    Swapped IC3 back with IC7 and adjusted SR1(trimpot 1) and SR2 by hear and sounds great.
    Thanks dmeek so much for your help!

    pete

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