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Thread: Crown IC 150 Preamp popping noise - source of problem?

  1. #21
    Join Date: Oct 2016

    Location: Bolton, England

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    I'm Andrew.

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    Quote Originally Posted by jimkarl View Post
    Disconnected phono board r13 line - voltage is present on Phono board (no longer present on main board volume pot) so voltage is coming from phono board. Voltage is present at C12/R20 junction, which I don't understand how that could be, given a brand new 4.7 tant installed.
    You're right - with a good capacitor and a good resistor for C12/R20 there should be no DC. That means either a dodgy cap or the 100k resistor has gone open circuit. Measuring the resistor should be easy enough.
    Actually, there's one other possibility - a DC path in parallel with the capacitor caused by dirt/dampness or some other contaminant on the circuit board in the vicinity of the capacitor.

  2. #22
    Join Date: Sep 2013

    Location: North Island New Zealand

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    I'm Chris.

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    Quote Originally Posted by RothwellAudio View Post
    You're right - with a good capacitor and a good resistor for C12/R20 there should be no DC. That means either a dodgy cap or the 100k resistor has gone open circuit. Measuring the resistor should be easy enough.
    Actually, there's one other possibility - a DC path in parallel with the capacitor caused by dirt/dampness or some other contaminant on the circuit board in the vicinity of the capacitor.
    And one more, earth is not earth at all !

  3. #23
    Join Date: Oct 2016

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    Quote Originally Posted by Light Dependant Resistor View Post
    And one more, earth is not earth at all !
    Sorry, I don't understand that - could you elaborate?

  4. #24
    Join Date: Sep 2013

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    Where earth we presume to be zero volts, is not at all.

    A measurement from the chassis to the 0V point should confirm.
    American equipment fails today on a number of fronts concerning isolation
    and best practice.

    We really need to see the full schematic, or provided a link to assist further.

    Cheers / Chris

  5. #25
    Join Date: Feb 2013

    Location: W Lothian

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    I'm Grant.

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    Ive seen tvs that had chassis at half mains potential too or something similar. Had to pull earth from scope or blow the damn thing up. Those were a pain in rectum.
    Regards,
    Grant ....

    I've said it before and I'll say it again: democracy simply-doesn't-work
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  6. #26
    Join Date: Apr 2017

    Location: maryland

    Posts: 32
    I'm jim.

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    Quote Originally Posted by Light Dependant Resistor View Post
    Where earth we presume to be zero volts, is not at all.

    A measurement from the chassis to the 0V point should confirm.
    American equipment fails today on a number of fronts concerning isolation
    and best practice.

    We really need to see the full schematic, or provided a link to assist further.

    Cheers / Chris
    Full schematic included, though the source PDF quality is quite poor. The board layout diagrams are almost completely unreadable....

    crown_full.jpg

  7. #27
    Join Date: Apr 2017

    Location: maryland

    Posts: 32
    I'm jim.

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    Quote Originally Posted by RothwellAudio View Post
    You're right - with a good capacitor and a good resistor for C12/R20 there should be no DC. That means either a dodgy cap or the 100k resistor has gone open circuit. Measuring the resistor should be easy enough.
    Actually, there's one other possibility - a DC path in parallel with the capacitor caused by dirt/dampness or some other contaminant on the circuit board in the vicinity of the capacitor.
    Pulled and checked resistor - measures correct. Over -2V at that junction, over -4V on the other side of C12, so it is doing SOME DC reduction. Mesured that new cap both for capacitance and esr. good. Checked every (i think) resistor on that board - all good. Checked the diode. Good. Put my magnifying glasses on and don't see any bridging that would cause the leakage. My soldering skills are not the best, so I could easily attributed it to that, except it's been doing this since before I ever even touched the board whern it was completely stock.

  8. #28
    Join Date: Sep 2013

    Location: North Island New Zealand

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    I'm Chris.

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    Quote Originally Posted by jimkarl View Post
    Pulled and checked resistor - measures correct. Over -2V at that junction, over -4V on the other side of C12, so it is doing SOME DC reduction. Mesured that new cap both for capacitance and esr. good. Checked every (i think) resistor on that board - all good. Checked the diode. Good. Put my magnifying glasses on and don't see any bridging that would cause the leakage. My soldering skills are not the best, so I could easily attributed it to that, except it's been doing this since before I ever even touched the board whern it was completely stock.
    Hi
    A few checks on your measurements. You are measuring with a multimeter DC -not AC, and DC relative to ? - usually earth , and you state you have negative 2 volts( - not mv ? ) at the upper side of
    R13 leading to the switch. If so can you check the same voltage is measured across the tape out RCA sockets ?

    The design is extensively AC capacitor coupled somewhat negating any possibility of DC being coupled
    Check the op amp IC1 is not inserted around the wrong way, there should be a dot meaning pin 1
    The IC150 has a voltage regulator check for steady 12v + and steady 12v neg at the emitters of the transistors
    located near the bridge rectifier ( sorry cannot read the part numbers )

    Hear from you soon.


    Cheers / Chris

  9. #29
    Join Date: Apr 2017

    Location: maryland

    Posts: 32
    I'm jim.

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    Definitely DC measurement, definitely full volts, not mv. With the R13 line connected back, yes it is present at the tape output to (direct line from the switch there so it should be)
    Note that when connected, that -2V does grow the longer I leave the unit on. That's why you see me state 4v 5v etc in previous posts. It starts at about 2v though on power up.
    Measurement taken relative to the chassis. Same connect point the main amp board connects to via .1uf cap. (also have done same connect point as phono board connect via .1uf cap too, since it's the same ground (chassis) no difference.

    IC is inserted correct. keep in mind, i didn't even swap them until recently and the voltage occurs at the phono board, with R13 line disconnected to switch/main board so not clear how an incorrect IC on the amp board would have any impact there anyway.

    I forgot to check the 12V source, but that would be for the main amp board anyway. I can check that tomorrow. I did check the 18v source for the phono board though - reading about 19v. Again with chassis as the ground reference.

    Another thing I might try tomorrow is to take that 4.7uf tant out of the circuit on the phono board, just to confirm there is no voltage present on the R13 side of it. If there is, I guess that would confirm a voltage leak onto that part of the circuit as RothwellAudio mentioned as a possible trace hop, though I couldn't find any today and I looked really close.

  10. #30
    Join Date: Sep 2013

    Location: North Island New Zealand

    Posts: 839
    I'm Chris.

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    Quote Originally Posted by jimkarl View Post
    Definitely DC measurement, definitely full volts, not mv. With the R13 line connected back, yes it is present at the tape output to (direct line from the switch there so it should be)
    Note that when connected, that -2V does grow the longer I leave the unit on. That's why you see me state 4v 5v etc in previous posts. It starts at about 2v though on power up.
    Measurement taken relative to the chassis. Same connect point the main amp board connects to via .1uf cap. (also have done same connect point as phono board connect via .1uf cap too, since it's the same ground (chassis) no difference.

    IC is inserted correct. keep in mind, i didn't even swap them until recently and the voltage occurs at the phono board, with R13 line disconnected to switch/main board so not clear how an incorrect IC on the amp board would have any impact there anyway.

    I forgot to check the 12V source, but that would be for the main amp board anyway. I can check that tomorrow. I did check the 18v source for the phono board though - reading about 19v. Again with chassis as the ground reference.

    Another thing I might try tomorrow is to take that 4.7uf tant out of the circuit on the phono board, just to confirm there is no voltage present on the R13 side of it. If there is, I guess that would confirm a voltage leak onto that part of the circuit as RothwellAudio mentioned as a possible trace hop, though I couldn't find any today and I looked really close.
    So what measurement do you get when measuring R13 upper and lower when switched to AC ?
    It could be there is a very low frequency oscillation from the phono board and the duty cycle nature of
    multimeters is translating an average DC voltage like a RMS ( Root Mean Square )
    because it is averaging then resident within a much larger AC voltage.

    The typical cause of low frequency oscillation is positive feedback
    As I see it Q4 causes emitter degeneration at Q3 which couples to the base of Q1 and then Q2 giving amplification
    R16 and R11 are feedback and signal related with a lot of reliance on Q4 collector to present opposite phase, as the emitter of
    Q4 is also a feedback path having emitter degeneration. So its not great at keeping positive feedback from starting.
    At C14 we see ability for Q4 to not be steady as its emitter is AC coupled. And emitter of Q4 taking shorter path
    through C14 and C16 for delivering amplified audio. Q6 is a cap multiplier

    I would check R29 variable as there is a lot of reliance on establishing a form of frequency summing point via C16
    In preference using a mosfet with a tiny bit of resistance prior to ground establishes a much better ability for Q4 to
    be predictable.

    I really need to breadboard the phono circuit to see what its characteristics are.

    Cheers / Chris

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