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

  1. #41
    Join Date: Oct 2016

    Location: Bolton, England

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    I'm Andrew.

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    Quote Originally Posted by Light Dependant Resistor View Post
    How wrong you are, 0v in the Crown iC150 is not zero volts as stated, as 0v is always in series
    with C40. Connect 2 capacitors in series you get voltage at their junction... no mystery to that.

    C40 is on the schematic is a capacitive connection to chassis ground,
    hence a zero volt reference up to the currents involved with a preamp.

    Crown's mistake was to take the floating ground concept a bit too far by relying on capacitors
    alone. Two diodes should retain Crowns concept and allow connection at a semiconductor junction
    at the same time, allowing residual voltages to properly dissipate.

    Cheers / Chris
    0 volts is whatever you choose as the reference voltage - there's nothing absolute about it. All voltages are measured with respect to something else, ie a voltage difference between two points is what is measured. It might be useful to the OP and to anyone who cares to contribute to this thread to establish what is being used as the reference point for voltage measurements. I suggest the negative side of the power supply should be taken as 0V, and that should be the same voltage as the negative end of R17 and the negative end of R20 and the negative side of C17 etc.
    The design of this pre-amp is not really the issue here, it's how to rectify the fault so the pre-amp functions as it did when new.

  2. #42
    Join Date: Sep 2013

    Location: North Island New Zealand

    Posts: 866
    I'm Chris.

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    It is a dual rail design having positive voltages , earth in the middle- and negative voltages
    Some of the voltages are regulated with zeners and good use is made of capacitor multipliers as well.
    The transistor amplifier stages borrow some single ended techniques, rather than split rail
    so has the appearance of being single ended. Negative voltages though are put to good effect.

    Earth then can exist either with sufficient capability of its own, usually termed a virtual earth
    and this is probably what you are referring to. A multimeter will flash 00.00 when encountering
    a virtual earth. But I see no deliberate virtual earth here.

    Rather capacitance via numerous branches of resistance are negotiating a common point
    that sadly contains other currents, they all are trying to settle to be a zero volt point we think
    is zero volts. The circuit board traces may have more resistance now, than when new hence
    the very fine see saw balance they achieved way back then, might not be as good

    Crown have gone a step further trying to separate chassis ground which
    they have designed as signal ground by using C40 so all those 0v resistance smallish capacitances
    see C40 and are desperately trying to get to ground but cannot quite get there.

    The sensible option is a pair of back to back diodes, it is a very professional way
    of accessing chassis earth by semiconductor lifting,

    If voltages are still residual after that, I would head to replacing the bridge rectifier and
    carefully examining the power supply components including the transformer.

    We are being very nice to each other trying to help, and I am proud to
    be part of a forum that goes about discussions this way.

    Cheers / Chris

  3. #43
    Join Date: Oct 2012

    Location: NE England

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    I'm Jez.

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    The 100pF cap is fine and supposed to look like that. The 0V is 0V as in any other unit. If only phono effected try R10, 10M resistor.

  4. #44
    Join Date: Oct 2016

    Location: Bolton, England

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    I'm Andrew.

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    BTW, this page might be of interest to the OP
    http://kenrockwell.com/audio/crown/ic-150.htm

    Apologies if you've seen it already.

  5. #45
    Join Date: Oct 2012

    Location: NE England

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    I'm Jez.

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    Getting off topic but the DC300A power amp actually measures very well indeed in spite of it's crude (but clever) OPS which runs pretty much in class B! I recall measuring around 0.003% THD from one of mine! Their bass performance is superb and in fact I can't name an amp with better bass... pity about the top end!

  6. #46
    Join Date: Apr 2017

    Location: maryland

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    I'm jim.

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    (Some) Progress!

    First some old business:

    Ground - According to the phono schematic, Earth ground and Schematic ground are tied to together. The phono board has a direct line from it's (schematic) ground right to the chassis. It also has another line to the chassis off via C9 (0.1uf 200v filmatic) on the phono board. And of course C40 on the main board etc etc.. My chassis point of ground reference is valid. My meter is working (tried another fluke today- same readings).
    I later today actually broke that C9 cap with all my mucking around on the phono board and I don't have a direct replacement for it. ** What is a simple (readily available) equivalent that i can get for that? It broke off after the majority of my testing so did not impact results, but may now be causing other issues.

    R20 100K resistor on phono board - replaced with fresh just to confirm no effect on voltage present.

    1ufs by the ICs - These were replaced. They did not correct the voltage on the phono board nor at the volume control. They did not correct the popping when switching from phone to tuner or back. They did not correct popping when moving the volume control though MAY of corrected popping when leaving the volume control at a static 85% worth phono selected. I say "may" because it's been a while since testing this specific artifact/ A number of components have been changed since the original post in effort to reduce the post C12 voltage present, including general power supply cap replacements throughout.

    Back to back diodes on C40 main baord - I did not have 007's, only 005's to try this. No effect or change on post C12 voltage. I did not expect any. You may have missed the post where it was confirmed this voltage is present on the phono board with the line from R13 (on the phono board) completely disconnect from the main board (via selector switch) C12/C40 are not in series when the line from R13 has been disconnected. There is no audio feed nor connectivity between board on that channel, other than the power line.

    Which brings me to my partial solution. I stated earlier the Phono board had good voltage. What I failed to realize (then) was there were actually TWO voltage feeds to that board. I had measured one a 19 volts (schematic says 18) I did not realize to check the other side until just now. It was -15V (schematic says -18v) too low!! traced that back to the power supply section and found a trimmer R28 and adjusted that via the main board -18v test point and WHAMO Problem solved! NO popping on right channel when using controls, when selecting between phono/tuner. None. Clean and sounds great. this is the channel with the fully working EQ and as far as I can tell, this channel is golden now. No voltage (mv's) on R13 anymore (connected or not to the main board). No popping. Nothing to do with anything except for the fact the voltage was incorrect (though I don't understand the "why" here, just that it was the source of the problem.).


    That would be it and onto the Left channel tone problem except.....
    Now there is voltage on the left channel, even more than before on that channel (now 1.5v), and of course popping. My immediate assumption was I just need to adjust that 19 volts down to 18v, which i did with it's trimmer, but it did not change anything. I went through the same test processes I had done on the other channel. Disconnected R13 (equivalent other channel's) from selector switch/main board. Confirmed a 1.5V dc still there so not induced by main board, must be sourced by phono board. Took C12 (equivalent) cap out, confirmed R13 voltage gone so no board leaks are happening. Back in and updated the R20 (Equivalent) 100K resistor. No change. Back to the same problem to resolve - unexpected DC voltage post C12 - just on a different channel now.

    So how did swing up the left channel voltage 3 volts move this problem to the right channel? Or is the Lack of C9 now playing some role?
    Another item of note - when Powering up or down and phono selected by default there is a substantial POP that occurs. Again manybe related to C9 missing. Not sure I noticed this happening before C9 broke or not.

  7. #47
    Join Date: Oct 2016

    Location: Bolton, England

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    Quote Originally Posted by jimkarl View Post
    I later today actually broke that C9 cap with all my mucking around on the phono board and I don't have a direct replacement for it. ** What is a simple (readily available) equivalent that i can get for that?
    Any 100nF cap should be fine - there's nothing special about it.
    Quote Originally Posted by jimkarl View Post
    Which brings me to my partial solution. I stated earlier the Phono board had good voltage. What I failed to realize (then) was there were actually TWO voltage feeds to that board. I had measured one a 19 volts (schematic says 18) I did not realize to check the other side until just now. It was -15V (schematic says -18v) too low!!
    I didn't see a -18V supply marked on the phono diagram though Q5 and C17 make a lot more sense if the supply to Q5 is -18V. It looks like you have tuned out the DC offset by balancing the positive and negative supply voltages. However, they shouldn't need to be balanced because the DC blocking capacitor (C12) should take care of any DC offset. It remains a mystery to me.
    One clue is that adjusting the supply voltage has affected a cure on one channel but created a problem on the other. I'm suspecting an issue with 0V and whether both channels are connected to it properly. I recommend not using the chassis as your voltage reference - it's really only connected to the circuit for safety and for screening and the metalwork should not be used as a conductor within the audio circuit. It might be convenient to stick your meter probe onto the chassis when making readings but it might also be misleading you.
    Last edited by RothwellAudio; 20-04-2017 at 08:47. Reason: typo

  8. #48
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    BTW, there is a way to check if the problem does have something to do with various bits of circuit not having the same 0V. Look for any voltage between various points that should be at 0V. For example, check for any voltage between the 0V end of R20 (phono section) and the 0V end of R27 (line section). Check between other points where you would expect there to be no voltage.

  9. #49
    Join Date: Apr 2017

    Location: maryland

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    Quote Originally Posted by RothwellAudio View Post
    Any 100nF cap should be fine - there's nothing special about it.

    I didn't see a -18V supply marked on the phono diagram though Q5 and C17 make a lot more sense if the supply to Q5 is -18V. It looks like you have tuned out the DC offset by balancing the positive and negative supply voltages. However, they shouldn't need to be balanced because the DC blocking capacitor (C12) should take care of any DC offset. It remains a mystery to me.
    One clue is that adjusting the supply voltage has affected a cure on one channel but created a problem on the other. I'm suspecting an issue with 0V and whether both channels are connected to it properly. I recommend not using the chassis as your voltage reference - it's really only connected to the circuit for safety and for screening and the metalwork should not be used as a conductor within the audio circuit. It might be convenient to stick your meter probe onto the chassis when making readings but it might also be misleading you.
    18 v vs -18v - The line off of R56 on the phono board that says 18V is a typo (or a PDF anomaly) All other board revisions pdf show this as -18v. That's what threw me and why I never checked it.

    Board ground vs chassis ground - If you look at the bottom of R17 where the ground symbol is, on the phono board at that location, there is a wire that comes off the board direct to the chassis. there is no difference on my meter whether I measure using my probe direct on the board at that point, or on the chassis. The meters readings are identical. I understand that in some designs, the chassis is not the same as the circuit ground and may have a different potential but it appears in this case (no pun intended) they are one and the same.

  10. #50
    Join Date: Apr 2017

    Location: maryland

    Posts: 32
    I'm jim.

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    Quote Originally Posted by RothwellAudio View Post
    BTW, there is a way to check if the problem does have something to do with various bits of circuit not having the same 0V. Look for any voltage between various points that should be at 0V. For example, check for any voltage between the 0V end of R20 (phono section) and the 0V end of R27 (line section). Check between other points where you would expect there to be no voltage.
    Checked all the 0V (ground) points on the phono board. All good. they are common trace points on both channels. I'll have to map out the Main board as it's hard to read the layout schematic, but keep in mind the voltage occurs on the phono board with the input line disconnected from the main board.

    Did add a (temporary) 0.1uf for C9 - no change.

    What's interesting about the voltage is it starts at about .5v (same as before when i was troubleshooting the other channel) I/knew it was always wrong but it the was lesser of two evil channels, so to speak. After turn on, it climbs fairly rapidly up to the roughly 1.2-1.5v, maybe about 0.1th volt every 15-30 seconds. I don't recall it climbing originally (on that channel). This happens with the R13 (equivalent) line disconnected from the main board , so it's building up a charge on the phono board.

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