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Thread: Crate BX50 noise

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    Crate BX50 noise

    Resoldered the noisy intermittent input jack, and upon reassembly there’s a loud swooping distortion that sounds only right when you turn it on. Only happens with the internal speaker, doesn’t happen with a speaker hooked up externally. Not sure where to start with finding the source of the noise. Both 4700uf caps read 22 volts.

    Schematic: http://web.archive.org/web/200305231...+/23701H9_.PDF

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by sunshine_bassman View Post
    Resoldered the noisy intermittent input jack, and upon reassembly there’s a loud swooping distortion that sounds only right when you turn it on. Only happens with the internal speaker, doesn’t happen with a speaker hooked up externally. Not sure where to start with finding the source of the noise. Both 4700uf caps read 22 volts.

    Schematic: http://web.archive.org/web/200305231...+/23701H9_.PDF
    Ok few suggestions. A bigger screen than a smartphone is most helpful for schematic work. It you don't have one I can get you one( I like to trade stuff so if your interested).

    Second a scope would be handy here. One for see the waveform on the speaker an after that you can probe pin 2 on the 12pin ribbon cable to see if noise is present there at turn on. What we are doing here is halving the problem. If it is present here it's further back in the preamp. If it is not here at pin2 it is coming from the output transistor.
    Being that problem disappears when an external speaker is connected makes me think load. Check the resistance on the speaker.
    Let us know what you need there is more than enough abundant knowledge here.

    nosaj

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    Supporting Member The Dude's Avatar
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    Have you checked the speaker itself? Try subbing in another speaker to the existing speaker connections (not external speaker). Try hooking up the existing speaker to something else.

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    No way to hook up the speaker to another source, not here in the shop anyway. The internal speaker is an 8 ohm, as is the external speaker I used before.

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    So I was just able to test a different speaker, and it still had the swooping distortion. There’s also noise on pin 2 of the ribbon cable when probed

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    Last edited by sunshine_bassman; 12-10-2018 at 08:15 PM. Reason: New info

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by sunshine_bassman View Post
    So I was just able to test a different speaker, and it still had the swooping distortion. There’s also noise on pin 2 of the ribbon cable when probed
    Ok using the scope on TP1 and TP2 See if you see that swoop at those points. After that I'd suggest reattaching the PCB back to chassis and putting it back in the cab and see if it's still there.
    nosaj

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    Don't forget the joker g1's Avatar
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    A definition of 'swooping distortion' would also be helpful. I'm completely confused as to what that might mean.

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by g1 View Post
    A definition of 'swooping distortion' would also be helpful. I'm completely confused as to what that might mean.
    It's real hard to describe, when I was there it's just about a second of noise. Maybe Sunshine can record it.

    Also look back over your soldering on the input jack make sure there's no excess solder that could be bridging something,

    nosaj

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    Quote Originally Posted by g1 View Post
    A definition of 'swooping distortion' would also be helpful. I'm completely confused as to what that might mean.
    It’s half like a buzzer on a game show, half like a fuzzed-out guitar bending a whole step. It lasts about half a second.

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    Stray Cap DrGonz78's Avatar
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    I would measure for any DC offset on the output at the time the amp is turned on. Probably not but just curious if there is any DC offset that relates to the noise. Also curious what that opto OC1 function is in this amp.

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    Stray Cap DrGonz78's Avatar
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    I know the optocoupler must be part of the limiter circuit. Curious how it functions.

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    Lifetime Member Enzo's Avatar
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    The output of one of the power amp ICs is sampled back through D5 D6. It is kinda convoluted, but they light the opto when needed. The resistor side of the opto is simple shunt across the signal going to the PA input.

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by Enzo View Post
    The output of one of the power amp ICs is sampled back through D5 D6. It is kinda convoluted, but they light the opto when needed. The resistor side of the opto is simple shunt across the signal going to the PA input.
    Yes.
    The typical Crate/Ampeg (SGM will hate me for this ) optical limiter is a VERY clever passive comparator.
    I LOVE elegant minimalistic solutions to problems.

    1) you have a 3 resistor string from +V to -V rails, here some 25+25V DC.
    We are talking R26/27/29 here.

    2) R26 and R29 drop somen 10V each, so they set up "reference voltage" points 10V below and above respecyive rails.

    3) optocoupler Led is connected to speaker out and both reference voltages through a diode network, so when speaker out peak is "near one rail or the other" LED turns on and LDR limits drive voltage.

    How near?

    a) to light a red Led you need 1.9V across it.
    Plus 2 series diodes (0.7V each) you now need 3.3V
    Add typical power transistor loss of about 4V and we need some 7.5V

    Add maybe 0.5 or 1V for various losses (usually emitter resistors ) and we are *almost* at maximum possible undistorted pek voltage.

    So this clever circuit reliably stops clipping, only "wasting" about 1 V peak ... nothing.

    Best feature: it actually tracks rail voltage, no matter what.

    As of my SGM mention: this Crate amp is basically same as diamond blue tolex clad "Ampeg" Rocket 50

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    Juan Manuel Fahey

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    Stray Cap DrGonz78's Avatar
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    Okay got it thanks for the great explanation. I was thinking some sort of comparator with the -/+ voltage rails on either side.

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    Is the optocoupler supposed to prevent noise like what I’m getting? I shorted the resistor side with alligator clips and still got the sound.
    Prolly worth noting that it doesn’t happen as soon as the amp turns on, but anout three seconds afterward

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by sunshine_bassman View Post
    Is the optocoupler supposed to prevent noise like what I’m getting? I shorted the resistor side with alligator clips and still got the sound.
    Prolly worth noting that it doesn’t happen as soon as the amp turns on, but anout three seconds afterward
    Did you ever mount the PCB back in the chassis with the nuts on the pots and jacks?
    From Juan
    optocoupler Led is connected to speaker out and both reference voltages through a diode network, so when speaker out peak is "near one rail or the other" LED turns on and LDR limits drive voltage.

    nosaj

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    Lifetime Member Enzo's Avatar
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    The opto is there to prevent the power amp from clipping...period.

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    I screwed everything back together and put it back in the cab, still makes the same sound. Could the opto possibly be bad? Maybe some of the resistors are open?

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by sunshine_bassman View Post
    I screwed everything back together and put it back in the cab, still makes the same sound. Could the opto possibly be bad? Maybe some of the resistors are open?
    Use that Multimeter to check the resistors there really aren't that many on the board. But it is unlikely a resistor opened just by you fixing the jack.
    Just a what if, What if you removed the jack you repaired would the noise still be there? Completely desolder it remove it turn it on and see.
    nosaj

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    Lifetime Member Enzo's Avatar
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    Why are we so focused on the opto? For tests, we could just remove the opto, amp still noisy?

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    Old Timer J M Fahey's Avatar
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    I still have no clue what
    loud swooping distortion that sounds only right when you turn it on
    means and no,
    It’s half like a buzzer on a game show, half like a fuzzed-out guitar bending a whole step. It lasts about half a second.
    does not explain it at all.
    A YT video or it does not exist

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    Juan Manuel Fahey

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    Stray Cap DrGonz78's Avatar
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    Yeah sorry to get anyone thinking it might have been the opto, not my intention. I was more curious about the limiter function. My advice still stands to measure any DC offset voltage at the output while it makes the noise. I would also check for any related DC voltage at pin 2 just to know. My idea involves the amp waking up stupidly notion and instead of a turn on pop there is a different fizzy sound going on. Maybe I am way off but the simple video Juan suggested is a great idea.

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