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Thread: CEM3381 Dual VCA

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    Old Timer nosaj's Avatar
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    CEM3381 Dual VCA

    Where might one find a CEM3381 Dual VCA or an equivalent?

    Thanks,
    nosaj

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    What are you using it in? That may provide some clues as to what might substitute.

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    Old Timer J M Fahey's Avatar
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    I bet Enzo has a couple in deep storage.
    It might take a Welsh coal miner to dig it out, of course

    EBay says:

    CEM3381 DUAL LINEAR VCA CHIP

    Essential IC for the following instruments:
    ADA MP-2, Alesis Microgate & Quadraverb,
    Doepfer A-100 series, Oberheim Echoplex
    Digital Pro, OB-MX, and more...

    Just checked datasheet, hoping function could be replaced by a commomn OTA or something (thimk LM13600) but no, itīs quite unique

    Hereīs datasheet:
    http://www.synfo.nl/datasheets/CEM3381-CEM3382.pdf

    If available, Iīd pay through the nose what seller wants.

    Just in case, are you sure itīs dead?
    Expensive/difficult part to be subject to shotgunning "just hoping it solves the problem".

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    Juan Manuel Fahey

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    Old Timer nosaj's Avatar
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    Quote Originally Posted by Mark Hammer View Post
    What are you using it in? That may provide some clues as to what might substitute.
    Baldwin gps2500 electric grandpiano IC7 on schematic http://www.shema.ru/cd/us/2/data/ps2500.pdf

    Thanks,
    nosaj

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    Old Timer nosaj's Avatar
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    Quote Originally Posted by J M Fahey View Post
    I bet Enzo has a couple in deep storage.
    It might take a Welsh coal miner to dig it out, of course

    EBay says:

    CEM3381 DUAL LINEAR VCA CHIP

    Essential IC for the following instruments:
    ADA MP-2, Alesis Microgate & Quadraverb,
    Doepfer A-100 series, Oberheim Echoplex
    Digital Pro, OB-MX, and more...

    Just checked datasheet, hoping function could be replaced by a commomn OTA or something (thimk LM13600) but no, itīs quite unique

    Hereīs datasheet:
    http://www.synfo.nl/datasheets/CEM3381-CEM3382.pdf

    If available, Iīd pay through the nose what seller wants.

    Just in case, are you sure itīs dead?
    Expensive/difficult part to be subject to shotgunning "just hoping it solves the problem".
    When tracing the signal with the scope getting no output on pins 1 and 7. Also on another note. Could I inject signal at pin1 and 7 to follow the signal through the rest of the circuit?


    Thanks,
    nosaj

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by nosaj View Post
    When tracing the signal with the scope getting no output on pins 1 and 7. Also on another note. Could I inject signal at pin1 and 7 to follow the signal through the rest of the circuit?
    Just a couple days ago we had a similar doubt on an amp or mixer,trying to see what a scope canīt actually show..

    IC7 has two VCA stages inside (it looks REALLY elegant and simple, now I understand why everybody and his Brother used them, all over the place, I would happily use them today if available) BUT , as datasheet says, are current input current output stages, so no voltage (so no scope image) either on inputs 3 and 5, or outputs 1 and 7.

    Want to see signal voltages? : scope either left side of R55 and R65 to check whether signal actually reaches this VCA block or IC11 out 1 and 7 .

    Also check that voltages are fine, meaning IC7 and IC11 get +/-15V rails, that there is no DC present on inputs and outputs.

    IMPORTANT: also check that IC7 is actually getting the Control Voltage it needs on pins 2 and 6 , it will NOT pass Audio if not ordered to. Check needed values (which might be voltage or current) in the datasheet.

    Not sure how itīs physically implemented, but schematic shows an option to disconnect it from IC11 mixer and drive 1k resistors instead R75-77 , by means of a couple jumpers JP1-2 .

    IF they are PC board type jumpers, itīs easy to flip them; if those are soldered links it might be messier.

    But I suspect that VCA block isnīt even being turned on, just a hunch.

    EDIT: just checked, Control Voltage on pins 2 and 6 goes from 0V (mute) to +5V (max gain), so : no control voltage=no audio.

    It comes from an external controller board which is beyond my scope.

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    Juan Manuel Fahey

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    Maybe the PA381 (lin control) or PA382 (exp control) may work - you need to check the spec sheets.

    This company shows them;
    https://www.dfsales.com/items/PA381

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    Old Timer nosaj's Avatar
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    Quote Originally Posted by J M Fahey View Post
    Just a couple days ago we had a similar doubt on an amp or mixer,trying to see what a scope canīt actually show..

    IC7 has two VCA stages inside (it looks REALLY elegant and simple, now I understand why everybody and his Brother used them, all over the place, I would happily use them today if available) BUT , as datasheet says, are current input current output stages, so no voltage (so no scope image) either on inputs 3 and 5, or outputs 1 and 7.

    Want to see signal voltages? : scope either left side of R55 and R65 to check whether signal actually reaches this VCA block or IC11 out 1 and 7 .

    Also check that voltages are fine, meaning IC7 and IC11 get +/-15V rails, that there is no DC present on inputs and outputs.

    IMPORTANT: also check that IC7 is actually getting the Control Voltage it needs on pins 2 and 6 , it will NOT pass Audio if not ordered to. Check needed values (which might be voltage or current) in the datasheet.

    Not sure how itīs physically implemented, but schematic shows an option to disconnect it from IC11 mixer and drive 1k resistors instead R75-77 , by means of a couple jumpers JP1-2 .

    IF they are PC board type jumpers, itīs easy to flip them; if those are soldered links it might be messier.

    But I suspect that VCA block isnīt even being turned on, just a hunch.

    EDIT: just checked, Control Voltage on pins 2 and 6 goes from 0V (mute) to +5V (max gain), so : no control voltage=no audio.

    It comes from an external controller board which is beyond my scope.
    Dumb question here Juan. Where does the voltage go? The resistor sucks up the voltage and lets current pass to the VCA?

    I will check this out shortly and report back.
    Thanks,
    nosaj

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    Old Timer J M Fahey's Avatar
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    Where does the voltage go? The resistor sucks up the voltage and lets current pass to the VCA?
    Which voltage?
    Audio or Controller?

    Which IC and what pins are we talking about?

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    Juan Manuel Fahey

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    Old Timer nosaj's Avatar
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    Quote Originally Posted by J M Fahey View Post
    Which voltage?
    Audio or Controller?

    Which IC and what pins are we talking about?
    This is what I was referring to "
    Quote Originally Posted by J M Fahey View Post
    Want to see signal voltages? : scope either left side of R55 and R65 to check whether signal actually reaches this VCA block or IC11 out 1 and 7"
    Thanks,
    nosaj

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    Last edited by tboy; 05-22-2018 at 03:55 AM. Reason: Quote cleanup.

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    Old Timer nosaj's Avatar
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    Quote Originally Posted by J M Fahey View Post
    But I suspect that VCA block isnīt even being turned on, just a hunch.

    EDIT: just checked, Control Voltage on pins 2 and 6 goes from 0V (mute) to +5V (max gain), so : no control voltage=no audio.

    It comes from an external controller board which is beyond my scope.
    Juan, Since I only have the poweramp board. I see no reason why I couldn't just use an external dc supply and turn it on?
    Thanks,
    nosaj

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    Last edited by tboy; 05-22-2018 at 03:55 AM. Reason: Deleted redundant quote.

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    Old Timer nosaj's Avatar
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    Quote Originally Posted by nosaj View Post
    But I suspect that VCA block isnīt even being turned on, just a hunch.

    EDIT: just checked, Control Voltage on pins 2 and 6 goes from 0V (mute) to +5V (max gain), so : no control voltage=no audio.

    It comes from an external controller board which is beyond my scope.

    Juan, Since I only have the poweramp board. I see no reason why I couldn't just use an external dc supply and turn it on?
    Thanks,
    nosaj

    EDIT: just checked, Control Voltage on pins 2 and 6 goes from 0V (mute) to +5V (max gain), so : no control voltage=no audio.

    It comes from an external controller board which is beyond my scope.
    Ok the Ic is a pa381.
    Wasn't sure of the voltages I was getting so I popped it out and checked the empty pin holes.
    Rails look good at -+12vdc Gonna go backwards on pins 2,5,and 6 as I shouldn't have any negative voltages there. On the schematic c73 connects to r53 what is the symbol for the other side of c73? Will check voltages there also .
    I suspect c75 is no good as well.

    Pin1 .001
    Pin2 -.356
    Pin3 1.30
    Pin4 -12
    Pin5 -.446
    Pin6 -.404
    Pin7 .005
    Pin8 12


    Thanks for any insights
    nosaj

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    Old Timer nosaj's Avatar
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    I just cannot figure where the DC is coming from to IC7.
    I subbed in new caps in c73 and c75
    And I still get the same voltages. I tried a trick from another post but not sure it applies to this IC.. Diode check on pins 1 and 3 and pins 5 and 7 I get readings both ways. 1and2 diode check ok. 5and6 diode check ok.

    Any insights anyone?

    Thanks,
    nosaj

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    Lifetime Member Enzo's Avatar
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    Not sure what you are chasing. Signal into the INPUT jacks on the right?

    Nothing out IC7? yes, of course you can inject signal after IC7.

    Note IC7 is controlled, see pins 2 and 6? Follow them over to the connector upper right, pin 7 MIC VOL. What control voltage have you put there.

    IC7 is a VCA - voltage controlled amplifier. That means it amplifies to the amount of control voltage you feed it. Check the data sheet.


    I suspect your odd half volt negative is just whatever the IC floats to.

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by nosaj View Post
    Ok the Ic is a pa381.
    Schematic mentions CEM3381 .
    Hope they are the same (they should)

    Just to refresh things up: these are VCA chips.
    Here are datasheet example and keyboard schematic side by side:

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    They work with current input, so they must get audio voltages through resistors, which "turn voltage into current"

    In datasheet example, inputs are 5 and 3 , get audio current through 24k resistors.
    They make it easy to mix and match different audio sources, just give each its own resistor.
    In keyboard, inputs are again 5 and 3, they get audio voltage present at points (1) and (2) through resistors R55 and R65.
    Do you have audio at (1) and (3)?

    These are VCA (Voltage controlled amplifiers) so they will amplify IF proper voltage is applied to control pins 2 and 6 .

    Datasheet says voltage goes from 0V (mute) to +5V (full volume)

    Keyboard schematic says that control voltage comes from an *external* controller, outside that board, through connector CN5 pin 7 , labelled, not surprisingly : "Mic Vol" .
    Said control voltage coming through track (3) is RC filtered by R53 C73 and reaches pins 2 and 6.
    Do you have +5V there? At least, say, 3V?
    "No control voltage no audio"

    IF you donīt have the external controller board, just send +5V yourself.
    Not sure they are high impedance pure voltage inputs or they take some current, but try a 15k/10k voltage divider from +12V to ground, connect junction to top of C73 and measure what you get, it should be around +5V
    If way lower, say 1 v or so, then it pulls significant current, Iīll suggest another divider or plain get +5V from somewhere else, if available.

    Wasn't sure of the voltages I was getting so I popped it out and checked the empty pin holes.
    Rails look good at -+12vdc Gonna go backwards on pins 2,5,and 6 as I shouldn't have any negative voltages there. On the schematic c73 connects to r53 what is the symbol for the other side of c73?
    European symbol for ground. Notice they use it all over that schematic.
    Will check voltages there also .
    I suspect c75 is no good as well.
    Why do you suspect C75?
    Ok, no chip pad voltages:
    Pin1 .001 check
    Pin2 -.356 not enough to turn volume on and should be exact same as pin 6 , they are joined by a track !!
    Pin3 1.30 you should have 0 voltage there (if no PA381 is present; if it is maybe itīs some internal bias voltage, there must be a reason its input is capacitor coupled, what DC voltage do you read at the other end of C80?)
    Pin4 -12 check
    Pin5 -.446 you should have 0 voltage there (if no PA381 is present; if it is maybe itīs some internal bias voltage, there must be a reason its input is capacitor coupled, what DC voltage do you read at the other end of C75?)
    Pin6 -.404 should be exact same as pin 2, see comment
    Pin7 .005 check
    Pin8 12 check
    I just cannot figure where the DC is coming from to IC7.
    I subbed in new caps in c73 and c75
    Hey, you said:
    I popped it out and checked the empty pin holes
    Are the above voltages with or without IC mounted? ??????????????

    And I still get the same voltages. I tried a trick from another post but not sure it applies to this IC.. Diode check on pins 1 and 3 and pins 5 and 7 I get readings both ways. 1and2 diode check ok. 5and6 diode check ok.
    Not sure about those tests, an IC is very complex, typical diode or transistor tests do not apply, except to confirm that inside is a molten blob of silicon (0 ohms among any 2 pins) which I hope is not the case.

    Fastest and most valid test is a **functional** one, parts are there for something, we check whether they do whatīs expected or not.

    Still not sure what the original complaint is: does the keyboard work but misses some function? / not work? / works but mic/line/aux input is mute? : pins 3 and 5 on Cn10, coming through ribbon from Cn3 in another board, from preamp and gain control IC3, from jacks "Input 1 , Input 2"

    IF you are troubleshooting just the power amp board, you must feed proper audio and control voltages to PA381 or itīs just a volume control set to 0.

    Inject audio and follow it, it should reach points (1) and (2), reappear at the other end at points (4) and (5), in the middle they are lost in the mists of "Current Land", your scope wonīt help you there.

    EDIT: IF you have a dead PA381 and it is ABSOLUTELY IMPOSSIBLE to get another (or they ask 100 bucks for it, same thing), I can suggest an "Argentine kludge" with *similar* functionality, volume will go from 0 to max with control voltage from 0 to +5 , not very linear but good enough to save keyboard from corner dumpster, all you need is a couple P Channel Fets and a couple thingies.

    But Iīm not yet sure the VCA is dead, go figure.

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    Last edited by J M Fahey; 05-31-2018 at 10:02 AM.
    Juan Manuel Fahey

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    Old Timer nosaj's Avatar
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    Quote Originally Posted by J M Fahey View Post
    Schematic mentions CEM3381 .
    Hope they are the same (they should)



    IF you are troubleshooting just the power amp board, you must feed proper audio and control voltages to PA381 or itīs just a volume control set to 0.

    Inject audio and follow it, it should reach points (1) and (2), reappear at the other end at points (4) and (5), in the middle they are lost in the mists of "Current Land", your scope wonīt help you there.

    EDIT: IF you have a dead PA381 and it is ABSOLUTELY IMPOSSIBLE to get another (or they ask 100 bucks for it, same thing), I can suggest an "Argentine kludge" with *similar* functionality, volume will go from 0 to max with control voltage from 0 to +5 , not very linear but good enough to save keyboard from corner dumpster, all you need is a couple P Channel Fets and a couple thingies.

    But Iīm not yet sure the VCA is dead, go figure.

    Why do you suspect C75?
    Ok, no chip pad voltages:
    Pin1 .001 check
    Pin2 -.356 not enough to turn volume on and should be exact same as pin 6 , they are joined by a track !!
    Pin3 1.30 you should have 0 voltage there (if no PA381 is present; if it is maybe itīs some internal bias voltage, there must be a reason its input is capacitor coupled, what DC voltage do you read at the other end of C80?)
    Pin4 -12 check
    Pin5 -.446 you should have 0 voltage there (if no PA381 is present; if it is maybe itīs some internal bias voltage, there must be a reason its input is capacitor coupled, what DC voltage do you read at the other end of C75?)
    Pin6 -.404 should be exact same as pin 2, see comment
    Pin7 .005 check
    Pin8 12 check
    New meter battery Pins 2 and 6 read the same right now.
    Pin3 read a -1.97 C80 -1.97 one side and -.008 on the other
    Pin5 reads -1.98 c75 -1.98 other side -7.20

    I guess what is throwning me is Mic Volume CN5 pin7 being a negative DC voltage.
    Tomorrow I can input +5v to the mic volume and follow the signal.

    Do you have an analogy for a resistor turning voltage into current? Earlier at some point I asked there the voltage went. You stated it in your reply, I'm just trying to wrap my head around the how.
    I am just troubleshooting the power amp board. initially leaking caps on the board which I cleaned up and replaced 2 tl072 ic's.

    Thank you so much for your help and all the other Giants here.
    When I think of Enzo it's always his avatar or his Santa Claus picture.
    You on the otherhand(Fahey) A Salvadore Dali figure comes to mind.

    Thanks guys,
    nosaj

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    Lifetime Member Enzo's Avatar
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    The circuit runs on what, +/-12v? So those control inputs are floating. They need to have a control voltage present. You are all focused on the voltage the unterminated pins float to. So they are half a volt negative instead of averaged at zero. So what? They are hanging in mid air - take control of them with a voltage.

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by nosaj View Post
    When I think of Enzo it's always his avatar or his Santa Claus picture.
    You on the otherhand(Fahey) A Salvadore Dali figure comes to mind.

    Thanks guys,
    nosaj
    Oh, not that "artist" looks but quite the boring average Engineer type.

    You want a picture? ... hereīs a fresh one :

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    Old Timer nosaj's Avatar
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    Quote Originally Posted by Enzo View Post
    The circuit runs on what, +/-12v? So those control inputs are floating. They need to have a control voltage present. You are all focused on the voltage the unterminated pins float to. So they are half a volt negative instead of averaged at zero. So what? They are hanging in mid air - take control of them with a voltage.
    Yay.. Input .115 volts and getting 1volt out on onme channel, will check the other channel later today. Wife wants the hot tub wire up so gotta get that first.

    Thanks guys and I'll update you.

    nosaj

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    Old Timer nosaj's Avatar
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    Quote Originally Posted by nosaj View Post
    Yay.. Input .115 volts and getting 1volt out on onme channel, will check the other channel later today. Wife wants the hot tub wire up so gotta get that first.

    Thanks guys and I'll update you.

    nosaj
    Other Channel Output on Right power amp is small. Tracing back at R55 Also shows on C75 I get what I believe is oscillation ( Looks like a AM signal on a scope. ) Do not remember seeing it at R108 will have to reverify. Gonna follow the traces to make sure they are good and nothing is wrong on the tracks.

    Thanks,
    Jason

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    Lifetime Member Enzo's Avatar
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    Does the other channel also have a non-terminated control voltage input?

    Juan or someone said the control voltage is 0-5vDC, so put 4 or 5 volts on that control input and I bet it is loud.

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    Old Timer J M Fahey's Avatar
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    We are dancing around without getting to the point, which is to know whether this Voltage Controlled Volume Control actually volumecontrols some signal fed to it.
    So feed it 1V RMS 1kHz at points (1) and (2), scope outputs (4) and (5) , you should find nothing there , then apply 0 to 5V to point (5) , use a 10k pot , signal should appear at points (4) and (5) following voltage fed at (3).

    Do that test and nothing else, and answer only the answer to that question. Please.

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    Old Timer nosaj's Avatar
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    Quote Originally Posted by J M Fahey View Post
    We are dancing around without getting to the point, which is to know whether this Voltage Controlled Volume Control actually volumecontrols some signal fed to it.
    So feed it 1V RMS 1kHz at points (1) and (2), scope outputs (4) and (5) , you should find nothing there , then apply 0 to 5V to point (5) , use a 10k pot , signal should appear at points (4) and (5) following voltage fed at (3).

    Do that test and nothing else, and answer only the answer to that question. Please.
    Can I use the pot on the variable DC supply or should I just it at 5v and then use the 10k pot to adjust it?

    Thanks,
    \nosaj

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  24. #24
    Old Timer J M Fahey's Avatar
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    IF you have available a power supply which can be adjusted between 0 and 5V thatīs the best solution.
    Connect it to ground and point (3) and vary it, while feeding audio at (1) and (2) , which may be parallel, and alternately scoping (4) and (5) to see what happens.

    I had suggested the 10k pot in case you used some existing fixed +5V supply.

    Something to check: remember those auxiliary inputs *can* be disabled and the option is shown straight in the schematic, as: "Audio/Video interface installed/not installed" depending on 2 jumper settings, Jp1 and Jp2.
    If "not installed" PA3381 outputs (which supply current) are derived to grounded resistors; R75/R77 , probably to not let them "floating"; if it IS installed, so signal appears at points (4) and (5), they must be linking PA381 outputs to TL072 inputs.
    Just for kicks check you have continuity between pins IC7-1 to IC11-2 and IC7-7 to IC11-6 ... what is shown in the schematic.

    What was the actual complaint ?
    What did de customer say when he left the keyboard?
    I bet he didnīt say: "signal dissappears on IC7 pin 3 and 5"

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    Last edited by J M Fahey; 06-04-2018 at 10:41 AM.
    Juan Manuel Fahey

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    Old Timer nosaj's Avatar
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    Quote Originally Posted by J M Fahey View Post
    We are dancing around without getting to the point, which is to know whether this Voltage Controlled Volume Control actually volumecontrols some signal fed to it.
    So feed it 1V RMS 1kHz at points (1) and (2), scope outputs (4) and (5) , you should find nothing there , then apply 0 to 5V to point (5) , use a 10k pot , signal should appear at points (4) and (5) following voltage fed at (3).

    Do that test and nothing else, and answer only the answer to that question. Please.
    This does exactly as you state, except for no signal at point 5.
    The input is not paralleled at cn10 pin3 and pin5, so I have to switch the input between the pins.
    jason

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  26. #26
    Old Timer nosaj's Avatar
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    Quote Originally Posted by J M Fahey View Post
    IF you have available a power supply which can be adjusted between 0 and 5V thatīs the best solution.
    Connect it to ground and point (3) and vary it, while feeding audio at (1) and (2) , which may be parallel, and alternately scoping (4) and (5) to see what happens.

    I had suggested the 10k pot in case you used some existing fixed +5V supply.

    Something to check: remember those auxiliary inputs *can* be disabled and the option is shown straight in the schematic, as: "Audio/Video interface installed/not installed" depending on 2 jumper settings, Jp1 and Jp2.
    If "not installed" PA3381 outputs (which supply current) are derived to grounded resistors; R75/R77 , probably to not let them "floating"; if it IS installed, so signal appears at points (4) and (%), they must be linking PA381 outputs to TL072 inputs.
    Just for kicks check you have continuity between pins IC7-1 to IC11-2 and IC7-7 to IC11-6 ... what is shown in the schematic.

    What was the actual complaint ?
    What did de customer say when he left the keyboard?
    I bet he didnīt say: "signal dissappears on IC7 pin 3 and 5"
    The audio/video is not install so signal arrives at point 4 and 5.
    There is contiunuity where you suggest.

    Actual complaint was a lot of hum, replaced the filter caps and 2 tl072's
    There was still residual hum after that.
    nosaj

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  27. #27
    Old Timer nosaj's Avatar
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    I think I found something but not quite sure what I found.
    Signal input CN10 pin5 I can trace signal to With the jumper removed. I can put the jumper back pin2 and 3 on and signal disappears like it grounds out.
    Guessing next step is to check contiuity from pin3 to ground. Got a lightning storm moving in so I'm done for tonight. Hot days here in FL seem to provoke the lightning in the evenings.

    Thanks for your Guidance.

    nosaj

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  28. #28
    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by nosaj View Post
    I think I found something but not quite sure what I found.
    Signal input CN10 pin5 I can trace signal to With the jumper removed. I can put the jumper back pin2 and 3 on and signal disappears like it grounds out.
    Guessing next step is to check contiuity from pin3 to ground. Got a lightning storm moving in so I'm done for tonight. Hot days here in FL seem to provoke the lightning in the evenings.

    Thanks for your Guidance.

    nosaj
    No,you are wasting time.

    You have **already** proven VCA works as expected, move on.

    Proof:

    * I asked:
    Just for kicks check you have continuity between pins IC7-1 to IC11-2 and IC7-7 to IC11-6 ... what is shown in the schematic.
    You answered:
    There is contiunuity where you suggest.
    so I am satisfied that Jumpers Jp1/Jp2 are installed in the "B" position, which DOES feed VCA output into the TL072
    All that remains is to confirm that:
    IF you have available a power supply which can be adjusted between 0 and 5V thatīs the best solution.
    Connect it to ground and point (3) and vary it, while feeding audio at (1) and (2) , which may be parallel, and alternately scoping (4) and (5) to see what happens.
    to which you do not answer in full:
    Signal input CN10 pin5 I can trace signal to With the jumper removed
    What does this mean?
    Did I ask you to do this?
    Did I tell you to remove the jumper?
    Do EXACTLY what I asked above and answer EXACTLY that.

    The "which may be parallel" bit means that you can momentarily jumper them together so you are not moving your signal generator between them, makes your life a little simpler but mainly avoids one possible mistake, that itīs working properly but you feed signal to point (1), scope point (5) and report "no signal found" ; or do same feeding to (2) and reading (4).

    This is already confusing, worse because it involves typed non real time remote control and to boot text communication does not seem to be working very well.

    In any case, if nobody complained about that VCA, Iīm donīt know what are you doing there.

    In fact, I *guess* (itīs incredible having to guess after 33 posts) that it works fine, but although you report some signal out (in an incomplete way), you donīt say a word about how it reacts to a 0>5V control input, not sure you even applied fixed 5V there.

    You are obsessed with signal dissappearing inside the VCA module, while all that matters is that signal reaches it at points (1) and (2) , and reappears at points (4) and(5), varying in level depending on voltage (between 0 and 5V) applied to point (3).
    Period: just that, only that and nothing but that so help you God.

    PS: when referring to IC pins name IC and pin, as in : "pins IC7-1" while "point (1)" refers to a Test Point **I** marked on the schematic.

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    Juan Manuel Fahey

  29. #29
    Old Timer nosaj's Avatar
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    Quote Originally Posted by J M Fahey View Post
    No,you are wasting time.

    You have **already** proven VCA works as expected, move on.

    Proof:

    * I asked:
    You answered:

    so I am satisfied that Jumpers Jp1/Jp2 are installed in the "B" position, which DOES feed VCA output into the TL072
    All that remains is to confirm that:

    to which you do not answer in full:

    What does this mean?
    Did I ask you to do this?
    Did I tell you to remove the jumper?
    Do EXACTLY what I asked above and answer EXACTLY that.

    The "which may be parallel" bit means that you can momentarily jumper them together so you are not moving your signal generator between them, makes your life a little simpler but mainly avoids one possible mistake, that itīs working properly but you feed signal to point (1), scope point (5) and report "no signal found" ; or do same feeding to (2) and reading (4).

    This is already confusing, worse because it involves typed non real time remote control and to boot text communication does not seem to be working very well.

    In any case, if nobody complained about that VCA, Iīm donīt know what are you doing there.

    In fact, I *guess* (itīs incredible having to guess after 33 posts) that it works fine, but although you report some signal out (in an incomplete way), you donīt say a word about how it reacts to a 0>5V control input, not sure you even applied fixed 5V there.

    You are obsessed with signal dissappearing inside the VCA module, while all that matters is that signal reaches it at points (1) and (2) , and reappears at points (4) and(5), varying in level depending on voltage (between 0 and 5V) applied to point (3).
    Period: just that, only that and nothing but that so help you God.

    PS: when referring to IC pins name IC and pin, as in : "pins IC7-1" while "point (1)" refers to a Test Point **I** marked on the schematic.
    Ok jumpered CN10 Pins 3and5 which are In1 and In2, Applied 1khz sine. Applied +5vDC to R53 (TP3) Scoped IC11-1 and varied the DC input(which makes the sine go up and down like a Volume pot) jumping over to IC11-7 no signal but varying the DC voltage The scope line stays flat but jumps to different points on the scope while staying flat.

    I hope I answered your questions without straying. Waiting on your reply before continuing.

    Thanks so much,

    Jason

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  30. #30
    Old Timer nosaj's Avatar
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    Quote Originally Posted by nosaj View Post
    Ok jumpered CN10 Pins 3and5 which are In1 and In2, Applied 1khz sine. Applied +5vDC to R53 (TP3) Scoped IC11-1 and varied the DC input(which makes the sine go up and down like a Volume pot) jumping over to IC11-7 no signal but varying the DC voltage The scope line stays flat but jumps to different points on the scope while staying flat.

    I hope I answered your questions without straying. Waiting on your reply before continuing.

    Thanks so much,

    Jason
    Anythoughts on what I'm seeing? Here's my thoughts on this IC7 pins are not working correctly. I tested IC11 by inputting 1k tone at jp1 bside and get signal at ic11pin7. Anything else I can try ?


    Thanks,
    Jason

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    Last edited by nosaj; 06-06-2018 at 01:30 AM.

  31. #31
    Old Timer nosaj's Avatar
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    Hi Juan,

    What did you have that could substitute for the PA381?
    Thanks,
    nosaj

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  32. #32
    Old Timer J M Fahey's Avatar
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    Sorry missed a couple posts, will answer in sequence even if later ones change the situation, so we leave no gaps in troubleshooting.
    Quote Originally Posted by nosaj View Post
    Ok jumpered CN10 Pins 3and5 which are In1 and In2, Applied 1khz sine. Applied +5vDC to R53 (TP3) Scoped IC11-1 and varied the DC input(which makes the sine go up and down like a Volume pot)
    OK, thatīs what I expected to see, the VCA IS functioning as expected, at least that half.
    As you see, it does not matter what happens "inside it", we have a "black box" called "VCA" and it does the function itīs supposed to.
    Now to the second half which in principle looks damaged:
    jumping over to IC11-7 no signal but varying the DC voltage The scope line stays flat but jumps to different points on the scope while staying flat.
    Ouch, looks like we have a damaged PA381 ... which of course is unavailable.

    As a last ditch test, replace IC11 TL072.
    Usually against shotgunning but itīs available, cheap, you certainly have a few in stock and it takes less than 5 minutes.
    Not much hope but since itīs so easy and quick

    Repeat above tests and if no good news, we go to Plan B: a crude but workable VCA which can reasonably do the deed.

    EDIT:
    now answering your second post:
    Anythoughts on what I'm seeing? Here's my thoughts on this IC7 pins are not working correctly. I tested IC11 by inputting 1k tone at jp1 bside and get signal at ic11pin7. Anything else I can try ?
    Ok, IC11 seems to be working, so .... we abandon the dead PA381 and since no replacements are available, we continue with Plan B.
    Let me edit the schematic so I include the Mod.
    stay tuned.

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  33. #33
    Old Timer nosaj's Avatar
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    Quote Originally Posted by J M Fahey View Post
    Sorry missed a couple posts, will answer in sequence even if later ones change the situation, so we leave no gaps in troubleshooting.

    OK, thatīs what I expected to see, the VCA IS functioning as expected, at least that half.
    As you see, it does not matter what happens "inside it", we have a "black box" called "VCA" and it does the function itīs supposed to.
    Now to the second half which in principle looks damaged:

    Ouch, looks like we have a damaged PA381 ... which of course is unavailable.

    As a last ditch test, replace IC11 TL072.
    Usually against shotgunning but itīs available, cheap, you certainly have a few in stock and it takes less than 5 minutes.
    Not much hope but since itīs so easy and quick

    Repeat above tests and if no good news, we go to Plan B: a crude but workable VCA which can reasonably do the deed.

    EDIT:
    now answering your second post:

    Ok, IC11 seems to be working, so .... we abandon the dead PA381 and since no replacements are available, we continue with Plan B.
    Let me edit the schematic so I include the Mod.
    stay tuned.
    Thanks so much Juan.

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  34. #34
    Old Timer J M Fahey's Avatar
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    Ok, hereīs the FET VCA:

    Click image for larger version. 

Name:	Nosaj03.gif 
Views:	38 
Size:	18.9 KB 
ID:	49131

    We ditch the dead PA381, we use the FET as a voltage controlled attenuator .
    Attenuator is a series resistor (in principle we leave the original 39K) and a FET to ground, working as a variable resistor.
    Its value will go from around 100 ohm (which is not perfect Zero but in any case *a lot* of attenuation, should be more than enough) to infinite, so no attenuation, when we vary Gate voltage from 0V to +5V.
    NOTE: since available control voltage is positive, we need a P channel Fet, not the more common N channel type.
    Most Fets dissappeared from the market, but J176 is still available.

    We pull the dead PA381 but use its PCB holes as reference and we can mount a couple "new component" legs on them; extra component legs should be tack soldered to needed pads, if already occupied.
    I guess you will have to add the two J176 from below, the copper side, should not be a problem.
    If too close to chassis, then you will have to solder it from above, do whatīs most comfortable to you.

    This will vary volume as expected, maybe not as smoothly, but acceptable.

    If level is too high or too low, gain can be adjusted; with my values it "should" be about unity gain with +5V, which I guess was the original gain, but if not, itīs adjustable, up or down, varying a couple resistors.

    Ok, get the P Fets and try it.
    Good luck.

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  35. #35
    Old Timer nosaj's Avatar
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    Quote Originally Posted by J M Fahey View Post
    Ok, hereīs the FET VCA:

    Click image for larger version. 

Name:	Nosaj03.gif 
Views:	38 
Size:	18.9 KB 
ID:	49131

    We ditch the dead PA381, we use the FET as a voltage controlled attenuator .
    Attenuator is a series resistor (in principle we leave the original 39K) and a FET to ground, working as a variable resistor.
    Its value will go from around 100 ohm (which is not perfect Zero but in any case *a lot* of attenuation, should be more than enough) to infinite, so no attenuation, when we vary Gate voltage from 0V to +5V.
    NOTE: since available control voltage is positive, we need a P channel Fet, not the more common N channel type.
    Most Fets dissappeared from the market, but J176 is still available.

    We pull the dead PA381 but use its PCB holes as reference and we can mount a couple "new component" legs on them; extra component legs should be tack soldered to needed pads, if already occupied.
    I guess you will have to add the two J176 from below, the copper side, should not be a problem.
    If too close to chassis, then you will have to solder it from above, do whatīs most comfortable to you.

    This will vary volume as expected, maybe not as smoothly, but acceptable.

    If level is too high or too low, gain can be adjusted; with my values it "should" be about unity gain with +5V, which I guess was the original gain, but if not, itīs adjustable, up or down, varying a couple resistors.

    Ok, get the P Fets and try it.
    Good luck.
    2 pchannel jfets and 2 100k resistors for r74 and r76 looks like the only changes on your schematic that I'm seeing.

    Will check parts I have shortly. Fooey..all I have are N channel Jfets, just ordered some off the bay Vishay j176 50 for $10.


    Thanks Juan

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    Last edited by nosaj; 06-06-2018 at 04:07 AM.

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