CEM3381 Dual VCA

[nosaj]

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

Thanks,
nosaj

[Mark Hammer]

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

[J M Fahey]

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".

[nosaj]

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

[nosaj]

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

[J M Fahey]

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.

[Mick Bailey]

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

[nosaj]

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

[J M Fahey]

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?

[nosaj]

Which voltage?
Audio or Controller?

Which IC and what pins are we talking about?

This is what I was referring to "

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

[nosaj]

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

[nosaj]

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

[nosaj]

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

[Enzo]

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.

[J M Fahey]

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:





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.