CEM3381 Dual VCA

[nosaj]

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

[Enzo]

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.

[J M Fahey]

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 :

[nosaj]

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

[nosaj]

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

[Enzo]

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.

[J M Fahey]

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.

[nosaj]

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

[J M Fahey]

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"

[nosaj]

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

[nosaj]

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

[nosaj]

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

[J M Fahey]

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.

[nosaj]

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

[nosaj]

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