Page 2 of 2 FirstFirst 12
Results 36 to 59 of 59

Thread: CEM3381 Dual VCA

  1. #36
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    30,934
    Thumbs Up/Down
    Received: 887/5
    Given: 0/0
    I checked my inventory sheet, only Curtis chips I stock are:
    CEM3340, 3360, 3372, and 3394.

    But the thought occurs to me, didn't you say you ONLY have the amp module and will be using it alone - not with the rest of the keyboard?

    If so, then why have the remote voltage control over volume in the first place? Replace the VAC with a pot.

    0 Not allowed! Not allowed!
    Education is what you're left with after you have forgotten what you have learned.

  2. #37
    Master Destroyer nosaj's Avatar
    Join Date
    Sep 2011
    Location
    Pace, FL
    Posts
    2,133
    Thumbs Up/Down
    Received: 450/100
    Given: 71/32
    Quote Originally Posted by Enzo View Post
    I checked my inventory sheet, only Curtis chips I stock are:
    CEM3340, 3360, 3372, and 3394.

    But the thought occurs to me, didn't you say you ONLY have the amp module and will be using it alone - not with the rest of the keyboard?

    If so, then why have the remote voltage control over volume in the first place? Replace the VAC with a pot.
    No as soon as it's working I want to put it back in the piano. I only just have the power amp right now.

    Jason

    0 Not allowed! Not allowed!

  3. #38
    Master Destroyer nosaj's Avatar
    Join Date
    Sep 2011
    Location
    Pace, FL
    Posts
    2,133
    Thumbs Up/Down
    Received: 450/100
    Given: 71/32
    Quote Originally Posted by J M Fahey View Post
    Ok, hereīs the FET VCA:

    Attachment 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.
    Why would I need to put jfets on opposite side of board. i was thinking about using a 8pin DIP socket to just plug the jfets into. That should be fine I would think, no?

    nosaj

    0 Not allowed! Not allowed!

  4. #39
    "Thermionic Apocalypse" -JT nickb's Avatar
    Join Date
    Dec 2009
    Location
    Devon, UK
    Posts
    3,008
    Thumbs Up/Down
    Received: 281/1
    Given: 267/1
    Kinda missed this thread. I'm 95% sure I have some PA381's if you still want one...

    0 Not allowed! Not allowed!
    Experience is something you get, just after you really needed it.

  5. #40
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    30,934
    Thumbs Up/Down
    Received: 887/5
    Given: 0/0
    There y'go^^^


    Why would I need to put jfets on opposite side of board. i was thinking about using a 8pin DIP socket to just plug the jfets into. That should be fine I would think, no?
    It is the circuit that matters, put the parts where you like.

    0 Not allowed! Not allowed!
    Education is what you're left with after you have forgotten what you have learned.

  6. #41
    Old Timer J M Fahey's Avatar
    Join Date
    Oct 2007
    Location
    Buenos Aires, Argentina
    Posts
    10,413
    Thumbs Up/Down
    Received: 560/15
    Given: 456/13
    Quote Originally Posted by nosaj View Post
    Why would I need to put jfets on opposite side of board. i was thinking about using a 8pin DIP socket to just plug the jfets into. That should be fine I would think, no?

    nosaj
    It was not an "order" but a "suggestion", accompanied by "do whatīs most comfortable to you."
    That said:
    1) I donīt trust IC sockets and even less here, they wonīt firmly grab the thin Fet legs.
    2) in any case some Fet connections are NOT to IC holes/pads anyway.
    3) in holes 3 and 5 youīll need to insert BOTH a resistor leg and a Fet leg .... hard to solve with an IC socket.
    4) top Fet Source goes to R77 ground pad (on bottom) or scratched and tinned top leg ... your choice.
    5) bottom Fet Source goes to R75 ground pad (on bottom) or scratched and tinned top leg ... your choice.
    6) thereīs more than one way to skin a cat ... I suggest a couple just in case
    7) just read nickbīs post ... if an original is available, by all means use it

    0 Not allowed! Not allowed!
    Juan Manuel Fahey

  7. #42
    Master Destroyer nosaj's Avatar
    Join Date
    Sep 2011
    Location
    Pace, FL
    Posts
    2,133
    Thumbs Up/Down
    Received: 450/100
    Given: 71/32
    Quote Originally Posted by J M Fahey View Post
    It was not an "order" but a "suggestion", accompanied by "do whatīs most comfortable to you."
    That said:
    1) I donīt trust IC sockets and even less here, they wonīt firmly grab the thin Fet legs.
    2) in any case some Fet connections are NOT to IC holes/pads anyway.
    3) in holes 3 and 5 youīll need to insert BOTH a resistor leg and a Fet leg .... hard to solve with an IC socket.
    4) top Fet Source goes to R77 ground pad (on bottom) or scratched and tinned top leg ... your choice.
    5) bottom Fet Source goes to R75 ground pad (on bottom) or scratched and tinned top leg ... your choice.
    6) thereīs more than one way to skin a cat ... I suggest a couple just in case
    7) just read nickbīs post ... if an original is available, by all means use it
    i just didn't understand quite why 3 explained it for me.

    Just waiting for nickb response, jfets are on the way.Can't hurt to have parts to instantly try something instead of waiting in the future.

    Thanks all.

    nosaj

    0 Not allowed! Not allowed!

  8. #43
    Master Destroyer nosaj's Avatar
    Join Date
    Sep 2011
    Location
    Pace, FL
    Posts
    2,133
    Thumbs Up/Down
    Received: 450/100
    Given: 71/32
    Quote Originally Posted by nickb View Post
    Kinda missed this thread. I'm 95% sure I have some PA381's if you still want one...
    That's a definite PM sent with my info.
    Thanks,
    Jason

    0 Not allowed! Not allowed!

  9. #44
    "Thermionic Apocalypse" -JT nickb's Avatar
    Join Date
    Dec 2009
    Location
    Devon, UK
    Posts
    3,008
    Thumbs Up/Down
    Received: 281/1
    Given: 267/1
    Quote Originally Posted by nosaj View Post
    That's a definite PM sent with my info.
    Thanks,
    Jason
    ..and responded to. I'll have to venture into the dustiest & darkest places of my Old Parts Emporium.

    0 Not allowed! Not allowed!
    Experience is something you get, just after you really needed it.

  10. #45
    Old Timer J M Fahey's Avatar
    Join Date
    Oct 2007
    Location
    Buenos Aires, Argentina
    Posts
    10,413
    Thumbs Up/Down
    Received: 560/15
    Given: 456/13
    Quote Originally Posted by nosaj View Post
    i just didn't understand quite why 3 explained it for me.

    Just waiting for nickb response, jfets are on the way.Can't hurt to have parts to instantly try something instead of waiting in the future.

    Thanks all.

    nosaj
    P-Fets are an ignored and very useful tool, great advantage being they can be used as switches or attenuators being contolled by **positive** voltages, which are more easily available (specially on pedals ) .

    Look at Phasers; in order to use more common (specially way back then) N Fets as variable phase shifters, they are *forced* to "elevate" Sources so Gates can be made "negative" (actually "less positive" ) for control.

    P Fets provide elegant simple solutions, they can be straight driven by, say, Logic voltages, output of microprocessors, etc. , which can provide *any* voltage you imagine ..... between 0 and +5V that is

    I always have at least a dozen handy, saved my bacon many times.

    0 Not allowed! Not allowed!
    Juan Manuel Fahey

  11. #46
    Old Timer J M Fahey's Avatar
    Join Date
    Oct 2007
    Location
    Buenos Aires, Argentina
    Posts
    10,413
    Thumbs Up/Down
    Received: 560/15
    Given: 456/13
    Quote Originally Posted by nickb View Post
    ..and responded to. I'll have to venture into the dustiest & darkest places of my Old Parts Emporium.
    They-were-somewhere-around-here


    0 Not allowed! Not allowed!
    Juan Manuel Fahey

  12. #47
    Master Destroyer nosaj's Avatar
    Join Date
    Sep 2011
    Location
    Pace, FL
    Posts
    2,133
    Thumbs Up/Down
    Received: 450/100
    Given: 71/32
    Quote Originally Posted by J M Fahey View Post
    They-were-somewhere-around-here
    FETS are here will wire them up in the morning. Still waiting on response from Nickb on payment. Never had to figure foreign exchanges before.

    nosaj

    0 Not allowed! Not allowed!

  13. #48
    Old Timer J M Fahey's Avatar
    Join Date
    Oct 2007
    Location
    Buenos Aires, Argentina
    Posts
    10,413
    Thumbs Up/Down
    Received: 560/15
    Given: 456/13
    Quote Originally Posted by nosaj View Post
    FETS are here will wire them up in the morning. Still waiting on response from Nickb on payment. Never had to figure foreign exchanges before.

    nosaj
    https://www.x-rates.com/calculator/?...o=USD&amount=1

    Rather than exchange rate, problem is how to wire them to a physical person.

    Shops or EBay sellers already have setup some system, maybe Paypal or similar.

    The lowest/simplest system, quite crude and which has some minimum fees no matter what you transfer, is Western Union or Moneygram.

    Designed for and most used by undocumented aliens (no kidding) who work "here" and want to send money to Mom or Family "there" , works like this:

    sender deposits money, plus , say, 2% to 5% (so far, "nothing", unless you are transfering, say, U$10000) plus a fixed fee, of, say, 5 or 6 U$, which is nothing on a large amount, a lot if you have to transfer $3.

    You mail receiver what office address you sent $ to, how much, who sent it (may be a nickname), all this as a crude security system, and sometimes a serial number or password, all of which will be asked by cashier at the receiving end.

    Actually simpler than it sounds, and works very well for small to mid amounts, say less than $100.

    We are "Paradise Country" compared to all around us, so we are chock full of undocumented immigrants from all of Latin America (even Colombians and Venezuelans who are actually quite far away) , refugee Africans (Nigerians, Camerunese), Syrians, Koreans, Chinese, Ukrainians, the works, all use this system so itīs well oiled and works flawlessly.
    Even for Dominican prostitutes .... no kidding.

    Being US created and run Services, they should work well with UK.

    0 Not allowed! Not allowed!
    Juan Manuel Fahey

  14. #49
    "Thermionic Apocalypse" -JT nickb's Avatar
    Join Date
    Dec 2009
    Location
    Devon, UK
    Posts
    3,008
    Thumbs Up/Down
    Received: 281/1
    Given: 267/1
    Quote Originally Posted by J M Fahey View Post
    They-were-somewhere-around-here


    Found them!!!

    Click image for larger version. 

Name:	PA381.jpg 
Views:	32 
Size:	275.3 KB 
ID:	49214

    Jason: I'll send you a Paypal doohickey. That will take care of exchange rate calcs.

    0 Not allowed! Not allowed!
    Experience is something you get, just after you really needed it.

  15. #50
    Master Destroyer nosaj's Avatar
    Join Date
    Sep 2011
    Location
    Pace, FL
    Posts
    2,133
    Thumbs Up/Down
    Received: 450/100
    Given: 71/32
    Quote Originally Posted by J M Fahey View Post
    It was not an "order" but a "suggestion", accompanied by "do whatīs most comfortable to you."
    That said:
    1) I donīt trust IC sockets and even less here, they wonīt firmly grab the thin Fet legs.
    2) in any case some Fet connections are NOT to IC holes/pads anyway.
    3) in holes 3 and 5 youīll need to insert BOTH a resistor leg and a Fet leg .... hard to solve with an IC socket.
    4) top Fet Source goes to R77 ground pad (on bottom) or scratched and tinned top leg ... your choice.
    5) bottom Fet Source goes to R75 ground pad (on bottom) or scratched and tinned top leg ... your choice.
    6) thereīs more than one way to skin a cat ... I suggest a couple just in case
    7) just read nickbīs post ... if an original is available, by all means use it
    Juan, part is one the way from nickb.
    But on another how critical in your VCA replacement is the 39k resistors? I have 33k then 47k's.

    nosaj

    0 Not allowed! Not allowed!

  16. #51
    Old Timer J M Fahey's Avatar
    Join Date
    Oct 2007
    Location
    Buenos Aires, Argentina
    Posts
    10,413
    Thumbs Up/Down
    Received: 560/15
    Given: 456/13
    Oh, not critical at all, any will do.
    I suggested 39k simply because there was another same value already there.

    0 Not allowed! Not allowed!
    Juan Manuel Fahey

  17. #52
    Master Destroyer nosaj's Avatar
    Join Date
    Sep 2011
    Location
    Pace, FL
    Posts
    2,133
    Thumbs Up/Down
    Received: 450/100
    Given: 71/32
    Quote Originally Posted by J M Fahey View Post
    Oh, not critical at all, any will do.
    I suggested 39k simply because there was another same value already there.
    This is the jfet setup you suggested. The replacement chip is on the way from nickb. While waiting I'm considering breadboarding up the pa381 I do have.
    I wired them in used 33k instead of 39k. Still not picking up signal on IC11 pin7. If I remove jp1 I get signal on the middle pin. Now If I move the scope into the millivolt I can see a litttle signal there. But it is fuzzy looking. I decided to check voltage at jp1 no dc on pins 1 and 2 on pin3 getting 4mvDC. To me it looks like IC12 might be the only place to pick that up at.

    Any suggestions or am I wasting time again?

    Thanks,
    nosaj

    0 Not allowed! Not allowed!

  18. #53
    Old Timer J M Fahey's Avatar
    Join Date
    Oct 2007
    Location
    Buenos Aires, Argentina
    Posts
    10,413
    Thumbs Up/Down
    Received: 560/15
    Given: 456/13
    Quote Originally Posted by nosaj View Post
    This is the jfet setup you suggested. The replacement chip is on the way from nickb. While waiting I'm considering breadboarding up the pa381 I do have.
    I wired them in used 33k instead of 39k. Still not picking up signal on IC11 pin7. If I remove jp1 I get signal on the middle pin. Now If I move the scope into the millivolt I can see a litttle signal there. But it is fuzzy looking. I decided to check voltage at jp1 no dc on pins 1 and 2 on pin3 getting 4mvDC. To me it looks like IC12 might be the only place to pick that up at.

    Any suggestions or am I wasting time again?

    Thanks,
    nosaj
    Where? I see no image in this post.

    EDIT: SUPPOSING you are talking abot the edited schematic where I showed the P Fets replacing the PA381:



    1) are you injecting signal into point(2) ? .... Y/N
    Say, 100mV 1kHz.

    2) If you scope there, do you see 100mV 1kHz there? ..... Y/N

    3) are you applying +5V into point (3)? .... Y/N

    4) scope the right side of R55 , any leg of C75 , bottom J175 Drain leg, left leg of added "39K" (33k?, 47k? .... same thing) ... you should have signal, about 50mV, on ALL of those points ... do you? .... Y/N
    IF signal present on some but not all ... where?

    5) measure DC at Gate legs on both P Fets ... do you have +5V there? ... if not, what DCV you measure there?

    6) test the FET VCA effect: varying voltage at point (3) from 0 to +5V ... does audio voltage vary on points mentioned in question 4) ?

    Thanks.

    EDIT 2: notice I am not even *asking* about what happens on IC11 (yet) , not even at JP2 (same thing).

    0 Not allowed! Not allowed!
    Last edited by J M Fahey; 06-13-2018 at 04:44 AM.
    Juan Manuel Fahey

  19. #54
    Master Destroyer nosaj's Avatar
    Join Date
    Sep 2011
    Location
    Pace, FL
    Posts
    2,133
    Thumbs Up/Down
    Received: 450/100
    Given: 71/32
    1) are you injecting signal into point(2) ? .... Y/N
    Say, 100mV 1kHz.
    Yes

    2) If you scope there, do you see 100mV 1kHz there? ..... Yes

    3) are you applying +5V into point (3)? .... Yes

    4) scope the right side of R55 , any leg of C75 , bottom J175 Drain leg, left leg of added "39K" (33k?, 47k? .... same thing) ... you should have signal, about 50mV, on ALL of those points ... do you? .... Yes
    IF signal present on some but not all ... where?

    5) measure DC at Gate legs on both P Fets ... do you have +5V there? ...yes

    6) test the FET VCA effect: varying voltage at point (3) from 0 to +5V ... does audio voltage vary on points mentioned in question 4) ? Yes

    Thanks.

    EDIT 2: notice I am not even *asking* about what happens on IC11 (yet) , not even at JP2 (same thing).

    So your Jfet replacement works just like you want Good signal at IC11pin7. It all looks good inputting at R55. So I move injection point back to CN10 pin3 and 5 jumpered. And IC11pin7 goes back to almost no signal. So between cn10 pin5 and r55 something is killing signal.

    Thanks for your help Juan.

    nosaj

    1 Not allowed! Not allowed!

  20. #55
    Master Destroyer nosaj's Avatar
    Join Date
    Sep 2011
    Location
    Pace, FL
    Posts
    2,133
    Thumbs Up/Down
    Received: 450/100
    Given: 71/32
    Quote Originally Posted by nosaj View Post
    1) are you injecting signal into point(2) ? .... Y/N
    Say, 100mV 1kHz.
    Yes

    2) If you scope there, do you see 100mV 1kHz there? ..... Yes

    3) are you applying +5V into point (3)? .... Yes

    4) scope the right side of R55 , any leg of C75 , bottom J175 Drain leg, left leg of added "39K" (33k?, 47k? .... same thing) ... you should have signal, about 50mV, on ALL of those points ... do you? .... Yes
    IF signal present on some but not all ... where?

    5) measure DC at Gate legs on both P Fets ... do you have +5V there? ...yes

    6) test the FET VCA effect: varying voltage at point (3) from 0 to +5V ... does audio voltage vary on points mentioned in question 4) ? Yes

    Thanks.

    EDIT 2: notice I am not even *asking* about what happens on IC11 (yet) , not even at JP2 (same thing).

    So your Jfet replacement works just like you want Good signal at IC11pin7. It all looks good inputting at R55. So I move injection point back to CN10 pin3 and 5 jumpered. And IC11pin7 goes back to almost no signal. So between cn10 pin5 and r55 something is killing signal.

    Thanks for your help Juan.

    nosaj
    ..

    0 Not allowed! Not allowed!

  21. #56
    Old Timer J M Fahey's Avatar
    Join Date
    Oct 2007
    Location
    Buenos Aires, Argentina
    Posts
    10,413
    Thumbs Up/Down
    Received: 560/15
    Given: 456/13
    So your Jfet replacement works just like you want Good signal at IC11pin7. It all looks good inputting at R55.
    Cool.
    So FET VCA IS working as intended.

    So I move injection point back to CN10 pin3 and 5 jumpered. And IC11pin7 goes back to almost no signal. So between cn10 pin5 and r55 something is killing signal.
    Ok, youīll have to trace that, but now itīs standard troubleshooting: inject signal at connector, **check that itīs still there** (connector or following track might be shorted to ground) and scope along the path.

    You may have a cracked open track anywhere along the path; most suspect points are near Cn10 itself or R55 (less likely but not impossible).

    Connectors are often yanked and a crack develops between one of its pads and tracks leading away from them, or one might have developed where R55 pad receives a track.
    You may scratch solder mask about halfway the track joining both and check.

    0 Not allowed! Not allowed!
    Juan Manuel Fahey

  22. #57
    Master Destroyer nosaj's Avatar
    Join Date
    Sep 2011
    Location
    Pace, FL
    Posts
    2,133
    Thumbs Up/Down
    Received: 450/100
    Given: 71/32
    Quote Originally Posted by J M Fahey View Post
    Cool.
    So FET VCA IS working as intended.


    Ok, youīll have to trace that, but now itīs standard troubleshooting: inject signal at connector, **check that itīs still there** (connector or following track might be shorted to ground) and scope along the path.

    You may have a cracked open track anywhere along the path; most suspect points are near Cn10 itself or R55 (less likely but not impossible).

    Connectors are often yanked and a crack develops between one of its pads and tracks leading away from them, or one might have developed where R55 pad receives a track.
    You may scratch solder mask about halfway the track joining both and check.
    Found it. The junction between r55 to r56,r57 was not there. Fixed the track and with .3v 1kz in at cn10 pins 2 and 5 I get 6.5v out at CN4 left and right channels. Which sounds good to me as this is just the power amp and not the preamp.

    Juan Thanks so much for sticking with me. When the pa381 gets here I guess I will just put it in my parts bin. Your clever solution rocks and I hope to implement some of the j176's in other projects.

    nosajClick image for larger version. 

Name:	piano.jpg 
Views:	20 
Size:	142.8 KB 
ID:	49341

    1 Not allowed! Not allowed!

  23. #58
    Master Destroyer nosaj's Avatar
    Join Date
    Sep 2011
    Location
    Pace, FL
    Posts
    2,133
    Thumbs Up/Down
    Received: 450/100
    Given: 71/32
    Returned it to it's rightful place in the piano all is working good on the amplifier now. No display at all on So I gotta look at that. But it's playing.

    Thanks,
    nosaj


    Doe anyone know how to test a cold cathode florescent display? Here is the datasheethttp://www.beyondinfinite.com/lcd/Li...i/SP14Q001.pdf
    The dc/ac converter had q1 q2 c2 and sp1(fuse ) were burnt up. I replaced them but no display yet.

    0 Not allowed! Not allowed!
    Last edited by nosaj; 06-16-2018 at 10:51 PM.

  24. #59
    New Member
    Join Date
    Oct 2018
    Posts
    1
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    the hard to find CEM3381 is replaced by an identical chip PDA 381 /PA381 pin for pin. Its also a fraction of the cost. Not rare. Just remove the old one, put in a socket and the new chip.
    https://www.dfsales.com/items/PA381

    0 Not allowed! Not allowed!

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. VCA noise
    By Mick Bailey in forum Music Electronics
    Replies: 19
    Last Post: 07-05-2016, 08:14 AM
  2. Old Dual-Pro type dual OT
    By EFK in forum Theory & Design
    Replies: 3
    Last Post: 11-28-2012, 03:14 PM
  3. 18W Footswitchable Dual Channel Dual Cascade Head Build
    By ForcedFire in forum Build Your Amp
    Replies: 6
    Last Post: 12-02-2010, 08:06 PM
  4. Roland Juno-106 VCO/VCA/VCF issues.....
    By jrfrond in forum Music Electronics
    Replies: 8
    Last Post: 05-28-2010, 04:05 PM
  5. Replies: 0
    Last Post: 06-25-2007, 11:19 PM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •