CEM3381 Dual VCA

[J M Fahey]

..and responded to. I'll have to venture into the dustiest & darkest places of my Old Parts Emporium.

They-were-somewhere-around-here

[nosaj]

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

[J M Fahey]

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.

[nickb]

They-were-somewhere-around-here

Found them!!!



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

[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

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

[J M Fahey]

Oh, not critical at all, any will do.
I suggested 39k simply because there was another same value already there.

[nosaj]

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

[J M Fahey]

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).

[nosaj]

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

[nosaj]

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

..

[J M Fahey]

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.

[nosaj]

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.

nosaj

[nosaj]

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.

[compaid]

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