OBERHEIM SYNTH

U503 is a CA3080, not a 3090!� I think a short between the control input 
and the output should not damage the 3080.

I would not replace chips blindly. Better measure signals and check 
whether the signals look reasonable:

do you have audio signal at pin 6 and 9 of theU502 (CEM3360)?
if no -> you have some completely different problem
if yes:
break the pcb line between U503-pin6/T501 on one side and 
U502-pin5-and-10 on the other side. Feed 1.5 Volt to pins 5 and 10. This 
should make the CEM3360 send out its signal (check at pins 2 and 13).
If no -> 3360 dead
If yes:
reconnect the split connection.
Check the voltages arount U503:
do you see a proper voltag at the "VOL" line (R509)?
do you see a proper voltage coming from the "Volpot" (R510)?
if no -> fix those
if yes
do you see something happening at pin6, when turning the main volume?
if no -> T501 might be shorted.

Am 19.01.2018 um 12:45 schrieb apu2010@... [oberheim]:
>
> Hi all,
>
> new to the board, proud owner of an OB8 wants to say hello and ask a 
> question.
>
> I recently calibrated my OB8 and all went fine. No at the end of the 
> procedure I was taking a look at the 3090 on one of the cards in the 
> output section (U503) and by a dumb move I shorted pins 5 and 6 :-( 
> Consequently after that whole voice card went quite. As I supposed I 
> killed off the chip in the process I replace the 3090 on the output 
> section but no sound anyway.
>
> As U503 is connected to the CEM3360 my suspicion was that it was 
> affected by that short, too. As I had a spare CEM I swapped it. The 
> problem now is that there is only a very, very low output on that 
> card. As the TL082 is last in line I thought it was not working either 
> so I replaced it, too but still the signal is low as before.
>
>
> I am a bit out of ideas for now as looking at the schematics there is 
> not much left to work on.
>
> The only thing that crossed my mind is that there is a 2n3905 a 
> ttached to the 3090 and that is connected to the VOL pot so maybe it 
> blew. What is its function in this?
>
> Another thing is that now on the upper board the panning is not 
> working anymore. I would suspect that the CEM3360 and the TL082 are to 
> be inspected. As I changed both of them again not sure whats left.
> Voices and tuning seem to work fine. (I suppose the U504/TL081 is 
> delivering the signal fromt the voices to the tuning unit?)
>
>
> BTW I have an oscilloscope and the schematics so if anyone could give 
> me an adivce what to look out for I would really appreciate.
> Thanks!
>
> JF
>
> 


-- 
http://www.florian-anwander.de

Well, since no replies yet… I’m not an expert but…

 

(Read all of this first) 

 

I think you should go through all the calibrations again, especially the output volume offset. 

 

If that doesn’t do it, compare the output level on the good board after the pan pots, to the same level on the bad board. If the bad one is really low in comparison, work backwards to find where that happens. 

 

Also, when you say 3090, do you mean CA3080? That’s what’s in mine for U503.

 

Next I was going to suggest looking at U504 but  I realized I need more information since that’s only the lower card.

 

Which card were you working on when this happened? Upper or lower?

 

Which card is now quiet? Upper or lower?

 

I’m going to guess this all happened to the upper board since since you didn’t mention checking U504, and that board is now the quiet one.

 

(If it IS the lower board, try U504.)

 

So now that I’ve looked at some pictures and figured out a bit more:

 

First, I’d check on that “3090”. It really should be a CA3080.

 

Second, I’d say there’s a chance the 3080 (if that’s what you actually used) isn’t working.

 

Third, maybe the replacement 3360 isn’t working, but it seems unlikely 2 would be dead.

 

Definitely do the calibrations again.

 

Could be the Q501 transistor.

 

And it could be U501 (which you changed).

 

Last I guess might be the electrolytic capacitors in the area.

 

Like I said, I’m not an expert, I’m just providing food for thought. But after looking at the schematics and pictures I’d say it has to be one of those things. But since you said you changed the 3360 and it went from dead to very quiet my guess is that calibration might do it. 

 

Also, make sure you try creating a sound in manual mode, then save it somewhere and go back and forth from old saved sounds to the new one. Sometimes memory gets corrupted and it looks like physical damage (I got a “dead” Matrix 6. Initialized a patch and suddenly it was alive! There was also a power supply problem that most likely corrupted the memory but that was an easy fix).

 

Let me know how it goes!

Scott

 

www.scottfromcanada.com

From: oberheim@yahoogroups.com [mailto:oberheim@yahoogroups.com] 
Sent: Friday, January 19, 2018 6:46 AM
To: oberheim@yahoogroups.com
Subject: [oberheim] Oberheim OB8 Panning and Voice Card issue

 

  

Hi all,

new to the board, proud owner of an OB8 wants to say hello and ask a question.

I recently calibrated my OB8 and all went fine. No at the end of the procedure I was taking a look at the 3090 on one of the cards in the output section (U503) and by a dumb move I shorted pins 5 and 6 :-( Consequently after that whole voice card went quite. As I supposed I killed off the chip in the process I replace the 3090 on the output section but no sound anyway.

As U503 is connected to the CEM3360 my suspicion was that it was affected by that short, too. As I had a spare CEM I swapped it. The problem now is that there is only a very, very low output on that card. As the TL082 is last in line I thought it was not working either so I replaced it, too but still the signal is low as before. 


I am a bit out of ideas for now as looking at the schematics there is not much left to work on.

The only thing that crossed my mind is that there is a 2n3905 attached to the 3090 and that is connected to the VOL pot so maybe it blew. What is its function in this? 

Another thing is that now on the upper board the panning is not working anymore. I would suspect that the CEM3360 and the TL082 are to be inspected. As I changed both of them again not sure whats left.
Voices and tuning seem to work fine. (I suppose the U504/TL081 is delivering the signal fromt the voices to the tuning unit?)


BTW I have an oscilloscope and the schematics so if anyone could give me an adivce what to look out for I would really appreciate.
Thanks!

JF