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
>
> 


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