Help Studer 169 Filter Section

[Shakusky]

Hi guys!

I'm restoring an old Studer 169. I've replaced  all electrolytic and tantalum caps with fresh ones but I still have trouble with the filter section in three channel modules. When I turn the Filter Switch on in one channel volume drops  significantly,  low Filter makes some crackling when I push it to the max and the mid EQ (called Presence/absence in the Studer manual) doesn't work.

In the other two, when I turn the filter on vumeters get crazy and sound doesn't show up (Its like some kind of short-circuit). I also replaced all switches with good quality ones. I'm suspecting that maybe could be faulty  ICS  2 and 3. Any help?  I attach the Filter EQ Schematics. Thank you so much in advance guys  !

Joan

 

[moamps]

Your attachment doesn't work for me. Could you resend it?

 

[Shakusky]

Yes sure!  Hope you can see it properly now. Thanks for your reply!

 

[mg73]

Have you tried switching out the IC's with ones from a working channel?

 

[moamps]

..... When I turn the Filter Switch on in one channel volume drops  significantly,  low Filter makes some crackling when I push it to the max ....when I turn the filter on vumeters get crazy and sound doesn't show up (Its like some kind of short-circuit)..

Most likely one IC oscillates, so as mg73 pointed out, try to replace ICs. That can be an easy fix.
Sometimes the pots are mechanically damaged what can be a bigger problem. Do you have a scope?

 

[Shakusky]

It worked!!! Faulty sluty ICS 2 and 3.  Swapped them with the ones from a working channel  and bingo!!! working like a charm!Thanks mg73 and moamps for your advice!!!! Its been a great help.  One last question if you don't mind. I want to replace this ICS with ones that have the same specs. The Studer Service manual it tells that this op amps  are 8 pin  LM301AN but nothing more.  Is there anything else I should consider? Bandwidth? Voltage range? Op amp brand?  I've checked in Farnell and LM301AN are no longer in production, but I found Texas Instruments  LM301AN/NOPB that it has the same specs.  So do you think I should be able to use the NOPB version as a direct replacement?

Thanks

 

[zamproject]

So do you think I should be able to use the NOPB version as a direct replacement?

Yes, just a "no lead" version, which is the rule today.
Best
Zam