Help Studer 169 Filter Section

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

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

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

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

Shakusky said:

..... 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)..

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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?

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

Shakusky said:

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

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