Studer 089 line amp problem 7.091.36

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Aref

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Feb 4, 2015
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Hi everybody,
I'm struggling to fix my 089 mixer for many years now. Recently asked a tech expert to take a look at line amp boards.( I have one working on a mixer and few spares which are not working) Surprisingly, he believes the amp design is so bad which makes it so unstable and unreliable. He offered me to design a different amp in the same size format with the studer dla transformers.
I asked him to explain a bit to get some advice from experts.
He says that stablizing the middle point of the op amp with potentiometer is problematic in design. In his test, this middle point of the op amp (-11v) is changing when the board gets hot or when the gain is changing.
IMG-20220213-WA0022.jpg
IMG-20220213-WA0021.jpg
Please let me know if you have a solution for that.
Thanks
 

Aref

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More info from studer manual
 

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Aref

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It's very disappointing to hear that Zam...
He's an expert in repairing Studer gears. This is a better claim from himself;
"although the regulated -22 volt supplies the unit, during operation and heating and also changing the gain, the center point of supply ( -11 volts) varies considerably.
For more, I also changed all the parts of the unit totally ( except the mounted transformer), but this failure happened again."
Any idea?
 

moamps

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I serviced some older Studer consoles and racked some 089 modules, and in fact the biggest problem was the mechanical damage of few switches in the EQ section, but never that there was any instability. Can you describe in detail how the failure of that module manifests itself?
P.S. When you changed parts, did you change the LM301 or did you keep it?
 

Bo Deadly

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Using a separate network to bias the amp as opposed to using a proper signal 0V is not ideal. Also this circuit is of an era where op amps were just starting to become usable. I don't see any obvious reason to immediately reject your tech's claims. It's very possible that the tech's conclusions are incorrect but the path of least resistance might be to just use the offered alternative. I doubt it can have a negative impact on performance since, again, the circuit is not ideal for various aforementioned reasons.
 

abbey road d enfer

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It's very disappointing to hear that Zam...
He's an expert in repairing Studer gears. This is a better claim from himself;
"although the regulated -22 volt supplies the unit, during operation and heating and also changing the gain, the center point of supply ( -11 volts) varies considerably.
That maybe what he observes, but it shouldn't be so.
His failure is not being able to find what causes the drift.
I've never seen anything designed by Studer to fail operationally. It may not be the best sounding, but reliability is the proverbial Swiss.

For more, I also changed all the parts of the unit totally
Really, down to the last resistor and capacitor?
I would check the electrolytics first.
 
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zamproject

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May 11, 2010
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Hi Aref
It's very disappointing to hear that Zam...
He's an expert in repairing Studer gears.
Sorry to have been harsh and probably over concise without explanations in PM as in public here.

I just can't get how expert can have such claim about Studer engineering without contextualisation.
If such design is not his cup oft tea I'm fine with it, it's almost 50 years old and some expect different things/sound today but "unstable and unreliable" is just a lie.
Now if his agenda is to charge you with premium modern design as replacement it's up to you.

I'm not an expert audio electronic at all, it's more a side passion (and a way to keep my "daily job" tools alive), I lack lot of knowledge regarding electronic, especially related to audio design. But I restore/rebuilt about four x89 series, including 2 large 289 (and services few other more), one of them (289) being in service for 12 years 7/24 until 1990 ! at the final room for french national classical music radio.
THIS specific line amp probably daily wave hundred million ears in eastern europe in the 70' an 80'...so for sure it work.

the center point of supply ( -11 volts) varies considerably.
We need a value here, +/- 1mV or 5V ??? and from what ref ???

About heat instability, depending on past life of your console (use, storage, humidity), some carbon comp resistors can be tired...


I would check the electrolytics first.
That's for sure !!!
Those gold Frako caps are now in the 100% fail rate after such time.
Also everything up to 47u (and sometime 100u) in those consoles are tantalum, they should be cheeked too (and replaced when needed)

Cheers
Zam
 

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