Need help with a line amp (studer dual line amp)

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Aref

Well-known member
Joined
Feb 4, 2015
Messages
60
Hello,
I'm in the learning process of electronics since I got an old 089 studer mixer. Most parts recapped and cleaned. By an accident, I damaged the dual line amp board and now one channel sounds ringing at higher levels. I have a few extra dual line amp boards, but all with one faulty channel.

So now I need your help to understand this line amp in order to fix them.

I have a multimeter, oscilloscope, little soldering skill, knowing components and what they do, learned op-amps, transistors....also made an extention cable in order to work on the board while it's working!
Please help me to learn it...
 
The pcb
 

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Sorry, this is the schem
 

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Given it's a dual line amp, and only one channel appears faulty on each card, I'd start by feeding an identical signal to both channels and probing identical spots on each channel to try to narrow down where the trouble is.

There aren't too many active devices here. Assuming you don't see any visible damage (scorched resistors, etc...), And don't have any tantalums that failed short, it ought not take too much to narrow things down.
 
Components seems allright visually. I don't know how to check tantals on the board.
Once I tried to follow the signal path with the oscope and don't know why suddenly that channel stopped working!
 
Aref said:
Components seems allright visually.

Doesn't mean anything really

Aref said:
I don't know how to check tantals on the board.

Same way you would regular capacitors. Do you have a capacitance or lcr meter????

Aref said:
Once I tried to follow the signal path with the oscope and don't know why suddenly that channel stopped working!

Are you saying you have no working boards now? Like bjoneson said, if you have a working channel, it should be pretty easy to check where things go wrong. Just study the schematic and be careful where you probe ....

Plenty of checks can be done with no power to the boards too.....

You can test the transistors using the diode function for example........

 
scott2000 said:
Same way you would regular capacitors. Do you have a capacitance or lcr meter????
I have a multimeter with a capacitence meter option, but I'm not sure if it's precise enough.
Can I measure them on the board with no power?
Numbers seem so different. For instance 220uf16v caps are about 600uf to 800uf!

I have some working spares, but as I said I had bad experience last time.

You can test the transistors using the diode function for example........
Can you describe how to check the transistors without taking them out?
 
> follow the signal path with the oscope and don't know why suddenly that channel stopped working!

Do you remember WHAT you poked to kill it?

One possibility: the '301 opamp's normal inputs are not used. Instead Q103 Q104 bypass the '301's IN transistors and feed the BAL pins which go to the 2nd stage. The BAL pins have NO protection against clumsy poking. Shorting to adjacent pin, or even stray static, will blow-out the guts of the '301.

There's no alternative to a '301 used this way (except the '101/'201 Mil/Commercial up-rated parts). As the '301 is End Of Life, you should stock-up while you can. Mouser has some in DIP and even in pretty metal cans.
 
PRR said:
As the '301 is End Of Life, you should stock-up while you can. Mouser has some in DIP and even in pretty metal cans.

Hi PRR

Where did you see that ?
At TI it's still active ... no EOL so far
But tks anyway for the warning, I still have 3 console massively using those and as it's a 40+ years op amp it will be EOL one day...
I should have more than dozen spare but will pick more at the next order !

For the OP, as already stated, usual suspect is LM301, shorted tants and sometimes "power" transistor BC140/160, maybe once I got one of the input stage transistor
All the old Frako alu (axial, over 100u value) should be changed and don't forget to check supply and the fuse...
I never see other issue for those line out in almost 8 year using this console, a large 289 quadrophonic version with 11 or 12 stereo line out like this one.

For the record 2 weeks ago I got a dead channel, I quickly swap 301 as I usually do first (as diode test at 140/160) opamp was dead, easy fix, but what I don't notice at first look is that a small corner of the opamp was gone, and a metal part pushed out for about 1mm !!! don't know what it is, inner metal layer or internal part of a leg ...

Best
Zam
 
PRR said:
As the '301 is End Of Life, you should stock-up while you can. Mouser has some in DIP and even in pretty metal cans.

I have to ask.....Is there any benefits in using the metal ones????? They sure are cool looking..... but, is there a significant benefit???


zamproject said:
For the record 2 weeks ago I got a dead channel, I quickly swap 301 as I usually do first (as diode test at 140/160) opamp was dead, easy fix, but what I don't notice at first look is that a small corner of the opamp was gone, and a metal part pushed out for about 1mm !!! don't know what it is, inner metal layer or internal part of a leg ...

Best
Zam

I had  a similar experience that I traced down to a tiny fracture in the ic socket. Another time in same unit was a bent wiper in the socket..... Not saying we're talking sockets, just saying because they happened to be 301 too and it reminded me...

Aref said:
I have a multimeter with a capacitence meter option, but I'm not sure if it's precise enough.
Can I measure them on the board with no power?
Numbers seem so different. For instance 220uf16v caps are about 600uf to 800uf!

Not sure about your meter's accuracy. Maybe compare a known good cap to see.....Many times you have to remove at least a leg to get an accurate reading.....power off when checking values......

Aref said:
Can you describe how to check the transistors without taking them out?

Here's one article.....there is tons of info on the web....

https://vetco.net/blog/test-a-transistor-with-a-multimeter/2017-05-04-12-25-37-07



Comparing readings from known good ones is useful info too......

Also....

I've trashed my handful of op amps accidentally as well however..........

 
PRR said:
> follow the signal path with the oscope and don't know why suddenly that channel stopped working!

Do you remember WHAT you poked to kill it?

Yes, I marked it and it happened when my prob was around q203.

I swapped almost all lm301 on the non-working channels with no good result!
 
zamproject said:
Hi PRR

Where did you see that ?
At TI it's still active ... no EOL so far
But tks anyway for the warning, I still have 3 console massively using those and as it's a 40+ years op amp it will be EOL one day...
I should have more than dozen spare but will pick more at the next order !

For the OP, as already stated, usual suspect is LM301, shorted tants and sometimes "power" transistor BC140/160, maybe once I got one of the input stage transistor
All the old Frako alu (axial, over 100u value) should be changed and don't forget to check supply and the fuse...
Best
Zam

Hello zam,
I learned how to check the transistors with a multimeter. Do you always take them out for checking? What about the tantals?
Is there any check points on this board?
Thanks
 
Aref said:
Hello zam,
I learned how to check the transistors with a multimeter. Do you always take them out for checking? What about the tantals?
Is there any check points on this board?
Thanks

If I get hokey readings on anything, I always pull to retest and verify.  If you have a working duplicate channel, you can always compare to avoid pulling components. For example if you have a 10uf cap reading 400uf, and the working channel is reading the same 400uf for a 10uf, chances are it's ok because it's reading other parts of the circuit......

Don't know about test points...I was just jumping in to share....

Good Luck!
 
Aref said:
I learned how to check the transistors with a multimeter. Do you always take them out for checking? What about the tantals?
Is there any check points on this board?

I usually check transistor first onboard for dead junction
You can more or less test tants for short with ohm select DMM and/or check voltage at both leg across ref (0V)
depending of situation it can give you a clue..
e.g. for C101 if you have -11 at both leg you have a problem...
Really, except the LM301 input stage bypassed and substituted by Qx03/x04 there is no mystery here...
Also keep in mind when you track signal that in most situation where the card is used in the desk the signal coming in is asymmetrical, sometime at - input (to flip previous flip) so you'll trace nothing at + pin and further path.

As manual and schemo (don't) say, no TP for this card as there is no calibration specifically requested here, the only gain trim pot is accessible at panel hole and calibrated by probing output.

Best
Zam
 

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