Repair of Studer mic pre

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Menno1000

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Hi all,
I have here a Studer euro-card board 1.915.301.00. One of these board is faulty. Schematic is attached.

What i found so far:
Fuse F1 was blown, because C21 was blown and created a short.
Also the IC3 was installed reversed. This could have been done after it stopped working with a quick check. The board worked flawless for many years.

C21, fuse and ic 3 are replaced an ok but it still does not work.
Now the bipolar transistor Q1 (BD517) gets very very hot. Too hot to touch after 2-3 minutes. Also the output transformer gets hot. Hotter than the other cards. I measured all components on board and i find no odd values.

Is this just Q1, BD517 that died or am i missing something here? Any advice and tips what to check?

Thanks for your help and best,

Menno
 

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gyraf

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If IC3 was compromised, it's more than probable that Q1/Q2 were both blown. And if failed shorted emitter-to-base, it could kill IC3 again-again.

I'd replace IC3, Q1, Q2 and try again.

(don't let the transformer get hot, isn't good for him)

/Jakob E.
 

zamproject

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I'll add to that, maybe change all other lythic, especially C22 23 24 if you already change faulty C21, great chance they suffer more or less the same issue.

Best
Zam
 

Bo Deadly

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Before you get too deep into it, you might want to check to see if the transformer is ok because that cannot be replaced. Specifically, measure the DC resistance of the primary on cards known to be good. Make a table of all the values. The circuit is such that you should be able to get a meaningful value with the transformer in the circuit. If the transformer on the bad card measures significantly lower, something might have melted inside. But hopefully the transistor took most of the power and save the OT.

Another thing to consider is that the core could now be magnetized. If you get it all working and the even order distortion is unusually high, that could indicate that the core got magnetized. But that might be fixed by running a high level low frequency sine through it. Although I don't know if you can do that in-cirucit. Maybe you can feed it into the output being careful not to overdrive the output circuitry.

As for the rest of it, you just need to go through it with a meter and a scope and the usual gadgetry and figure out what's bad. Look at the voltages around the transistor bases and figure out if they're blown or if the op amp is putting out nonsense or what.
 

moamps

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I would just add that the problem may be in the incorrectly set trimmer R26 which can lead to instability of the output amplifier (positive feedback). Check that the trimmer slider is set fully ccw during testing (+ input of IC is connected to ground) and measure the DC voltage at pin 6 of IC3. It should be 0V.
 

Whoops

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Hi all,
I have here a Studer euro-card board 1.913.301.00. One of these board is faulty. Schematic is attached.

I think you wrote the wrong model number,
isn't it 1.915.301.00 and not 1.913.301.00?

I repaired a lot of those modules.
Common problems with blown fuses in these preamps are failed Electrolytic capacitors.
By this time the ones that still didn't failed will fail soon enough.

C21, fuse and ic 3 are replaced an ok but it still does not work.

Don't replace just C21, replace all the Electrolytic capacitors in the module.
And while you're at it replace also all the Tantalum capacitors because when they fail they fail short circuit.
 

Whoops

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Also the IC3 was installed reversed. This could have been done after it stopped working with a quick check. The board worked flawless for many years.

That's not good that someone would take the IC out and install it reversed.
If that happen also change IC3, Q1, Q2 like Jakob recommended.

I never seen one of these preamps fail alone besides problems with Lytic and tantalum capacitors.
 

jokeramik

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At first I would replace the faulty capacitor. Then I would remove all active components (ic´s, transistor and so on). Then I would measure all dc-voltages at relevant points (f.i.: ic´s: pin 4 and 7) to ground. After this and power off I would replace ic 1. Now you can power on. and look to the dc-voltages at pin 4 and 7. If this is right you can inject a testtone and with an oscilloscope you can track the signal. So you can check every stage because they are dc-decoupled. Please use new ic´s and transistors.
Best regards.
jokeramik
 

Menno1000

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Thanks a lot for all your replies!

I put work in it and found the fault. First I got back and tested components. Q1 and Q2 tested ok. Transformer was the same as on the good boards. Put everything back.

I missed that with replaced c21 and c22 the trace was on the component side of the pcb and did no connection between -15V supply and Q2 . I reinforced all traces with hard wire and now it works fine again.
compromised, it's more than probable that Q1/Q2 were both blown. And if failed shorted emitter-to-base, it could kill IC3 again-again.

I'd replace IC3, Q1, Q2 and try again.
I ordered new BD517/518, pretty hard to find.
So best i replace these even if it now works now?
I replaced C21/22 1000uF with Panasonic FC. While I am at it, is it a good idea to add a film cap parallel?
I'll add to that, maybe change all other lythic, especially C22 23 24 if you already change faulty C21, great chance they suffer more or less the same issue.

Best
Zam
Yes gonna do this.
I think you wrote the wrong model number,
isn't it 1.915.301.00 and not 1.913.301.00?

I repaired a lot of those modules.
Common problems with blown fuses in these preamps are failed Electrolytic capacitors.
By this time the ones that still didn't failed will fail soon enough.



Don't replace just C21, replace all the Electrolytic capacitors in the module.
And while you're at it replace also all the Tantalum capacitors because when they fail they fail short circuit.
Yes, i will. Funny the Frako EP J 1000uf created a short. I never knew a lytic could create a short circuit. But this one did.
I really like these mic pre's and i am on the lookout for more.

I modded two boards by replacing the 5534 for an opa627 en shorted tantalum C5 en and C8 to make it dc coupled. C15 I kept in place to have at least one dc block in the chain. Maybe it can go? These boards sound a lot faster. In the unmodded boards i could replace the tantalum decoupling caps with Panasonic FC or FM. Or leave this in for the sound reasons?

Any. Thanks again and have a good day.
 

Whoops

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I ordered new BD517/518, pretty hard to find.
So best i replace these even if it now works now?

No. Jakob just advised you to change them in case they were shot.
You can also test transistors with a cheap MK-328 component tester, I advise you to buy one as it's a really useful unit. (You can also test transistor with a multimeter in Diode mode).

If the transistors are good and working, there's no reason to change them.

You should replace all Electrolytic capacitors and Tantalum capacitors in the modules for precaution, because they're pretty old and fail. If you change them you can be confident these mic pres will work well for many years.

I replaced C21/22 1000uF with Panasonic FC. While I am at it, is it a good idea to add a film cap parallel?

I don't see why, they are not in Studer's schematic, and the original circuit is incredibly good, why change something that works and sounds great?

. In the unmodded boards i could replace the tantalum decoupling caps with Panasonic FC or FM. Or leave this in for the sound reasons?

I replace the Tantalum capacitors for the same dielectric, so Tantalum for tantalum and Electrolytic for Electrolytic. Changing the dielectric will also change the original sound, and the original sound is really good.

These are extremely good and well sounding Preamps, I used them in many records and was impressed by the sound from the first time I used them. A Studio I used to work had 16 of them, and truly I became a fan and liked them so much I bough some units for myself.

You seem to be eager to change and modify parts in the circuit, but I never found anything that needs to be modified, they're great stock.
Faster Opamps doesn't mean that it will sound better, Neve preamps are slow and it's still the most well know and regarded mic preamp circuit in the world.
Also modern IC based mic preamps are really fast and people don't regard those so well in terms of sound.

Replace the polarized capacitors in the boards and have Fun recording with them for the next 30 years.

My 2 cents
 

Menno1000

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Dec 9, 2014
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No. Jakob just advised you to change them in case they were shot.
You can also test transistors with a cheap MK-328 component tester, I advise you to buy one as it's a really useful unit. (You can also test transistor with a multimeter in Diode mode).

If the transistors are good and working, there's no reason to change them.

You should replace all Electrolytic capacitors and Tantalum capacitors in the modules for precaution, because they're pretty old and fail. If you change them you can be confident these mic pres will work well for many years.



I don't see why, they are not in Studer's schematic, and the original circuit is incredibly good, why change something that works and sounds great?



I replace the Tantalum capacitors for the same dielectric, so Tantalum for tantalum and Electrolytic for Electrolytic. Changing the dielectric will also change the original sound, and the original sound is really good.

These are extremely good and well sounding Preamps, I used them in many records and was impressed by the sound from the first time I used them. A Studio I used to work had 16 of them, and truly I became a fan and liked them so much I bough some units for myself.

You seem to be eager to change and modify parts in the circuit, but I never found anything that needs to be modified, they're great stock.
Faster Opamps doesn't mean that it will sound better, Neve preamps are slow and it's still the most well know and regarded mic preamp circuit in the world.
Also modern IC based mic preamps are really fast and people don't regard those so well in terms of sound.

Replace the polarized capacitors in the boards and have Fun recording with them for the next 30 years.

My 2 cents
Excellent, sounds like a smart plan. I am a fan too. Any preferred brands for tantalum? C11 and C47 are also green quite big tantalums.
 

Menno1000

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Dec 9, 2014
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No. Jakob just advised you to change them in case they were shot.
You can also test transistors with a cheap MK-328 component tester, I advise you to buy one as it's a really useful unit. (You can also test transistor with a multimeter in Diode mode).
Allright, I was hoping for this. I have this MK-328 and they tested fine.

Thanks everyone! Really helpfull.

PS
I anyone has a source where to find these boards i would be happy very to acquire some.
 

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