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My ARC D70 get trouble

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Dear All,

After I changed two old OPAMP TL071 with a new TL071CP in my ARC D70, powered on 30 seconds the speaker output have a background constant noise “zzzz” continuously. However, the music was still be clearly listened with this noise. The great problem is I had lost the original TL071, so I couldn’t do comparative on them.

Here are the questions that I want to know.
1) How to determine the noise come from which OPAMP ?
2) What the function of these two OPAMPs ? Is it only for voltage control ?
3) If it is the significant causing the noise then how to stop it happen ?

Schematic is placed into this path :
http://home.netvigator.com/~sosowong2/ARC_D70/

Thanks for your great help.

CK
 
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I've worked on lots of M100, D70, and other ARC amplifiers. The TL071 is usually used in ARC amps as the gain element in series pass regulators and in many cases as a bias servo in some of the early amplifier stages.

Usually when one of these fails there is a lot of other damage as well.
Check all of the pass transistors, zeners and other components associated with the stage causing the problem.

Do you have a good scope, signal gen, and meter?

These amps are hard to repair. Failure to get everything right the first time usually means starting all over.

I won't touch one unless it is already broken, and I hate working on them with a passion I can't describe. :xeye: The boards are so thick it is almost impossible to remove parts without lifting traces.

Unless you are a very proficient tech I don't recommend working on one of these. Unfortunately your only reliable recourse in many instances is a factory approved service center.

Why did you replace the original op-amps? The replacements you chose would provide no sonic benefit at all??
 
Thanks
kevinkr

I have good scope, signal gen, and meter. I found the U2 output without the noise . But the Q2 emitter have the noise same as speaker output found. How do to the next step ?

Can you give me some ideas which component should I check at first ?

Another question about the singal broard. In order to get TP1-3 & 2-4 & 5-7 & 6-8 all voltage to 34mv, I adjusted V7,8,9,10. Therefore -37v can't be kept. Is it correct for my adjustment or alternatively select all bias become -37v ?

CK
 
Hi kevinkr,

One point forgot to tell you. D19 and D20 nearby the U1 I was change it to 1N4148, due to it had broken when I cleaning. Not sure it can be substituted by 1N4148 ?

From the schematic I can't see clear about Q2 model, what kind of transistor is using. Can you tell me ?

Except to checking on Q2, can you advise to me which points and components should I to check in advance ? If possible pls give me your expected result upon your advice.

Thanks for you support again !
CK
 
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Joined 2004
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Yes I think 1N4148 are fine for D19 and D20, not much difference between that and a 1N914 in most instances. Generally regarded as interchangeable.

You may need to replace Q1 as well with a new TIP41. I can't make out what type Q2 actually is in the schematic. Incidentally most if not all of these parts are still made so substitutions of other parts should not be necessary.

I would contact ARC for more guidance on what can be used to replace Q2. It will not be subjected to any more voltage than the op-amp itself so a particularly high voltage rating is not necessary. (Vce of 50V or so should be fine.) What is important is the beta and FT so that stability is maintained.

When cleaning be extremely gentle, I would use a small acid brush or large artist's paint brush and alcohol only for cleaning.
 
Dear kevinkr,

Just tell you a good news, after I reconnect the broken pin of Q2 the noise was gone. Anyway, if you find the model of Q2 pls leave it here.

Last question... do you know the optimal voltage drop across TP1,3 & TP5,7 etc with a best sonic.

Thanks for your friendly support.

CK
 
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Joined 2004
Paid Member
You can also use 7308 and ECC88/E88CC in this amplifier. I would recommend vintage and expensive Amperex from Holland for this honor.

IMHO I would not trade an SP9 for an SP11 of any flavor, but then that's me.. Any way you could borrow an SP11 and compare it to your SP9?

If you don't listen to records an LS2 might be nice.

To open a further can of worms :D you might want to listen to the CAT SL1 (Convergent Audio Technologies) or one of its more recent variants.
 
CAT SL1 phono is optional but I mainly listen to LP.
It's spec very serious espeical on Weight: Net 64 Lbs and 80W consumption.

Do you know how much of SL1 at second hand market ?

SP11 I know it have an innovative design at its age, not sure nowadays which product like as it.
 
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