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Old 14th August 2006, 04:56 PM   #1
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Unhappy My ARC D70 get trouble

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 couldnt 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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Old 14th August 2006, 05:07 PM   #2
kevinkr is offline kevinkr  United States
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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. 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??
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Old 14th August 2006, 05:39 PM   #3
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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
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Old 14th August 2006, 05:48 PM   #4
kevinkr is offline kevinkr  United States
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Check transistor Q2 and make sure it is not shorted. Voltage at the test points indicates matching of output tubes, and should prevail. The -37V is a guideline and will vary somewhat from tube to tube. You should use well matched tubes in this amplifier.
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Old 15th August 2006, 05:18 AM   #5
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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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Old 15th August 2006, 03:09 PM   #6
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Hi kevinkr,

I find Q2 base pin almost near broken. Do you know what model of it and which modern NPN transistor to substitute it ?

I have MPSU10 Vce 300v not sure it can replace it or not ?

CK
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Old 15th August 2006, 03:51 PM   #7
kevinkr is offline kevinkr  United States
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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.
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Old 15th August 2006, 04:52 PM   #8
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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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Old 15th August 2006, 07:02 PM   #9
kevinkr is offline kevinkr  United States
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Well that is good news!

I'd stick to the ARC idle current recommendation certainly I would not go much over 45 - 50mV on the TP (45 - 50mA) and only with SV6550 or JJ KT88.
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Old 16th August 2006, 05:19 AM   #10
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I have also replaced all 6DJ8 instead to use 6922. How do you see ?

In earlier I want to exchange my SP9 MKII with SP11 MKII. In second hand market SP11 selling price around HKD 17K-19K .
Is it really worth to spend ?

CK
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