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Old 23rd April 2012, 07:11 PM   #61
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R121 resistor is fine. This is all going over my head now!!
Does anybody live near Bolton i need help!!
regards

Richard Tonge
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Old 24th April 2012, 04:32 AM   #62
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I will do two more test. Remove the AC from the transformer that feed the +/- 18 volts. I know that it is no used but just to eliminate a possible problem. I still do not like the 7 volts in the +18 volts. Second remove the chassis ground and see if still the amplifier does the "thump ". You should do both test one at a time.
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Old 24th April 2012, 08:29 AM   #63
mjona is offline mjona  New Zealand
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Hello Richard,

Electronics repairs are a process of elimination during which it is important to adhere to some safety practices.

One of these is to ensure your chassis and everything contained is connected to safety earth. If the chassis or anything inside happens to become mains live there is protection against electric shock should anyone come into contact. You do not want that to happen to you.

Do not remove R121.

The function of R121 is to provide a low resistance path from amplifier ground to safety earth.

The fact there is a resistance in that position rather than a short to safety earth is to do with prevention of earth loops. This is a second reason to leave it alone.

tauro0221's aim is to eliminate the phono section.

Removing the transformer a.c. windings will not only depower the preamp but the power amp as well.

I offered more direct ways to deal with the issue in earlier posts.

Michael Jonassen
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Old 24th April 2012, 02:13 PM   #64
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I am only tell him that remove the chassis ground from the audio just for a test. I didn't said to remove it permanently. This is just for the test to see if the chassis ground it is causing the problem. Once he do the test he need to install it back. I had a problem issue grounding the chassis one time. This is just a test. Always safety it is first. Remember the old vintage amplifier didn't have the chassis ground.
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Old 24th April 2012, 06:15 PM   #65
mjona is offline mjona  New Zealand
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The use of a small value resistor between ground and safety earth has the same isolation effect. The simplest way to test that is to short across R121 temporarily with a test wire.I use ones for the purpose - insulated wire in various colours but an insulated wire stripped at either end will do the job. The exposed wire ends should be a few m.m. long and these should preferably be tinned so the individual wires stay together.

The proof of the pudding will be when Richard is able to report his findings on that point.

Michael Jonassen
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Old 24th April 2012, 06:21 PM   #66
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Is this while the amplifier is turned on or being turned on?
Thanks
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Old 24th April 2012, 06:53 PM   #67
mjona is offline mjona  New Zealand
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It should not make any difference. Any anomaly will be present after initial switch on when the amp has settled and this time will be more convenient to test.

Michael Jonassen

Last edited by mjona; 24th April 2012 at 06:58 PM.
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Old 24th April 2012, 08:50 PM   #68
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Quote:
Originally Posted by mjona View Post
It should not make any difference. Any anomaly will be present after initial switch on when the amp has settled and this time will be more convenient to test.

Michael Jonassen
No difference. With that wire .


But I have just been checking Q 31/32/33/34 and on start up the voltage jumps upto 58v ish then drops down and settles to 38v. Is this normall?

I would of thought it would build upto 38v not jump well above then settle. Could the large surge be the turn on noise?????
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Old 24th April 2012, 10:10 PM   #69
mjona is offline mjona  New Zealand
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Hello Richard,

I am working from a Cyrus I diagram.

I seem to recall the driver devices and the output power ones are doubled up in the Cyrus 2. The Cyrus 1 has one driver and one output power transistor. I assume you are talking about the ones connected to the positive rail.

The difference between your rails is 76 volts and that measurement should remain constant thoughout. Can you confirm that and check the voltage drop between the negative rail and earth during switch on.

Let me know what you find and I will get back to you this evening local time.


Michael Jonassen
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Old 25th April 2012, 06:52 AM   #70
mjona is offline mjona  New Zealand
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Hello Richard,

I downloaded a service manual.

It is clear Q37 and Q35 are part of a protection circuit which will disable the power amplifier if excess current flows through R107A. I believe that is being triggered into action but not working properly.

I noted your observation on the voltage rail in the power amplifier and an earlier one about VR2 the in the phono section.

Having referred again to the You Tube clip you put up, I noted a slight crackling noise in your loudspeaker before the cone receded and moved back again.

I think the crackling is a sign of instability either in the phono section or the power amplifier.

The surgical / desoldering procedures - already mentioned to disable the phono stages were on the basis that you may want to use the integrated whole without the PSX. At present it seems you have not done that. It is easier to identify faults if you are dealing with one piece at a time i.e. the Cyrus box.

Another option also mentioned would apply if the Cyrus 2 is used exclusively with the PSX.

The phono stage could be disabled by removing the mains fuse from the IEC at the rear of the unit and refitting the mains connector lead. It is not clear whether you have tried that yet.

I am concerned for the possibility of instability in VR2 or any op.amp in the phono stage, and, contamination of the ground line within the Cyrus unit.

Can you clarify please?

Michael Jonassen
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