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Old 25th April 2012, 07:31 AM   #71
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An option you may wish to consider is getting the amp repaired professionally. I noticed that there is someone on E-bay who does a fixed-price repair on Cyrus 1 and 2 amps with lots of good feedback.
Try a search on ars_ws on E-bay UK.

I have no association with the trader, and have never used his services, I simply noticed him this morning while looking at Cyrus listings.

Not very DIY, but sometimes time is better spent on other things....
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Old 25th April 2012, 11:13 AM   #72
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I do use a psx with it but this is not at fault so I'm using just Cyrus 2 as this is wer the fault lies.
I have done the test as where I take fuses out of Cyrus and run psx and still the noise. I unplugged Cyrus 2 from mains and therefore disabling phono section ( I think) and the noise on switch On without power to Cyrus as psx is powering Im sure fault with power side of Cyrus,hope you understand explanation .
Regarding vr2. I have replaced and is giving out 18v so allis fine.

What about replacing Q37&35 as you said may not be working correctly ?

Thanks Richard
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Old 25th April 2012, 08:01 PM   #73
mjona is offline mjona  New Zealand
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Hello Richard,

I think the problem lies elsewhere than with Q37&Q35 as the protection circuit is being triggered. This combination latches so that it still works after it is triggered and it will hold until the fault is cleared. It monitors the voltages across the emitter resistors R107A. R107B and will trip if there is difference of approx 3.5 volts.

An obvious cause of the tripping would be excessive current being flowing though the PT7 transistor in line with R107A.

Each PT7 output transistor is connected to the output by an emitter resistor which is fusible. The service manual suggests if one of these is blown then the PT7 in line will also have blown.

One of the problems with transistors is that excess current can cause failure before they take out the fuses.

It would still be possible for your amplifier to work if there is a blown transistor if it has gone open circuit. That might have happened without entirely disabling the protection circuit. I will spare you any analysis on that.

I recommend you first check R107A, R107B for continuity and then power up to check Q31 and Q41 for the voltage reading from base to emitter.

I recommend pulling the rail fuses and substituting 100 ohm safety resistors. A convenient way to fashion a safety resistor is to find fuses that fit your rail holders and deliberately blow them. You can then solder a 100 ohm 5 watt resistor across the metal end contacts.

After that you are done with the above you should repeat the procedure with the adjacent partners. The item numbers don't appear on my circuit diagram.

Michael Jonassen

Last edited by mjona; 25th April 2012 at 08:08 PM.
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Old 25th April 2012, 09:02 PM   #74
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Have you ever come across a transistor that has failed open circuit? I've come across ones that are a bit iffy, but still work to an extent. Usually they have been stressed and are due a fail sometime in the near future. Then they fail short circuit.

Pure sound - can you clarify if the problem is that same on both left and right channels? I think you said it was, but I just need to be sure.
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Old 25th April 2012, 09:24 PM   #75
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Quote:
Originally Posted by richie00boy View Post
Have you ever come across a transistor that has failed open circuit? I've come across ones that are a bit iffy, but still work to an extent. Usually they have been stressed and are due a fail sometime in the near future. Then they fail short circuit.

Pure sound - can you clarify if the problem is that same on both left and right channels? I think you said it was, but I just need to be sure.
yes its on both channels!

thanks richard
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Old 25th April 2012, 09:39 PM   #76
mjona is offline mjona  New Zealand
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Hello Richie,

The Cyrus work I did was over 20 years ago.

That was to do with transistor failures. I don't recall whether these went leaky or open circuit and from your more recent hands on you would know more about that than me.

A leaky device could be the cause of the initial crackling sound on the You Tube clip. It could be something that ramps up slowly then takes off. I notice the cone of the speaker is motionless during the crackling and then moves backward as if there is a d.c. hanging up on the negative rail - momentarily. The cone then moves forward again.

Are you able to say from your experience what the cause may be - I have to speculate the cause from studying the circuit diagram?

Michael Jonassen
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Old 6th June 2012, 07:30 AM   #77
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...chiming in!

Hi All!

I have the same kind of noise coming out of my C2/PSX combo, even with the C2 w/o PSX connected...Difference is, the humming noise is not dropping as in the video posted by Pure sound (Richard) but constant.

Any clue here?

Thanks and bet regards

Rad
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Old 6th June 2012, 09:25 PM   #78
mjona is offline mjona  New Zealand
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Hello Radiance,

Noise present on both channels? Cone displaced from centre at switch witnessed by Newsound?

Can you measure the d.c. and polarity of the voltage at the red speaker terminal for left and right channels.

In post #73 I suggested a convenient way to replace the supply rail fuses with 5 watt 100 ohm resistors to allow you to test various components to see if they are working properly without the risk of further damage.

With safety resistors in place instead of fuses you could start by measuring the voltages across all the resistors connecting the PT7's to the output point and noting the readings for each of these power transistors. You could then measure the voltage between the base and emitter of each PT7 and record that and record the measurements in a similar way.

You can get an idea of how much current the amplifier is drawing from the supply from ohms law by measuring the voltage drop across the safety resistors which will only get hot if there is a fault.

Michael Jonassen
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Old 6th June 2012, 09:46 PM   #79
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Do you have an scope.? I think by looking at the power supply with a scope probe and look at each of the voltages you can see if one of the voltage are coming up at the same rate. You have +/- 18 and +/- 41 volts. I will concentrate in the +/- 18 because it is the one powering the op-amps. Just put the probe in one of the voltage power up amplifier and see how it is coming up. Do the same for every one and determine if one it is too slow in coming up.
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Old 6th June 2012, 09:56 PM   #80
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Another test that you can do it is grounding resistors R43/R44. By grounding these resistors you will eliminate if the it is noisy it is coming from the preamp or from the amplifier. This is the input for the power amplifier.
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