Cyrus 2 amplifier

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.
 
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
 
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.
 
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
 
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?????
 
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
 
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
 
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....
 
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
 
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
 
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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.
 
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
 
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
 
...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
 
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
 
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.