Cyrus2 amp Left channel distortion - Page 2 - diyAudio
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Old 3rd March 2011, 06:13 PM   #11
Mooly is offline Mooly  United Kingdom
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The readings seem OK.

Have you swapped speakers over ? to be sure

If the amp really is faulty next step in the absense of test gear has to be to carefully adjust the preset bias pot to confirm that the output stage passes current correctly. Disconnect speakers, volume on minimum.
You need to very carefully measure the volt drop across R95/96 and see if it can be taken up to say 120mv DC. That would correspond to just over 0.5 amp. If it can, return it back to the original value. If the either of the two diodes I mentioned were faulty that could cause distorted sound. The amp will run fine with them out to test.
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Old 6th March 2011, 09:42 AM   #12
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I replaced R81 which has discoloured.
The amp is working fine, no more distortion from the left channel.

Thank you Eric, Mooly and Diyaudio for your help.

Regards
CS
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Old 6th March 2011, 01:46 PM   #13
Mooly is offline Mooly  United Kingdom
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Hmmm.... I'm pleased it's working of course

just make absolutely sure there are no dry/poor soldered joints around the output and driver transistors....... the resistor reading 122 ohm instead of 120 isn't the problem.... so something else has happened or "fixed itself".
It could even be a transistor failing open circuit which although unusual isn't unheard of!
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Old 7th March 2011, 03:07 AM   #14
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I am interested to find out more about the cyrus 2.

From thread #11 "carefully adjust the preset bias pot to confirm that the output stage passes current correctly. Disconnect speakers, volume on minimum."

Tell me more on how to carry out this test.

1) Is this measurement done after changing transistor etc.
2) to check both channel output is equal / balance.

Are you referring to RV3 ( and RV2 for right channel ) seen in photo as the biasing pot.

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Old 7th March 2011, 07:35 AM   #15
Mooly is offline Mooly  United Kingdom
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The "bias pot" which is RV2 and RV3 (shown in the circuit post#6) allow the output transistors to pass (conduct) a small current in the absence of any audio signal. This is done to minimise crossover distortion in a push/pull class b (or ab) output stage.

A typical value of current would be around 20 to 100 milliamps. To measure this, the normal method is to just measure the volt drop across one of the emitter output resistors and calculate the current from ohms law.

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The service manual wording is confusing... it says use the above method using emitter resistors R93 and R94 for left and right channels. The circuit (post #6) shows one channel with two pairs of output transistors and R93/94 in the same channel... so without me having the unit in front of me it's confusing to give you exact advice... do you see

If your amp is as per the circuit in post #6 I would locate resistors marked R95 and R96 in each channel. Make sure your amp is like this and that these resistors are this value as fitted.

Measuring the voltage across this resistor adjust for around 10 millivolts which will be a current of around 45ma. The value will drift wildly as the output stage warms so keep rechecking and adjusting, doing the same for the other channel.

In practice a current of a couple of millamps and above renders the crossover distortion inaudible, go to high and the output stage runs too hot.

If the service manual procedure makes sense with the unit in front of you, and you can identify their test points then follow that, otherwise aim for around 45 milliamps measuring the voltage across a known resistor such as the 0.22 ohm.

Edit... this adjustment has no effect (in the terms you are thinking of) of giving different "channel balance" etc if set to different values or L/R were not equally balanced etc.
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Last edited by Mooly; 7th March 2011 at 07:39 AM.
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