Cyrus2 amp Left channel distortion - diyAudio
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Old 23rd February 2011, 08:08 AM   #1
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Default Cyrus2 amp Left channel distortion

Hi

I have the cyrus2 (issue06), serial number 204037c.
After cranking the volume just above 2, distortion set in on the Left channel. Right channel is functioning normally.

I open up the amp to check: no bloated caps on the AMP-L circuit. I spotted a green resistor R81 having brown discolouration on the cap body. Measures 122ohms (with amp OFF), schematic of R81 is 120ohm.
Is this the cause for the distortion?

Click the image to open in full size.

Both poweramp 50v 10000uf cap looks alright.

What else can cause distortion?

Please help and give me your advise.

Thank you

CS law
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Old 23rd February 2011, 11:56 AM   #2
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check all transistors (b-e and b-c measuring like a diode, and c-e measuring open circuit.
Which one is R81 ?.

Eric.
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Old 23rd February 2011, 01:22 PM   #3
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Hi Eric
Attach photo from another angle of R81

Click the image to open in full size.

R81 is below the label AMP-L.

How to check the transistors. With power ON or OFF. And which transistor.
I have to admit i have little knowledge electronically, but am able to do component change.

CS Law
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Old 24th February 2011, 02:07 PM   #4
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Ok, fault finding 101 for newbies.....
First off, service manual is here - Cyrus Cyrus 2 pwr amp Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics ... in Acrobat Reader, right click>rotate clockwise might help.
First thing - you need a Multimeter that has a diode test position - even better if you have access to a Fluke 73/75/77.....if not, most cheapo meters have a diode symbol test position.
You are looking to test transistor junctions - with no power applied and a good transistor, B-E or B-C will test 0.6xx (or thereabouts) with correct probe polarity, and C-E will test open circuit.
If any junctions test short circuit or low value (isolate, desolder or remove the device to confirm) you have a rooted transistor.
Also test all resistors in the output stage and tell me what you find.

You give me the facts that I ask for and I will teach you how fix it.

Eric.
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Old 3rd March 2011, 09:45 AM   #5
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Measurement of the Left channel transistors Q45, Q43, Q39, Q41.
Q45: B-C 0.46, B-E 0.06, C-E ( value keeps increasing, i suppose its open circuit )
Q43: B-C 0.49, B-E 0.06, C-E ( value keeps increasing )
Q39: B-C 0.44, B-E 0.06. C-E ( value keeps increasing )
Q41: B-C 0.44, B-E 0.06, C-E ( value keeps increasing ).

Base is Left lead, Collector is middle lead, Emitter is Right lead ( info i got online ).

I cannot get proper ohm measurement from the resistors infront of the 4 output transistor. R91 /93 /87 /89, 95 measures about 0.6ohms each. ( base on schematic : should be 47ohm each, 120ohms ).

Are these measurement useful.

CS
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Old 3rd March 2011, 10:40 AM   #6
Mooly is offline Mooly  United Kingdom
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Is the circuit in the manual correct ? Look at Q24, there is no means of biasing the transistor. Q16 looks all wrong too.

Your measurements don't really tell much tbh and reading parts in circuit is no good. Also this doesn't really sound like a semiconductor failure, or not directly anyway.

Given limited test equipment I would measure and confirm the resistors in red here and remove for now the diodes in blue.

Measure the DC offset (voltage across the speaker terminals) and confirm it is zero.

Have you swapped speakers over to be sure it really is the amp at fault ?
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File Type: jpg Cyrus.JPG (130.2 KB, 73 views)
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Old 3rd March 2011, 11:13 AM   #7
Mooly is offline Mooly  United Kingdom
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R81! Is it discoloured in the other channel ? Does it get hot as the volume is increased ?
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Old 3rd March 2011, 12:15 PM   #8
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by cslaw View Post
I cannot get proper ohm measurement from the resistors infront of the 4 output transistor. R91 /93 /87 /89, 95 measures about 0.6ohms each. ( base on schematic : should be 47ohm each, 120ohms ).
CS
The resistors I have highlighted are 0.47 ohms so you reading 0.6 ohms is OK (meter lead resistance added to 0.47)
R95 is a 0.22 ohm
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Old 3rd March 2011, 12:39 PM   #9
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My mistake: R 91/93/87/89 is R47 ( 0.47ohm ), R95 is R22 ( 0.22ohm ).
I hope i got it correct this round. Measurement is 0.6ohm and 0.3ohm respectively.

(
Quote:
Originally Posted by cslaw View Post
Measurement of the Left channel transistors Q45, Q43, Q39, Q41.
Q45: B-C 0.46, B-E 0.06, C-E ( value keeps increasing, i suppose its open circuit )
Q43: B-C 0.49, B-E 0.06, C-E ( value keeps increasing )
Q39: B-C 0.44, B-E 0.06. C-E ( value keeps increasing )
Q41: B-C 0.44, B-E 0.06, C-E ( value keeps increasing ).

Base is Left lead, Collector is middle lead, Emitter is Right lead ( info i got online ).

I cannot get proper ohm measurement from the resistors infront of the 4 output transistor. R91 /93 /87 /89, 95 measures about 0.6ohms each. ( base on schematic : should be 47ohm each, 120ohms ).

Are these measurement useful.

CS
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Old 3rd March 2011, 12:51 PM   #10
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Hi Mooly, R91 /93 /87 /89, 95 is R47 and i measure 0.6ohm, R95 is R22 and i measure 0.3~0.4ohm.
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