Sony XM-2002GTR left channel bias - diyAudio
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Old 14th June 2011, 05:53 AM   #1
Dr Zeus is offline Dr Zeus  United States
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Default Sony XM-2002GTR left channel bias

I got this amp in for repair which the owner said the left channel makes a lot of distortion. Upon tear-down I found the bias is not entirely able to be set correctly; almost as if one of the drivers in this amp is leaking or having some other issue. The distortion is audible through a speaker and visible on the scope. If I set the bias to about 0.004v then about 90% of the distortion goes away, the outputs get a bit warm, and the right channel is still superior. The bad channel has three small transistors which should be the drivers: Q105, Q106, and Q107 leaning against the board's heat sync. Have not yet removed, but while on the PCB all drivers in both channels appear to be measuring the same.
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Old 14th June 2011, 12:26 PM   #2
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Post a photo of the distorted waveform.
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Old 17th June 2011, 03:16 AM   #3
Dr Zeus is offline Dr Zeus  United States
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Its the top half of the waveform with the abnormality. The scope is set to 2v/div ant 1mS. The input frequency is about 150hz sin wave. The channel that has this distortion gets a bit hotter than the one which is working fine. This is with a test speaker attached. Without the speaker the trace is thin throughout.

Click the image to open in full size.
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Old 17th June 2011, 05:31 AM   #4
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Did you use exact replacements when you replaced the transistors?

Is the bias set properly?
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Old 17th June 2011, 11:16 AM   #5
Dr Zeus is offline Dr Zeus  United States
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I actually did not replace anything in this amplifier yet as I could not find any degraded components.

I set and re-set the bias a few times measuring the voltage from center to outer leads of the 0.22ohm two-way resistors. Ive been setting the bias to just below 0.001v at idle. If I set the idle bias to 0.004v at idle the top half of the waveform improves but outputs get pretty hot quick.
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Old 22nd June 2011, 03:37 AM   #6
Dr Zeus is offline Dr Zeus  United States
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I blew it. Q801 by accidentilly shorting it to the pad right next to it. All I can read o nit is ASY and possibly D1 perpendicular to the ASY. I looked up a bunch of sony schematics and Q801 almost always seems to be a 2sa1162. Is this correct?

Also, I'm still trying to figure out what is wrong with the bias on the left channel of this amp.
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Old 22nd June 2011, 04:55 AM   #7
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KTA1504 in SOT-23

I have the GTW serivce manual. It lists the transistor above for Q801. Email me if you want the manual.
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Old 22nd June 2011, 12:40 PM   #8
Dr Zeus is offline Dr Zeus  United States
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MMBT2907 (2B) looks similar and slightly superior to KTA1504. I have MMBT2907 in my collection. I'll try it out and see if I can get this amp back to the last state.

I emailed you for the service manual. Thanks!
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