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Old 4th March 2007, 10:03 PM   #11
anatech is offline anatech  Canada
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Hi Arif,
Both outputs doing the same thing is good in a way. They probably both have a simliar or same fault.

The distorton LEDs are on because the diff pairs are both saturated in the same direction as they attempt to correct the DC offset. We know you most probably have a voltage amp issue since there is no excessive current draw.

Check Q157 and associated components. Also check R166 or R171 for open. Both are 100R. Either there is excessive current through Q157 (2SA1210 & I doubt this) or no current flow through Q158 (2SC2912). An open R171 would do that. Beware the shorted driver or predriver.

-Chris
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Old 4th March 2007, 11:31 PM   #12
K-amps is offline K-amps  United States
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2912's and 1210's seem ok. As do the resistors.... however the Q257 and Q307 (Bias sensors) seem open, would they cause this sticking to the rail?

It's a pain to open the whole thing up as it is buried under one OP device... , just wanna make sure before I do...
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Old 5th March 2007, 01:49 AM   #13
anatech is offline anatech  Canada
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Hi Arif,
If the bias network went open you would see very high currents. I'm betting that R121 & R171 went open. The bias could also go open, but then I'd expect damage. Of course, both may have occured.

Try to measure B-E and B-C junctions from the driver board on diode test. You could also use a transistor checker to confirm good gain and no leakage without touching the heatsink assemblies.

That's what I would do.

-Chris
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Old 5th March 2007, 10:51 AM   #14
K-amps is offline K-amps  United States
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K Thanks... off o the road, will be back Friday to check again..
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Old 10th March 2007, 07:05 PM   #15
K-amps is offline K-amps  United States
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Chris...

replaced the sa2240 (bias tracker trannie) no diff.

Tell me something, what voltages should I see at R120, 121 (The resistors feeding the predrivers.

Currently I see +80, -80 on all R120,121,170,171. Even when I pull out the heatsink assly. i.e. the voltage is being fed from the input sections not back fed from a shorted driver...

Is this normal?
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Old 12th March 2007, 06:06 PM   #16
anatech is offline anatech  Canada
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Hi Arif,
Those voltages should be within 3 VBE drops of your speaker output voltage while not delivering current. So it would appear that your negative current sources are turned off, or your thermal switches are open and there is something leaky turning the Q156 / Q157 combination on.

-Chris
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