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Sony XM-7547 muted- schematic error?
Sony XM-7547 muted- schematic error?
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Old 19th April 2018, 05:29 PM   #1
miniman82 is offline miniman82  United States
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Default Sony XM-7547 muted- schematic error?

Maybe my eyes arenít working properly, but page 14 of the service manual for this amplifier has the mute control circuits on it and itís showing 26 volts on Q812. Where the hell is 26v coming from???

The only thing that transistor does is forward bias the octet of Ďline swí transistors in the front end which will allow a signal to pass (not muted), when mute is active they have negative voltage at their bases to keep any signal from passing. But again, I canít see where Q812 is sourcing 28v from. Itís definitely biased off which is why I have no sound, but Iím currently scratching my head.
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Old 19th April 2018, 10:05 PM   #2
miniman82 is offline miniman82  United States
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Wound up being IC813, the optocoupler driving the mute circuits. But I still don’t see how 26v appears out of thin air here lol.
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Old 19th April 2018, 10:39 PM   #3
Perry Babin is offline Perry Babin  United States
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What voltage did you find on those points with muting active and defeated?
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Old 19th April 2018, 11:53 PM   #4
miniman82 is offline miniman82  United States
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Not sure which points you mean...

If youíre looking at the schematic, wire 7 at the top of page 14 comes out of the mute circuit and heads over to page 11 where the muting transistors live. When mute is active the bases of the 8 transistors on page 11 sit around -13.5v, which shuts them off and kills the audio.

With mute inactive you get like 24v or so at the same place, thus switching the transistors on and allowing audio to pass.

Just noticed another schematic error- The neg voltage for the transistor bases comes from apparently 2 places: thereís a 1-meg resistor from the -15v supply (R882 on page 14), and thereís another 1-meg on page 11 labeled R882... I believe these are in fact the same resistor, it wouldnít make sense to parallel a pair of 1-meg resistors when you could just as well have used a single 470k. Guess Sony didnít pay much attention to their drawings, luckily it was close enough to figure this one out.

Drawing also shows a fan and associated driving circuits on this model amp, mine doesnít have that stuff. Not sure whatís up with that, I know the 5-channel one had a fan.
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Old 20th April 2018, 12:49 AM   #5
Perry Babin is offline Perry Babin  United States
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I should have been more specific. I was asking about the points where the manual showed approximately 26v but it didn't appear that there was a source for the 26v.
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Old 20th April 2018, 01:53 AM   #6
miniman82 is offline miniman82  United States
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Yes, it’s there all right just like the schematic says, but I still can’t tell where it’s coming from. I think there must be a trace that’s not easily seen or something, I’m just glad I didn’t have to try and figure it out. There are numerous silkscreened lines and reference designators all over the board, a small trace could easily be hidden under all that. As for the schematic, I’m sure it’s just a simple error.
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