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2butterbeans 2butterbeans is offline
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Default Yamaha MX-800u

I have a Yamaha MX-800u and the power indicator light is getting dim. What does that mean? Is the amp getting weak? Or is it just the light? Any of you guys had this to happen to you? Or do I just need to replace the indicator light ? Any help ? Thank You
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Dave Platt[_2_] Dave Platt[_2_] is offline
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Default Yamaha MX-800u

2butterbeans wrote:

I have a Yamaha MX-800u and the power indicator light is getting dim.
What does that mean? Is the amp getting weak? Or is it just the light?
Any of you guys had this to happen to you? Or do I just need to replace
the indicator light ? Any help ? Thank You


It looks as if the "Power" and "Protection" indicators are a pair of
LEDs. They're wired up in series, and fed from the "protection"
board. The signal on the protection board which controls the output
relay is also fed to a transistor which bypasses ("shorts out") the
"Protection" LED, turning it on and off as appropriate.

Dimming of the "Power" LED could mean any of several things. It
could simply mean that the LED is getting old and its output is
dropping... this does happen to LEDs with age, especially if their
encapsulation wasn't perfect, and your MX-800 is about 25 years old.
This would be a harmless failure, and simply replacing the LED with a
similar one would probably fix the problem. I think this is the
likeliest cause of the problem.

Another possibility is that the current-limiting resistor for the LEDs
(R565, described as a "2P3.9k") has gone bad (value drifting high), or
the connection between the protection board and the LED assembly has
gotten dirty or oxidized and is cutting down on the current flow to
the LED. Again, harmless, and probably not too hard for a tech to
fix.

Dimming could also mean that the DC voltage fed from the "protection"
board has dropped - this could be a serious problem. You'd have to
check the voltages in the amp against those described on the schematic
to be certain. However, if the amp still sounds OK and still puts out
the expected amount of power, I think this is probably unlikely.

Due to the design of the LED circuit, if both LEDs are lit, they're
both being fed with the same amount of current, and should have
roughly the same brightness (the schematic and parts list says that
they're the same type of LED). This fact may help you figure out
whether the fault is in the LED itself, or elsewhere... if they're
both dim, it's the driver circuit or connection, while if Protection
is brighter than Power, it's the Power LED itself.

http://www.hifiengine.com/manual_lib...a/mx-800.shtml
has a copy of the service manual (free registration is required to
download it).
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