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Old 18th October 2007, 08:07 AM   #1
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Default yamaha mx 800 led problems please help

I have a yamaha mx 800 and the center diplay that lights up showing the wats for the power meters didnt work. I discovered the 2 leds were changed to red ones and they were not bright enough. I bought to clear leds from radio shack but every time i hook them up the burn out. I have tried reversing the polarity and the same thing they light up and burn out. Any help would be greatly appreciated.
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Old 18th October 2007, 08:51 AM   #2
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Location: Dona paula, Goa
The LEDs u are mentioning may be aged colored bulbs.

Looks like the originals were filament bulbs supplied by a separate winding.
If u want to install LEDs instead, u will have to rectify using brige rectifier and filter the supply, and add a series resistor (R) to each of the led.

How...
measure the ac voltage
multiply by 1.4, this is the dc voltage after filtering. Use about 470f cap and its voltage more then the calculated.

Each LED drops 1.6 volts.
Vdc minus 1.6/.02 =R for 20mA of LED current

Gajanan Phadte
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Old 18th October 2007, 08:56 AM   #3
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Missed...
If the supply is DC, then u have to watch the polarity and connect the LEDs.
Longer lead should be connected to positive.

Gajanan Phadte
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Old 18th October 2007, 03:16 PM   #4
gni is offline gni  United States
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I have drawn a 'schmatic' of the circuit for the Yamaha M-45 which
is very similar if not the same for all the Yamaha amps with the
front power meters.

Yamaha M-45 Power Meter Mods

The panel is AC lamps in series with resistors, while the LEDs are
driven by rectified DC and driver chips. The lamps drop about 8VAC
while the resistors drop about 3.7 VAC. . .or 123mA per lamp or
about 1W per lamp.

Check out my thread. . .this is all moot if Yamaha redesigned the
lamp circuit for the MX series. . .but I don't think the LEDs when
the MXs were made were that bright. . .

The fact that you installed LEDs in an AC circuit and they worked
briefly and died seems to indicate that it is still an AC circuit. . .that
should drop 8VAC across the lamp, but the LED is lower resistance
and draws too much current and die: either from excessive forward
voltage or too much reverse voltage. . .LEDs don't like AC.

Let's see a pic of the panel you have. . . and compare it to the
M-45. . .
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Old 19th October 2007, 03:46 PM   #5
gni is offline gni  United States
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The Yamaha M-70 used 14.5V 80mA lamps. All the meters in the
Yamaha amps are similar but not exact: variations include different
driver chips, load select, power ratio change, number of LEDs.

Changing the power ratio is an interesting topic. . .since most
power amplifiers operate at less than a watt when casual listening
is involved. . . changing by a factor of 10 or 100. . .more of the
meter would be used more of the time.
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Old 20th October 2007, 07:04 PM   #6
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Default You guys are correct

Radio Shack dont carry the filamant bulbs anymore so looks like I am converting them to led's probly better anyway as the life of leds are alot better. I have a pioneer surround sound receiver with low level outs for each channel. the fronts are going to an m-80 that drives my front pioneer cs-905's and cs-605's and the mx-800 I was going to drive the rear speakers which are a set of ar 13's. Watch allot of movies so the amps will be on quite a bit of time. Thanks for all the advice this site is awsome and the people on it are awsome.
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Old 20th October 2007, 07:04 PM   #7
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Default Here is the mx 800

When I got this amp a week ago it was filthy inside I spent about 5 hours cleaning the inside polishing all the contacts. From what ive read this amp is just as good and clean and powerful as the m80. I just hope I can do the led conversion without messing it up.
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Old 20th October 2007, 07:04 PM   #8
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Default Theater Room

The yamaha m-80 is on the bottom. I think I am going to try some of the mods I have seen in this forum. They are not too hard are they?
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Old 23rd October 2007, 02:45 PM   #9
gni is offline gni  United States
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Once you pull apart the meter assembly you will be could install
a single diode and extra resistor and LED to convert to LED instead
of lamp. . .if LEDs had been cheaper than lamps when it was first
made, then they would have used LEDs.

The amp should be capable of low impedance loads. . .four caps. . .
the M-45 has only two. . .but same mounting bracket; the M-45 has
the same heat sink brackets that alow narrow main boards in the
M-45 and the wider board for the M-85s.
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Old 23rd October 2007, 03:45 PM   #10
gni is offline gni  United States
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Here is a good article on how to run LEDs on an AC circuit:

http://ourworld.compuserve.com/homep...den/page10.htm
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