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Old 28th June 2012, 03:42 AM   #1
asmith is offline asmith  United States
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Join Date: Sep 2011
Default Droping Resistor.

I just picked up a Yamaha C-6 preamp and M-4 amp. These are the ones with the big square lighted buttons on the front. Now the problem, every bulb is out. Looking at the schematic of the preamp it looks like the bulbs are 14.5v but the copy is so bad I can not read the current rating.

After looking at Radio Shack's web site they carry a 12v 25ma bulb that looks like a good fit, now the question what is the value of the droping Resistor, Im having a serious mind blank this evening and can't figure it out. This is bad consider that I started my career as an electronic tech 30+ years ago.

Any help is highly appreciated.

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Old 28th June 2012, 04:52 AM   #2
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Join Date: Jun 2012
does each bulb have it's own resistor,
or do all the bulbs drop from one resistor? How many total bulbs on one resistor?
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Old 28th June 2012, 06:06 AM   #3
RJM1 is offline RJM1  United States
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Join Date: Jan 2010
Location: Titusville, Fl.
E=IR , voltage drop is 14.5-12=2.5V.
2.5V/.025A=100 ohms for each bulb.
If you have four bulbs in parallel you would have a current of 100ma and a resistor value of 25 ohms.
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Old 28th June 2012, 06:56 AM   #4
RJM1 is offline RJM1  United States
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Location: Titusville, Fl.
100/ ( number of bulbs in parallel ) = resistor value.
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