Droping Resistor.

asmith · 28th June 2012, 03:42 AM

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.

Alan

soundguruman · 28th June 2012, 04:52 AM

does each bulb have it's own resistor,
or do all the bulbs drop from one resistor? How many total bulbs on one resistor?

RJM1 · 28th June 2012, 06:06 AM

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.

RJM1 · 28th June 2012, 06:56 AM

100/ ( number of bulbs in parallel ) = resistor value.

28th June 2012, 03:42 AM	#1
asmith diyAudio Member Join Date: Sep 2011	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. Alan

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28th June 2012, 04:52 AM	#2
soundguruman diyAudio Member 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?

28th June 2012, 06:06 AM	#3
RJM1 diyAudio Member 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.

28th June 2012, 06:56 AM	#4
RJM1 diyAudio Member Join Date: Jan 2010 Location: Titusville, Fl.	100/ ( number of bulbs in parallel ) = resistor value.