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kumori
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Replace Incandescent Indicator Lights with LEDs

Thu Sep 22, 2016 5:24 am

I've replaced some incandescent indicator lights on my vintage amp with LEDs and, after the replacement, the LEDs don't light up. I'm looking for some pointers as to what I could have done wrong. My soldering skills are limited. I've done some soldering of wires inside of my PC (e.g., fans), but I've never done an LED.

My amp is a Yamaha B-4 from 1978. This is a amp that was only available in Japan, but the exterior is visually identical to the Yamaha M-4, the U.S. model.

Recently, the power indicator light burned out and I decided to replace it with an LED so I wouldn't have to deal with this problem again for 20 years. I followed this guide. The guide is in Japanese, but I've linked it because the pictures are useful.

The basics are as follows. The amp supplies 12v to the indicators. The LED I used was 17mA and 3.1v. I soldered in a 560ohm 1/4W resistor into the positive (red) wire and then soldered the positive (longer) terminal of the LED to the red wire and the negative (shorter) terminal of the LED to the white wire. After all that, the LED didn't turn on. I checked all the joints and they look good (i.e. shiny). I then replaced the LED with a different LED and still it didn't work.

Any hints?
 
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Re: Replace Incandescent Indicator Lights with LEDs

Thu Sep 22, 2016 5:40 am

Either the resistor is too big or the polarity is wrong. You'll need a multi meter to check the polarity is what you expect (red might not actually be positive)

My very rusty electronics skills suggest you should be using a 520ohm resistor so with a 560 you might not be getting enough voltage across the diode to turn it on. 

It might be a good idea to use something adjustable anyway since it will allow you tweak the brightness of the LED to a level that matches the other lights on the amp.
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Re: Replace Incandescent Indicator Lights with LEDs

Thu Sep 22, 2016 6:53 am

If anything, the resistor value is too small, not too large. Yes, it'll keep the LED current in spec; but it'll be running the LED pretty close to its maximum current rating, likely making it blindingly bright.

Four possibilities:

1. You've got the polarity reversed (LED installed backwards).

2. The problem was not with the indicator light; the problem is that the amp is no longer supplying power to the indicator light.

3. You're overheating the LED when soldering it, and damaging it.

4. The value of the dropping resistor is not what you think (e.g. you've got a 560 kOhm instead of a 560 ohm).

cheesyking wrote:
My very rusty electronics skills suggest you should be using a 520ohm resistor so with a 560 you might not be getting enough voltage across the diode to turn it on.

LEDs are non-linear (constant-voltage devices). The voltage across the LED will remain nearly unchanged over a very wide range of dropping resistor values; just the amount of current flowing will change. As long as the original voltage source exceeds the rated forward voltage of the LED, you can vary the dropping resistor value by an order of magnitude or more and the LED will still light, with just the brightness varying.

TBH I think you probably want something more like 2-10 kOhm here, unless your goal is for the LED to be REALLY bright.

Edit: Here's a generalized IV curve for an LED, illustrating the relationship between the voltage across the LED and current: https://en.wikipedia.org/wiki/Light-emi ... -Curve.svg

For the LED we're dealing with here, Vd is 3.1V.
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Re: Replace Incandescent Indicator Lights with LEDs

Thu Sep 22, 2016 7:27 am

I'm probably going to make myself look dumb here, but are you sure the amp is supplying DC power for the bulbs?  I would think such low voltages are DC, but you never know.
 
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Re: Replace Incandescent Indicator Lights with LEDs

Thu Sep 22, 2016 7:51 am

The Egg wrote:
I'm probably going to make myself look dumb here, but are you sure the amp is supplying DC power for the bulbs?  I would think such low voltages are DC, but you never know.

AC to the indicator is a possibility. Older designs with linear power supplies sometimes wired power indicators directly across the secondary of the step-down transformer.

However, the LED should still light in this case; it's a diode, after all -- so it will conduct during the half cycle of the AC waveform where it is forward biased. You'll only get half the brightness, and some 60 Hz flicker though. (With an incandescent indicator the flicker is minimal because it conducts during both halves of the AC waveform, and the thermal inertia of the filament smooths things out.)

Routinely reverse-biasing an LED may not be good for its long-term health. If using an LED in an AC circuit you may want to include an additional series blocking diode (e.g. generic 1N4001 silicon rectifier) to take the brunt of the reverse voltage.
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Re: Replace Incandescent Indicator Lights with LEDs

Thu Sep 22, 2016 8:25 am

just brew it! wrote:
cheesyking wrote:
My very rusty electronics skills suggest you should be using a 520ohm resistor so with a 560 you might not be getting enough voltage across the diode to turn it on.

LEDs are non-linear (constant-voltage devices). The voltage across the LED will remain nearly unchanged over a very wide range of dropping resistor values; just the amount of current flowing will change. As long as the original voltage source exceeds the rated forward voltage of the LED, you can vary the dropping resistor value by an order of magnitude or more and the LED will still light, with just the brightness varying.

TBH I think you probably want something more like 2-10 kOhm here, unless your goal is for the LED to be REALLY bright.

Edit: Here's a generalized IV curve for an LED, illustrating the relationship between the voltage across the LED and current: https://en.wikipedia.org/wiki/Light-emi ... -Curve.svg

For the LED we're dealing with here, Vd is 3.1V.

Thanks for the refresher, it's amazing how quickly you forget stuff when you don't use it.  :oops:
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Re: Replace Incandescent Indicator Lights with LEDs

Thu Sep 22, 2016 10:05 am

Are you sure the power light is not part of a larger circuit? I've seen designs where the indicator was actually required to be within a certain tolerance for a design to work properly, and LEDs can certainly interrupt that if that 12V was actually AC...
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Re: Replace Incandescent Indicator Lights with LEDs

Fri Sep 23, 2016 7:52 pm

Thanks for the help. I can't figure out how quote multiple posters using this new system, so I'm going to respond to JBI who seems to have covered most of the issues.

just brew it! wrote:
1. You've got the polarity reversed (LED installed backwards).


OK, I will check this one.

just brew it! wrote:
2. The problem was not with the indicator light; the problem is that the amp is no longer supplying power to the indicator light.


Before I started this quixotic project, two of the indicator lights worked. I replaced the burned out light and one of the functional lights with LEDs and now only one indicator light works. I don't think this is the issue.

just brew it! wrote:
3. You're overheating the LED when soldering it, and damaging it.


I've tried three LEDs. Either I overheated all of them or this is not the issue. Would there be anyway to visually determine whether this has happened?

just brew it! wrote:
4. The value of the dropping resistor is not what you think (e.g. you've got a 560 kOhm instead of a 560 ohm).


Actually, the first time the LEDs didn't work this was the mistake (i.e. 560K vs. 560). I tried the correct resistor and the LED still didn't light up.

just brew it! wrote:
TBH I think you probably want something more like 2-10 kOhm here, unless your goal is for the LED to be REALLY bright.


The guide that I followed (linked above) tried a bunch of different resistor values and settled on 560ohm. The specs of his LED were a little different (2mA / 3.2v), but I thought I'd give this a try. The original indicator lights were way to dim. This amp is a little weird because the LEDs slip into a thick rubber sleeve that gives the lights their color. This also acts to diminish the brightness of the LED.
 
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Re: Replace Incandescent Indicator Lights with LEDs

Fri Sep 23, 2016 8:06 pm

kumori wrote:
just brew it! wrote:
3. You're overheating the LED when soldering it, and damaging it.

I've tried three LEDs. Either I overheated all of them or this is not the issue. Would there be anyway to visually determine whether this has happened?

Not easily, unless the plastic of the LED is visibly deformed (in which case, yeah it's toast).

You could test the LED using a 9V battery and dropping resistor.

kumori wrote:
just brew it! wrote:
TBH I think you probably want something more like 2-10 kOhm here, unless your goal is for the LED to be REALLY bright.

The guide that I followed (linked above) tried a bunch of different resistor values and settled on 560ohm. The specs of his LED were a little different (2mA / 3.2v), but I thought I'd give this a try. The original indicator lights were way to dim. This amp is a little weird because the LEDs slip into a thick rubber sleeve that gives the lights their color. This also acts to diminish the brightness of the LED.

Ahh, OK. In that case a higher brightness would be appropriate. Efficiency and light output of LEDs does vary over a quite wide range though, so just be prepared for the possibility that you might need to swap in a different resistor to dial the brightness back.
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Re: Replace Incandescent Indicator Lights with LEDs

Fri Sep 23, 2016 8:12 pm

I would switch to color-matched LEDs and omit the bulb sleeves. Gives a bunch more options and the LEDs can be run with higher-value dropping resistors, ensuring long life.

The epoxy housing is a solid casting around the inner die, which means you can shave the "lens" tip with a file or sandpaper and generally abuse it a bit if you want the light to scatter rather than focus (typical requirement for incandescent substitutions).
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