From Brian Willoughby Sent Fri, May 3rd 2019, 03:09
If this were my Polymoog ( ;-) ), I would try to find the model number = of the original manufacturer=E2=80=99s LED and look for a data sheet. = Hopefully, that data sheet would list the exact color wavelength of = those LEDs in nanometers. I would personally want the =E2=80=9Csame=E2=80=9D= color of red, not just any old red. Modern LEDs are available in various shades, usually with good = documentation on the exact color wavelength. Of course, the really cheap = LEDs just say =E2=80=9Cred=E2=80=9D but you can still afford LEDs with = better quality control. Some have just a single wavelength, while others = have a wide range, but can still be =E2=80=9Cbinned=E2=80=9D by measured = wavelength in much narrower ranges. You could save a little money by = buying the unmeasured LEDs, but the savings isn=E2=80=99t worth the = tradeoff at your quantity. Instead, I think it=E2=80=99s worth paying a = little more to control the color. Assuming you can find the original specifications (or perhaps measure = them), then finding a modern match should be easy. Most modern data = sheets have graphs showing brightness versus color, and you can match = the brightness as well. As Murray pointed out, you=E2=80=99ll have to change the resistors, too. = The 180 ohm resistor sets the current, based on the LED supply voltage = and the LED forward voltage drop. If the modern LED has the same voltage = drop, then you=E2=80=99ll surely have to change the resistor to get = lower current. There=E2=80=99s a random chance that you could find a = modern LED with a higher forward voltage (that isn=E2=80=99t too high = for the existing LED voltage) and this would lower the current without = changing the resistor, but it seems unlikely that it would work out = precisely. One option that you should consider is buying modern LEDs with the same = current rather than lower current. The reason for doing this is that you = wouldn=E2=80=99t need to change the resistors. Of course, forward = voltage and efficiency are factors. One thing to note is that LED brightness can be measured in lumens or = candles, so you might have to convert between the standards if you want = to dial in the same brightness. Also look out for the dispersion pattern = - a directional LED will only be bright directly in front, while a = non-directional LED will spread out the brightness across a wider angle. = The difference between lumens and candles is related to dispersion in = different ways. I suppose that I=E2=80=99ve made this sound like an incredibly difficult = task, but it=E2=80=99s actually quite simple if you do the math. A good = LED data sheet will give all the information you need to calculate a = match to the Moog original and perhaps even save you from changing the = resistors. Good luck! Brian On May 2, 2019, at 1:51 PM, Dan Goldstein <xxxxxxxx@xxx.xxx> wrote: > I=E2=80=99ve got a Polymoog I=E2=80=99m restoring, and I=E2=80=99d = like to replace the old, original LEDs with new, modern ones. The = originals were 5mm bulbs, =E2=80=9CSSL-22=E2=80=9D, each with a 180 ohm = resistor hooked up to it. > =20 > I=E2=80=99d like to replace them with new, modern red LEDs of = equivalent brightness and lower current use. There are so many different = LED values these days, and I=E2=80=99m not sure how to know what would = be the ideal replacement LED. Can anyone offer any advice on specs for = replacement LEDs for a Polymoog? > =20 > Thanks! > Dan >=20