Re: [AH] Replacing LEDs in a Polymoog

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