Replacing Old LEDs

[Tony Duell]

> 
> Dear Classic Computers,
> 
> I regret that my EE knowledge was never deep and is
> a little rusty now. I am refurbishing some old PC
> boards and wanted to replace a few of the LED which
> had become damaged or broken off. Here is my lame
> question, how do I correctly pick replacement LEDs?
> I know colors, and size, but how do I make certain
> that I have picked correct wattage, etc.?

Unless they were doing something 'clever' like using the LED as a 
poor-quality voltage reference [1], then it's not going to be that 
critical. I assume thsse LEDs were used simply as indicators.

Pick the same physical size, same colour (the forward voltage drop does
depend on the colour), and for old stuff you don't want the modern 'high
efficiency' or 'low current' LEDs. If you have more choice, pick something
where the specified forward current (may be called If) is around 10-20mA.
That's the value for most, if not, all, general purpose LEDs. 

[1] At least one Zenith MDA monitor used that trick....

-tony