[time-nuts] Re: Unobtanium bulbs for HP506x: Found 'em!

[ed breya]

These midget flanged base style bulbs are extensively used in old gear. 
I used to have a pretty big collection of salvaged old indicator lamp 
fixtures and bulbs, but gradually used them up to now having only a few 
of the really nice ones left.

I have done a lot of LED swap-ins with these, to get virtually permanent 
bulb life in projects that I build. I gut the glass bulb and adhesive 
out of the base, and mount a suitable LED, and design or modify the 
electronics to work with the LED. The LEDs never look quite like the 
original lamps - sometimes worse, sometimes better - so it usually takes 
some fooling around with the jewel and LED colors and added diffusing 
materials to be satisfactory.

Doing this for existing equipment almost always needs some internal 
mods, which are typically quite easy. I have a fair number of certain 
bulbs still good, saved for replacements in various gear. Once they run 
out, it will be time for changing to LEDs. I save all the burned out 
bulbs as husks for LED transplants.

Here's an interesting aside (I think). The Tek 7000 series scopes use 
this style bulb (three of them) for graticule illumination. A few years 
ago, I made and installed some "white" LED replacements to see how they 
worked. It looked pretty good, and I was happy. Then I took some screen 
shots with a phone/camera. The graticules showed up nearly invisible in 
the pictures. The false spectrum of the LEDs fooled the eye OK, but the 
camera's response was way different.

The lesson is to make sure the LED "color" actually works properly in 
all of its applications. The LEDs I used back then were of some whitish 
unknown type. I think the more modern high power ones built for lighting 
should have a good spectrum for cameras too, but I haven't yet tried the 
obvious experiment to find out.

Ed