This picture shows the cover of the indicator housing removed showing where I soldered in a replacement bulb to the leads of the old bulb. The indicator assembly is an Industrial Devices Inc "Line-o-Lite" 3590A20 which the entire assembly was intended to be replaced when the bulb failed. As far as I can determine it is no longer available. If you know of a source please email me below. I repaired it by opening it up and soldering in a new bulb. The black frame around the cover has two tabs bent over at each end holding it in place. Pry them up and it and the cover comes off. It's tight trying to solder inside the bulb module so watch out for melting the housing. You can see I broke one of the leads to the old bulb while soldering in the new bulb. The bulb I used was a ME353-2182 which is now a CM2182 and available from Mouser. All the lights were burnt out in my drive so I didn't have to try to match the intensity of the original bulb. I also haven't seen another drive powered up to know how bright they should be. If you need to match I would buy a couple different bulbs so you can pick the correct brightness.
The bulbs run off the external -15V supply with a 15 ohm series resistor. The resistor drops about 1.3V with these bulbs so the -15V supply should be no more than -15.3V to prevent shortening the bulb life.
This picture was taken before I replaced the bulb again since it had failed after around 10 years of use. The bulb is in theory rated for 40,000 hours but it didn't last that long. For part of the time it may of been running over its rated voltage though. I am using a 799 supply for the TU56 which isn't well regulated. The -15V output is about -16.6V. The bulb would have been running with about 15V for a while before I added a -15V regulator on the supply output. Running a bulb above its rated voltage greatly shortens its life.
Feel free to contact me, David Gesswein firstname.lastname@example.org with any questions, comments on the web site, or if you have related equipment, documentation, software etc. you are willing to part with. I am interested in anything PDP-8 related, computers, peripherals used with them, DEC or third party, or documentation.
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