IBM 1130 DISPLAY PANEL HAS DARK LAMP POSITIONS AND BULBS ARE FAILING
The display panel of the 1130 uses incandescent bulbs with wire leads, placed in a nylon holder and plugged onto a board with an SCR driven by the logic signal associated with the lamp. There are no new manufacturers of these bulbs and new-old stock is severely limited.
The challenge is that the wires corrode where they exit the glass envelope, so many of these bulbs fail because the wire breaks off, rather than through a burned out filament. Handling the boards that hold eight to sixteen bulbs in order to change one bulb can result in broken wires on bulbs that previously were working.
I had to loan quite a few of my own bulbs to get the VCF 1130 panel usable, but there are still dead positions and each time I address this, more of the original bulbs snap their wires off. I decided to build a replacement that I can put in the nylon holder instead of the almost unavailable incandescent bulbs.
REPLACEMENT BULB PLAN
I decided to use a yellow LED, soldering on a 470 ohm resistor to limit the current, placed into the nylon holders. When I tested it, it appeared to glow satisfactorily and should look fine illuminating the display panel legends.
I tried to install them on the bottom left board (from the rear of the display panel) which has only two lamps, for Carry and Overflow status. The Overflow lamp has never illuminated but Carry was working. When I looked closely at the board, I saw two problems. First, the Overflow SCR had the ground lead broken off near the plastic body, so it would not work. Second, the Carry SCR had a lead that was never soldered by the IBM factory. Instead, it was just bent into contact with the PCB trace.
BAD WORKMANSHIP FROM THE FACTORY
You can see in the picture that a lead from the SCR is not actually soldered in place, it is just folded over the trace and solder only coated below it. I repaired it after this picture was taken.
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