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HP 7475A Plotter: LED Lighting

If white LED strips had existed in the early 1980s, the engineers responsible for the HP 7475A plotter would surely have done this:

HP 7475A Plotter - LED paper illumination

HP 7475A Plotter – LED paper illumination

Not, that’s not stretched vertically: I bought a ream of B-size paper (11×17 inches) just for plotter demos.

Although the power supply does have a +12 V output, it comes from a TO220 transistor without a heatsink. The +5 V supply uses a robust TO3 transistor on a huge quad heatsink that can surely dissipate another watt or two without getting any sweatier.

I powered the LEDs from a dirt-cheap boost converter that provides a convenient brightness adjustment; it’s set to 10.5 V and that’s plenty bright enough. The converter attaches to pair of wires soldered across VR1, which is probably a crowbar that blows F3 (not shown) in the event the regulator fails hot:

HP 7475A - LED power tap - schematic

HP 7475A – LED power tap – schematic

They don’t make power supplies like that any more.

The part locations (“O9” looks like a typo):

HP 7475A - LED power tap

HP 7475A – LED power tap

The PCB has holes in exactly the right spot for a zip tie anchoring the wires exiting to the bottom:

HP 7475A Plotter - LED power tap - PCB top

HP 7475A Plotter – LED power tap – PCB top

This vertiginous view shows the inside of the case atop the chassis, with the boost converter affixed to the galvanized steel pan with foam tape and the LED wires stuck down with Gorilla Tape:

HP 7475A Plotter - LED strip and boost converter

HP 7475A Plotter – LED strip and boost converter

Red silicone tape around a PCB-mount coax jack rounds out a true hack job.

Although I didn’t bring the plotter to the CNC Workshop, that venue’s dim light reminded me that you can never have enough light when you’re showing off your toys: the LED panels on the M2 and the LED light bars on the Model 158 sewing machine were the brightest spots to be seen.

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