Floppy drive spindle motor speed measurement

The floppy drive in my Apple III is not working well enough to boot floppies written on my Apple II. I think the spindle motor speed may be off. I’m hoping that’s what it is, since it’s the easiest thing to fix. I’m putting together a timing strobe light to measure it, using a microcontroller and an inexpensive Harbor Freight LED flashlight.

Using a strobe to adjust floppy disk drive spindle motor speed works the same way as the strobe lights on turntables. The drive spindle pulley has a sticker with 50 Hz and 60 Hz stripes. One looks at the stripes illuminated by the strobe light, and if the drive speed is set correctly, the stripes appear to stand still. Otherwise they will appear to rotate in one direction or the other, depending on whether the motor speed is low or high, and at a rate proportional to the speed error. Traditionally a neon lamp was used, powered by AC mains power, at 50 Hz or 60 Hz depending on your locale.

I will use a Harbor Freight LED flashlight, which comes in a two-pack (one with red housing, one with blue), currently on sale for $2.99.  The flashlights use three AAA “heavy duty” zinc-carbon cells. As Richard Ottosen has observed, there is _NO_ current limiting, other than the impedance of the AAA cells, so replacing them with alkaline cells may reduce the life of the LED(s).

Two weeks ago I bought a two-pack of the flashlights. They had a package with a single black housing flashlight for less money. It had nine LEDs that appeared to be conventional T1-3/4 package. The two-pack (red and blue) had a single LED. I compared them side-by-side, and the single LED flashlights were brighter, so I bought those. Today I went back for more, and discovered that the two-packs on the rack are actually a mix; some packs have nine-LED flashlights, and some have single-LED.

Today Richard modified a flashlight for me, drilling a hole in the side and soldering wires to the battery holder, so I can power and control it externally. He measured the current with the provided set of zinc-carbon AAA cells, and found that at initial power-on it draws over 450mA, but drops to closer to 400mA.

This evening I connected it to my bench power supply and took measurements at various forward voltages in 0.1V steps. I was in a hurry and did not let the LED reach thermal equilibrium at each step. I put the data into a spreadsheet with a graph, and exported it as a PDF. What surprised me is that a forward voltage of 2.2V actually did produce an extremely dim light, though the power supply (with 1mA resolution) still read 0mA. At 2.5V it was still pretty dim, but there was a significant jump in brightness between 2.5V and 2.6V, and the power supply read 2mA at 2.6V.

I had originally hoped that using only two AAA cells and a very inexpensive boost regulator, I could drive the enable input of the regulator to modulate the LED on and off. However, at 3.0V, the LED draws 73mA, and produces an impressive amount of light. I think I’ll either have to use a “true output disconnect” boot regulator, or use an nFET to switch the power, rather than the boost regulator enable.

Building a self-contained timing stobe light with a boost regulator is beyond the scope of this RetroChallenge project. For now, I’ll just power it from a bench power supply.

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