Floppy Disk Drives / Measurements / Disk Drive Speed

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How fast does my floppy drive spin?

As an introduction to the topic, here is a quote from Herb Johnson, whose website provides a wealth of information. Simply great!

Floppy drive rotation adjustment

There have been some questions about adjusting the rotation rate of floppy drives. Floppy drives rotate the floppy on a spindle, which is driven by some motor. Many drives allow for adjusting that speed via a small pot (potentiometer, a variable resistor) on the circuit board connected to the spindle motor. AC powered spindles don't have an adjustment, the AC line frequency and the mechanics set the speed.

That spindle often had a decal or lable which consisted of a series of black bars, radially around the edge of the spindle, a kind of star-burst pattern. Some of them were labled "60" and "50" with slightly different patterns. Typical rotation speeds are 300 RPM and 360 RPM, depending on the drive size and density of the diskette's format.

Those lables are actually stroboscopic patterns. If you use a neon lamp or some fluorscent lamps, the 60 HZ (or 50 HZ) line current variations in the lamp would cause the rotating pattern to stand still, IF the drive was at speed. If it was slow or fast, the pattern would appear to rotate slowly. The tech would adjust a small pot to change the speed. Most older floppy drives are not crystal controlled.

At one time, I made a photocopy of one of those patterns, and simply placed it on any drive which did not have such a decal to tune up the speed. Of course, you need different patterns for 300 rpm and 360 rpm, and physically different sizes for 5.25 and 8-inch floppy drives (the size is only a convenience, patterns for the same speed have the same number of stripes).

If response warrents I could scan these patterns and put them up on my Web site, otherwise just look over your old drives and see if you have some. Dual speed drives (later 5.25 inch floppies) would require the use of two of these patterns. Also, some diagnostic software on some systems simply time the index hole to compute spindle RPM; one could do the same with a counter electronically connected to the output of the index sensor on the drive.

(Herb Johnson: www.retrotechnology.com)

The question of how to measure the rotational speed of floppy disk drives is actually quite easy to answer. In the following I would like to describe three possibilities.

Please have a look at the corresponding floppy disk manual "where" to change the speed.

1) Software: RPM (by Sydex)

I have already described a short article about this DOS program. Just follow this link. RPM is suitable if you want to check the rotation speed quickly.

2) Software: ImageDisk / IMD (by Dave Dunfield)

IMD which also supports the measurement of rotational speeds, is much better. When I make adjustments to a drive, I use IMD because you can simply set the measurement to "zero" using the SPACE Bar without stopping the rotation.

(T) RPM

This function calculates the revolutions per minute of the diskette drive. Note that this is a simple function which employs only the standard DOS 55ms timer tick - The results will become more accurate the longer you allow the test to run.
Note that the result is cumulative, and will not accurately reflect changes to the drive speed. If you are adjusting the drive, press SPACE to restart the calculation after each adjustment.

(Dave Dunfile: www.classiccmp.org)

3) Hardware: Using a scope

If you want to measure the drive speed with an oscilloscope, it's like shooting sparrows with a cannon, but it works fine, as you can see in the following figure. The difference between two pulses are 4 x 50 ms = 200 ms. This corresponds exactly to 300 RPM (300 rounds per minute = 5 rounds per 1000 ms = 1 round per 200 ms). The measurement was carried out on my BASF 6106 (Siemens) with an OWON SDS1102.

Oscilloscope
Using an oscilloscope

The only difficulty was to find the test point for the INDEX pulse. Since the PCB of my BASF 6106 (Siemens) does not match the one in the manuals, I had to search a bit, but ... here it is.

Update: 2019-03-28

By chance I found the manual of the BASF 6116/6118 in the technical manuals of my floppy disk drives and see there ... The PCB used there is almost identical to that of my two BASF 6106. This concerns in particular the so-called test points (TP).

Oscilloscope
Testpoint (TP8) for the INDEX pulse

One centimeter to the left is the large test point (TP9) for grounding; not to be overlooked!

Oscilloscope
The potentiometer R47 for speed adjusting (Beckmann).
Oscilloscope
Checking with the program IMD. It fits.