Famicom — FDS Drive Calibration (WIP)

Head alignment on the Famicom Disk System drive is mission critical. Also, the level of tolerance for head calibration is minuscule at 50 micrometers (0.05mm). In my search to simplify and quantify this calibration process I have developed several techniques. However, I believe my newest technique is the simplest:

The farthest edge of the read-head​

from the center of the disk spindle​

should be 35.5mm from the center​

of the disk spindle.​

It's that simple. Right? Let's visualize what I mean to begin with.

Note the gap at the front of the read head sled? It should be the maximum size. The gap at the back on the end-stop switch should be as small as possible and the read-head should be as far back as possible.

With your drive in the home position you can proceed to measure the distance of the read head from the spindle.

The distance from the farthest​

edge of the disk spindle to the​

farthest edge of the read-head​

should be 37.95mm​

Go / No Go Jig (Don't Dance)
I've made a tool to simplify this process, it's essential a "go / no go" jig:

I have several of these jigs which will be listed on eBay. The design is available to download here.

Some may be wondering how this differs from the existing calibration technique which adjusts the set screw on the read-head sled. This screw exists to accommodate for manufacturing defects — the inclusion of this screw indicates there are known variances in the manufacturing tolerance.

However, the distance from the center of the disk spindle is always the same for every reader as this is the common truth among the drives and the purpose of the set screw — to adjust the distance from the center of the disk spindle. This go / no go jig is designed to tuck under the disk cover so that measurement and calibration is as simple as possible.

Epilepsy Warning
This guide utilizes a free stroboscopic app at frequencies of approximately 7 Hz. If you're at risk of seizures or epilepsy then please proceed with caution. It's possible to follow this guide without using a stroboscopic light.


Calibrating the motor can be done precisely with no other tools than a modern smart phone and a small screwdriver. It's only necessary to measure speed from the disk table shaft (see diagram below). Drive terminology is borrowed from Mitsumi's 1982 US patent filing for the Quickdisk drive.

Target Speeds:​

1. The disk table shaft should rotate at 400 RPM.
2. The motor belt holder should rotate at ~800 RPM.

It's sufficient to measure the speed of the Disk Table Shaft as it's a product of the motor speed adjustment.

Technique​

Before You begin:​

1. Contacting the outer metal shell of the motor while adjusting the inner potentiometer will result in a short if the tool is conductive. I recommend using a plastic screw driver or isolating the shaft of conductive screw drivers with heat shrink / tape.
2. I recommend downloading the free StrobeLight (iOS) app or any other stroboscopic app you prefer. You can also use a handheld contactless tachometer, like this one from Amazon.

​

Preparations​

If you're using a stroboscopic app then proceed to draw a line down the center of the disk table shaft. If you're using a contactless tachometer then this step is not necessary but you may need to apply a small piece of reflective tape for the contactless tachometer (refer to your tachometer's manual).

It's not necessary to open the drive when measuring only the disk table shaft.

Power The Motor​

The primary goal is to energize the motor. This can be accomplished several ways, including applying 4.95 VDC directly the tabs with polarity labeled on the motor or inserting a disk while hooked up to your complete famicom disk system.

Measure Speed​

Whether you're using a stroboscopic solution like the iOS app linked above or an alternative like a contactless tachometer you can now proceed to energize the drive and measure the rotational speed.

Target Speeds:​

1. The disk table shaft should rotate at 400 RPM.
2. The motor belt holder should rotate at ~800 RPM.

​

Using StrobeLight​

StrobeLight works by flashing the iPhone's camera light at fixed frequencies. Drawing a line down the center of the disk table shaft will allow you to visualize the stroboscope's proximity to the rotation target.

Tips:​

1. Before you begin, adjust StrobeLight's GUI to the target RPM.
2. It's necessary to have a high contrast line through the middle of the drive table shaft.
3. On StrobeLight press the "power" button. This will begin flashing the camera's light at the target frequency (e.g., 6.67 Hz).
4. If your line is slowly rotating clockwise then it's spinning too fast.
5. If your line is slowly rotating counter-clockwise then it's spinning to slow.
6. If the movement of the line is choppy and hard to follow then you may need to either speed up or slow down StrobeLight's RPM target in order to gradually adjust the motor's speed until it's at the target RPM.
7. The line will completely stop moving when the motor matches the target RPM set in StrobeLight.
8. You may have the best results visualizing the rotational speed by having your camera's light as close as possible to the marked surface of the disk table shaft.

​

Speed Adjustment​

Speed adjustment can be accomplished by rotating the potentiometer inside of the motor either clockwise or counter clockwise.

Tips:​

1. The potentiometer is sensitive to small adjustments, patience will be invaluable if you want precise speeds.
2. Counter-clockwise adjustments will increase RPM.
3. Clockwise adjustments will decrease RPM.
4. The potentiometer is ~8 mm deep within the motor.
5. The potentiometer is slotted for a flat-edge screwdriver.

Adjustment Examples​

• If the disk table shaft is rotating at 450 RPM then you must rotate the adjustment counter-clockwise. Physically, this will translate to ~2 mm of rotation "to the right" when the front disk bay is facing you.
• If the disk table shaft is rotating at 350 RPM then you must rotate the adjustment clockwise. Physically, this will translate to ~2mm of rotation "to the left" when the front disk bay is facing you.

This is the screwdriver I modified for adjusting FDS motor speeds.


Editor's Note:
I discovered the stroboscope app I was using made it easy to have a ~2x factor error. I had previously reported that the RPM was 873RPM however I can confidently say it is 400 RPM: