Motor Drive Calibration
Your turntable motor is pre-calibrated at the factory and in normal use, should never require re-calibration.
Since the initial release of this controller, we changed from the originally specified cloth belts to Kapton belts which perform similarly to Mylar but are much more robust and have a perfect splice.
Changing to Mylar (or Kapton) requires re-calibration of the controller due to the large difference in belt thickness, and this procedure was documented to facilitate re-calibration in the field.
The cloth belts are .035″ thick and Mylar (and Kapton) is .002″ thick. Thinner belts require the motor to turn slightly faster in order to achieve 33 and 45 rpm.
This influence of belt thickness on the pulley to platter gear ratio is counterintuitive and this is somewhat obscure knowledge. For the purpose of calculating the gear ratio, the radii of the pulley and platter need to add approximately 1/2 of the belt’s thickness (technically, it’s the belt’s neutral axis). So, a small and equal change to both the pulley and platter’s effective radius will have an effect on the platter speed, and this is the calibration we’re making when we change belt types.
The new controller design has a much finer, more precise speed adjustment and this was accomplished by specifying lower values for the precision, 10-turn potentiometers (the main speed trim pots you’re familiar with). This limits their adjustment range. To ensure a full range of adjustment, we added the blue trim/calibration pots (pictured above), and it is these pots which must be adjusted (re-calibrated).
These trim/calibration pots are accessed through the holes adjacent to the red and blue speed indicator LEDs (labeled in the photo above).
- A small, straight bladed jeweler’s screwdriver – one with a blade that is about half the diameter of the access hole. This is a bit larger than screwdrivers that come with eyeglass repair kits, and smaller than the screwdrivers that come with ZYX and Benz cartridges.
- Your speed strobe of choice.
- A small flashlight.
Overview: You will be installing the Mylar belt and practicing engaging the screwdriver in the screw slot of the blue, trim/calibration pots:
- Install the new belt and set its tension as you normally would.Remember that Mylar requires considerably less belt tension than the fabric belts.You will be returning to this lighter tension.
- Let the belt “settle” into it’s running position on the platter.Run the motor for a few minutes, turn it off, readjust belt tension and guide the belt onto the correct height on the platter as necessary – exactly as if you were initially installing/aligning any new belt.If you have any doubts, run the motor for 10-15 minutes. You’re well familiar with a belt that’s tracking correctly.
- Set both of your main pots (the large ones with the protruding shafts that you’re familiar with) to their middle position – 5 turns from either the clockwise or counterclockwise stop (10-turn pots).After calibration, you want to have an equal amount of upward and downward adjustability with your main adjuster pots.
- Turn off the controller
- Look at the photo below, and review the note at the end: “About the Calibration Pots and Engaging the Screwdriver”Familiarize yourself with the location and depth of the straight slotted screw head on the blue trim/calibration pots.Use a flashlight to look into the access hole.The screw should be in vertical alignment with the hole. If it’s not, refer to the note at the end of this guide.Using the screwdriver, feel for when it engages the slot in the adjustment screw (see photo). It takes a bit of a delicate touch.Use your fingertips for sensitivity and use the weight of the screwdriver to fall into the slot (don’t push on the screwdriver!).
Start by calibrating 33 rpm (the speed linked to the blue LED) and repeat these steps to adjust the 45 rpm speed (the red LED).
Note: the controller in the photo below has been removed from the motor pod to show the screwdriver engaging in the calibration pot. You do not have to remove the controller from your motor pod.
- Place your speed strobe on the platter.
- Turn on your controller and let the speed stabilize (wait 45 seconds).
- Turn on your speed strobe. You should observe that the turntable is running slow.
- If the screwdriver isn’t engaged in the slot of the screw in the calibration pot, do so now (remember – “light touch”).
The screwdriver will likely remain engaged in the slot if you let go of the screwdriver.
- Start by turning the screwdriver 3 turns clockwise (these are 25 turn pots) and observe the speed. The controller will take some time to lock speed.
- Continue adjusting slower (counterclockwise) or faster (clockwise) until you are very close to speed. Use the “regular” pots to perform the fine adjustment. You shouldn’t have to turn the large pots more than half a turn if you’ve taken a bit of time with the screwdriver and the calibration pots.
About the Calibration Pots and Engaging the Screwdriver
Note in the photo how the pot is standing on its narrow edge. There’s a tendency for it to tilt or wiggle a bit, and more so if you force the screwdriver into the screw slot. This is the reason for the “light touch”. The pots are quite robust, but we never encourage torturing any component unnecessarily.
If the the screw isn’t in vertical alignment with the access hole, It will be difficult to engage the screwdriver in the screw slot.
- If you have difficulty, view the screw head through the access hole with your flashlight. Observe its orientation and use the screwdriver to tilt the pot into alignment.
- You may need the next size smaller screwdriver from the one you’re using to adjust with which is why we’ve shown two screwdrivers in the photo above.