Refining the performance of the Hurst motor
The Merrill Mods
One of the quintessential questions facing mid-80’s AR turntable owners trying desperately to bring their turntables to the next level o fplay is how to quiet the noisy stock AR/Hurst motors. In a recent interview with George Merrill, the creator of the highly acclaimed Merrill turntables and the highly successful Merrill AR mods, he revealed the exact process by which he modified the Hurst motors sent to him for modification in the late 80’s to early 90’s. Though I have not yet carried out these modifications personally, I have seen (and not heard) many of these refined motors over the past 4-5 years. They are definitely the most quiet I have come across. Until speaking with George, however, I could never tell by just looking at the motors, what exactly he had done to improve their performance.
The trickiest part of the process is/will be getting the right tool for the job. I plan to explore getting a couple dozen made and selling them cheaply. If someone else would like to do this part instead because they have the tools/expertise, let me know. You can sell them free of charge from my site.
According to George, the tool needs to be fashioned from a piece of 1/4″ steel rod. Cut the rod to about 3″ long, and in one end, bore a hole that is just slightly larger than the shaft of the motor. George is pretty sure the diameter was 1/8″ but he said to double check. The hole needs to be deep enough for the rod to fit over the shaft and the “bottom” to touch the small “collar” that is at the base of the spindle. The end product will resemble a nail punch but with a deeper hole. You will also need a plastic mallet.
In general terms, according to George, the Hurst motor has a sleeve bearing, and the shaft is just pressed into place by this collar. With his fine measurment tools, he found that exerting pressure downward on this collar can reduce the “slop” or “play” by nearly half.
I will now describe the process in detail…
Before you use the bored rod tool, remove the pulley and put 3-4 drops of 30 weight oil right at the point where the spindle enters the collar. Then run the motor overnight. (You should already notice an improvement based upon that simple task.) The next day, turn off the motor. Then, remove the bottom of your turntable, and then remove the metal plate that covers the bottom of the motor. It should come off pretty easily by twisting it in either direction. Save the plate for reinstallation.
While the turntable is on its side note the way the shaft from above is visible from below. This area is shaped like a hexagon nut. Place the turntable on three to four tall cans, and then use a another piece of 1/4″ rod (non-bored) to support the hexagon area (especially the shaft in the center of the hexagon.) Ideally, the rod would be supported by a solid surface rather than just your hand. When I tackle this mod, I plan to use a small piece of two by four and some wood shims. Whatever you use, the rod needs to FIRMLY hold the spindle of the motor so it won’t pop out, when, in the next step, you strike the spindle from above.
You are now ready to clean up the slop. Place the bored rod over the spindle so that it makes contact with the gold/brass colored collar. Hold the rod as perpendicular to the motor as possible, and then give the rod two firm taps with the plastic mallet.
Remove the rod from below and the bored rod from above. Start the motor, and check to see if the spindle still turns. If it doesn’t you have tapped too hard, and you will need to use a squared off (use a file) nail to gently tap the spindle from below so it rises back up and allows movement once again. Repeat the setup explained above, but this time, strike the bored rod only once, and more softly. Hopefully this time you will have succeeded in getting rid of the “slop” without stopping the turning of the spindle.
George said he used to use fine measuring devices to take the play to 1/5000th of an inch. However, he said that if you carefully use a trial and error approach, you can tighten the collar without these tools and thus reduce the spindle “chatter.”
Once the collar was tightened, George said he placed another 3-4 drops of 30W oil on the spindle collar and left the motor running straight for 5-7 days. (Days!) After, he would measure the play to see if it had changed. If you don’t have fine measuring devices, you can just wiggle the spindle to see whether or not there is significant play. If you feel the play has increased over the 5-7 days, you can tap the collar again, using the same procedure as earlier described.
The final trick George mentioned was that instead of melting lead into the top of the motor pulley (as some advise)a good way to dampen resonances is to squirt some silicone rubber into the top of the pulley. You do this after the pulley is installed back on the spindle, overfill it slightly, and use a putty knife to remove excess and even off the surface. With silicone rubber you are less likely to inadvertently throw the pulley into an imbalance.
Update: May 2021
Instead of a bored rod, you can probably get away with a 3/16ths socket. Something like this.
I have used this process a couple hundred times since he covered it with me. I would guesstimate about a six or seven out of ten success rate. If you find you just can’t get yours to “behave” you might consider the Upgrade Motor George had spec-ed to solve most of the issues with the original. The one I sell is available here.