Slowing Motor

Thread Starter

Jchristie_1979

Joined Jun 29, 2021
15
Restoring vintage radio/record player. Record player plays only 78’s. Motor for player is 115 vac at 60hz. I need to slow down motor so that platter spins at 33 rpms for 33 records. I have been told an in-line switch with resistors to step down voltage but is 115 to high for resistors? Won’t they over heat? I have also seen light dimming switches installed to accomplish this for similar motors just not record players. Thoughts???
 

MaxHeadRoom

Joined Jul 18, 2013
23,906
The motor is a synchronous version, it requires a different dia pulley.
We used to convert ones comming from Europe 50hz to NA 60Hz.
 

Thread Starter

Jchristie_1979

Joined Jun 29, 2021
15
The motor is a synchronous version, it requires a different dia pulley.
We used to convert ones comming from Europe 50hz to NA 60Hz.
So are you referring to increasing the size of the idler wheel to decrease the rpms on the platter? Makes sense in my head just seeing if that’s what your talking about.
 

Lo_volt

Joined Apr 3, 2014
233
Keep in mind that the needle for a 78 rpm record is usually larger than that for a 33 rpm LP. When LP's first came out turntable arms had two needles, one for the "new" LP's and another for the older 78 rpm disks. The user would rotate a selector on the arm to choose which needle was to be used.
 

Thread Starter

Jchristie_1979

Joined Jun 29, 2021
15
I got new needles to make everything work. This darn motor is what’s hanging me up. Too old to be compatible with new stuff and no replacement stuff. Just trying to find a way to slow it down electrically or mechanically.
 

djsfantasi

Joined Apr 11, 2010
7,913
Keep in mind that the needle for a 78 rpm record is usually larger than that for a 33 rpm LP. When LP's first came out turntable arms had two needles, one for the "new" LP's and another for the older 78 rpm disks. The user would rotate a selector on the arm to choose which needle was to be used.
THAT’S what that knob / selector did!

Edit: clarification
 
Last edited:

Lo_volt

Joined Apr 3, 2014
233
There is no knob. It’s a 1942 zenith. Strictly 78’s. Trying to retrofit for 33’s
You misunderstand me. The knob was for changing needles. It had nothing to do with motor speed. It was present on turntables that were newer than yours. It was there for backwards compatibility with the 78 rpm turntables. djsfantasi was commenting because he'd seen the needle knob but didn't know what it was for.

I'm just highlighting the fact that the groove on a 78 rpm record is larger and deeper than that on a 33 rpm LP. The needle on an older 78 rpm turntable will gouge the groove of a 33 rpm LP. You may need to modify the arm to accommodate a needle that is designed for 33 rpm LP's.
 

MaxHeadRoom

Joined Jul 18, 2013
23,906
I got new needles to make everything work. This darn motor is what’s hanging me up. Too old to be compatible with new stuff and no replacement stuff. Just trying to find a way to slow it down electrically or mechanically.
Mechanically is the best option, what size intermediate pulleys do you have right now?
 

drc_567

Joined Dec 29, 2008
1,156
... The important consideration is the ratio of the drive wheel diameter to the driven wheel diameter. It looks like the best approach would be to fabricate a smaller diameter attachment to be fastened to the top of the original drive wheel, and then use an appropriate pulley band of some sort.
 

Thread Starter

Jchristie_1979

Joined Jun 29, 2021
15
Trying to work my way backward from platter since I know RPM of it as well as diameter of platter and idler wheel. Simple math but math isn’t simple for me. Scratch paper time!!
 

Thread Starter

Jchristie_1979

Joined Jun 29, 2021
15
So platter of 8” turning at 78 rpm is turned by 2.5” idler wheel at 250 rpm which is turned by motor shaft of 3/8” (.375) at 1666 rpm.
 
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