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 Post subject: convert 78rpm turntable to play 33's??
PostPosted: Nov Tue 21, 2017 2:30 pm 
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Joined: Dec Tue 06, 2011 1:27 pm
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Location: Wyoming, PA
Hello all, so sorry if this has been asked a million times, but why can't one convert a 78rpm turntable to 33rpm's? Can it be done successfully? Is it the counter EMF created when stepping down the voltage? Any info would be greatly appreciated, thank you!!


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 Post subject: Re: convert 78rpm turntable to play 33's??
PostPosted: Nov Tue 21, 2017 3:36 pm 
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Location: Annapolis, MD
If it uses a synchronous motor, then the voltage does not control the speed.

For any "traditional" record player...which includes all that are 78-only...I think you'd have to modify the spindle and/or the idler, or maybe install a different motor. In that vein, my turn for a question:
For a 60-Hz synchronous motor, what range of speed control is available by changing the frequency?

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 Post subject: Re: convert 78rpm turntable to play 33's??
PostPosted: Nov Tue 21, 2017 3:59 pm 
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Joined: Aug Sun 23, 2015 6:01 pm
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Location: South Jersey East of Philly
Here's a Google page of that topic on the AR forums......
https://www.google.com/search?q=+78+turntable+to+play+33rpm&sa=Search&cof=LW%3A460%3BALC%3A%23880000%3BL%3Ahttp%3A%2F%2Fantiqueradios.com%2Fgoogle.gif%3BLC%3A%23880000%3BLH%3A60%3BAH%3Acenter%3BVLC%3A%23888888%3BAWFID%3Ac6d45a6e1fd29e31%3B&domains=antiqueradios.com&sitesearch=antiqueradios.com


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 Post subject: Re: convert 78rpm turntable to play 33's??
PostPosted: Nov Tue 21, 2017 5:08 pm 
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Joined: Nov Wed 27, 2013 5:59 am
Posts: 381
Location: Metzger Oregon
There are several things that need to be considered if you are seriously thinking of doing this:

-Is this a changer or a manual play turntable?
-Do you want 33 only, or a way to switch between speeds? Okay with a non-reversible change?
-Can a more modern cartridge be adapted to the tone arm?

If you have a changer, it may not want to track light enough to play an LP record. Note that in the 78 era, there was no need to worry about the excessive tracking weight, and that the trip mechanism may not work with a lighter tracking cartridge.

If you have a manual play turntable, you might be able to reduce the diameter of the motor shaft to decrease the speed of the platter. I bought several motors used in these types of record players, and am going to attempted to use a lathe to do just that. Unfortunately I can't say just how successful I will be just yet, time will tell.

A final option, again with a manual play turntable, many many of them use a mechanism made by General Industries or Alliance. In later years, these companies also made 3 speed platters that mount in a similar fashion as their older 78 only cousins. You could get one of these and swap it in place of the old mechanism. I have a small radio with a record player under the lid, and I plan to do just that. The tone arm uses a standard 1/2" mount cart and was easy to lighten up, so it's just a matter of installing the new motor assembly. Be sure to keep the distance between the spindle and tone arm pivot the same as before.


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 Post subject: Re: convert 78rpm turntable to play 33's??
PostPosted: Nov Fri 24, 2017 1:42 am 
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Location: Tucson, Arizona U.S.A.
Here you go: http://americanradiohistory.com/Archive-Poptronics/50s/55/Pop-1955-05.pdf page 72. If this displays poorly, save the file to your hard drive and use a pdf reader that is separate from your browser. Many web browsers don't display pdfs properly.

And, as everyone else has said, just because someone has done this doesn't mean that it is a good idea.

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 Post subject: Re: convert 78rpm turntable to play 33's??
PostPosted: Nov Sat 25, 2017 4:48 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: San Jose, CA USA
I filed down the shaft of the motor in a 78 phonograph when I was a kid to make it play 33 records. I slightly overshot, so it ran a touch slow. But it worked well enough. I basically did as described in the magazine article linked above (on page 73).

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 Post subject: Re: convert 78rpm turntable to play 33's??
PostPosted: Nov Tue 28, 2017 9:54 pm 
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Joined: Dec Tue 06, 2011 1:27 pm
Posts: 128
Location: Wyoming, PA
Thanks for the info guys!!


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 Post subject: Re: convert 78rpm turntable to play 33's??
PostPosted: Dec Sat 02, 2017 3:04 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Warner Robins, GA
classicelectronicsguy wrote:
I bought several motors used in these types of record players, and am going to attempted to use a lathe to do just that. Unfortunately I can't say just how successful I will be just yet, time will tell.



Might I suggest sandpaper or some sort of file and use it with the motor running then checking it for proper speed on the intended record player. Repeat until the desired speed is achieved.


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