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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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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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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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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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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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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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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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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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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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 Post subject: Re: convert 78rpm turntable to play 33's??
PostPosted: Dec Sun 17, 2017 11:44 pm 
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I ground the motor shaft down on an RCA radio phono back in the late 1950's and it worked great with the original crystal cartridge installed a sapphire all speed needle. I also put inverse feedback which it didn't have in the amplifier which improved the sound greatly.

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 Post subject: Re: convert 78rpm turntable to play 33's??
PostPosted: Jan Wed 24, 2018 9:47 pm 
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Location: Metzger Oregon
Tube Radio wrote:
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.


I was planning to take a slightly different approach. I checked with a strobe disk and verified that the record player is currently running dead-on at 78 rpm. Knowing this, I measured the motor shaft diameter with calipers. I then did the math to figure out what diameter would give me 33 rpm. I might shoot for a little larger to start with, and sand the shaft down, but I'm thinking I can get close enough that any error would not be noticed.


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 Post subject: Re: convert 78rpm turntable to play 33's??
PostPosted: Jan Thu 25, 2018 12:33 am 
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There's an all-electrical method of lowering the speed in Popular Electronics, August 1956 page 89. http://americanradiohistory.com/Archive-Poptronics/50s/56/Pop-1956-08.pdf
This method undoubtedly works but I wonder how consistent the speed will be. Also, I would check VERY carefully that the motor isn't overheating. However, it is easy to try and is easily reversible if it doesn't work as desired..

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 Post subject: Re: convert 78rpm turntable to play 33's??
PostPosted: Jan Thu 25, 2018 5:21 am 
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Getting the speed down to 33 is the easy part. The problem is the arm, as previously mentioned. 78 players typically had big, massive arms with a nail for a bearing. The tracking force was often measured in ounces. A modern magnetic cartridge is not going to work very well in an arm like this.
Rick


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 Post subject: Re: convert 78rpm turntable to play 33's??
PostPosted: Jan Thu 25, 2018 8:53 pm 
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Location: Metzger Oregon
I disagree. I'm using an Alliance platter and motor assembly just like the one shown in the 1955 article mentioned above. I looked through my box of misc record player parts, and did find some tone arms that look to be pre-war - they are stamped steel and would track very heavy. Too bad as they look cool. I also have a few that are late 40s vintage, they are cast aluminum and are very light, especially with a more modern plastic bodied cartridge. I decided to go with a Shure Glider, but there are others that are similar. Some have a spring to adjust the tracking force. For those that don't, I've had good luck in the past just attaching a mass to the other side of the pivot as a counter weight. No problem getting them down to a few grams. I also have several plastic tone arms, that look like the ones you'd see on a low end 50s portable, but wanted to stick with metal for a more period look. As far as cartridges, I went with a modern ceramic, as I intend to use the record player as an attachment on a 30s or 40s radio ceramic/crystal input, and because I suspect I may get too much hum with magnetic anyway.


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 Post subject: Re: convert 78rpm turntable to play 33's??
PostPosted: Jan Fri 26, 2018 12:58 pm 
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Yes unless the motor is a four pole motor you would get a lot of hum with a magnetic cartridge.


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 Post subject: Re: convert 78rpm turntable to play 33's??
PostPosted: Jan Fri 26, 2018 3:58 pm 
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What gridleak said.

When I was younger I did just that, and it did work - and it quickly ruined my records.

If you can get your stylus to track around 3-5 grams and get an output from the cartridge and get your needle to track, then you will be all right. That won't be easy though.


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 Post subject: Re: convert 78rpm turntable to play 33's??
PostPosted: Feb Fri 02, 2018 12:50 am 
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I thought I would post a follow-up as I'm getting a back into this project a bit. First a little background, as information that applies to my project may not apply to others here. I have a collection of 78rpm records, but I mainly have them for use with my 78-only changers - the type of music I like is much more easily found on 45 or 33. I decided to build a small, pre-war style record player attachment for use with my 30s and 40s radios, and decided that my main objective was to play LPs. This will be a new wood cabinet with a vintage tone arm, motor, and platter.

I have several Alliance motor/turn-table assemblies identical to the one shown in 1955 article that Jim linked to previously (see picture). Most of these were cheap/free salvaged for old homebrew stuff that I got from the local radio club or from Craigslist. Note that Alliance made an 8" turntable and a 9", possibly others. I decided to use the 8" version for my project. Note that the author of the article used the 9" version, and I found that his 0.100" diameter matches my calculations for the correct diameter for 33.3rpm. For my 8" turntable, this would need to be a little smaller. As others have mentioned, there should be no issue reducing the speed. One side note, though, I was thinking about the all-electric method that Jim posted in the 1956 article. I was also concerned about heat or other issues caused by this method, as the motor would be less than half speed. One idea I had, though, was to reduce the motor shaft to make a 45 RPM speed, then use the electronic method to further reduce that to 33.3. This would run the motor at 75% of full speed and maybe reduce any ill affects. This would work for me as I have no interest in playing 78s on this record player.

For the tone arm and tracking issues, what I end up with will be very similar to the Columbia 105 that I used to have (see picture), only with a single speed instead of 3 speed turntable. I placed a modern stereo cart in mine and it tracked well with only a few grams pressure. I understand that this would not be the case for a changer, but definitely an option for those who can convert to this style of arm on a portable or suitcase single play type of record player.

I'll post an update once I have time to get the speed reduced, but unfortunately due to other priorities, it'll be a couple weeks.


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