Author Topic: Had one given to me today. Not sure what model.  (Read 939 times)

Bobby Basham

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Re: Had one given to me today. Not sure what model.
« Reply #30 on: August 10, 2018, 07:11:08 PM »
I don't know if I can upload a file, but the MAGNAVOX SERVICE MANUAL #5276, issued April, 1969 should look like this...

Someone was kind enough to do a PDF scan many, many years ago, and will try to upload it for you (if permitted).  Others can benefit from this also.  Hopefully, it will print.

If the moderator sees this, I hope it can be posted to the downloads section because I didn't see anything there (possibly my oversight) referencing these turntables.

Bill

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Re: Had one given to me today. Not sure what model.
« Reply #31 on: August 11, 2018, 08:36:28 PM »
I agree, I hope it gets transferred to the downloads section.

Bill

kd4pbs

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Re: Had one given to me today. Not sure what model.
« Reply #32 on: August 13, 2018, 08:06:38 AM »
Its like if your car comes with 15 inch wheels. The speedometer is calibrated to that size tires and wheels.  if you change to 13 inch wheels, your speedometer with show faster than you are actually going because the smaller wheels are spinning faster to achieve the same speed.  The RPMs of the smaller tires will always be more than the RPM's of the larger ones, given the same forward velocity.

Not exactly scientific, but it's what I got.  8) 8)
Oh, I understand that concept exactly.  But the thing is, the car's wheels are not an idler wheel; they are directly driven by the "motor" (engine if it's powered by fuel), not indirectly driven like an idler wheel.  They are analogous to the shaft of the motor on the turntable, and the rim of the turntable is the road.  Stick a wheel (the idler wheel) between the actual drive wheel and the road, and it won't matter the size of that wheel.  You will still go the same speed regardless of the size of that wheel.  Of course, we'll end up going backwards if we use an odd number of wheels, but other than this, the diameter doesn't matter.
Happy Monday, y'all.

rdnzl

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Re: Had one given to me today. Not sure what model.
« Reply #33 on: August 13, 2018, 08:35:35 AM »
Given the same motor speed, diameter IS everything.


kd4pbs

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Re: Had one given to me today. Not sure what model.
« Reply #34 on: August 13, 2018, 08:47:29 AM »
Except when it is an idler wheel.

kd4pbs

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Re: Had one given to me today. Not sure what model.
« Reply #35 on: August 13, 2018, 08:49:41 AM »
Do you think that the surface speed of the tires on a tractor trailer going 60 MPH is somehow a different speed than the surface speed of little 12" trailer tires on a trailer traveling at 60MPH?

kd4pbs

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Re: Had one given to me today. Not sure what model.
« Reply #36 on: August 13, 2018, 08:59:31 AM »
https://en.wikipedia.org/wiki/Idler-wheel

"Gear ratio
An idler gear does not affect the gear ratio between the input and output shafts. Note that in a sequence of gears chained together, the ratio depends only on the number of teeth on the first [the diameter of the motor shaft] and last [the diameter of the hub on which the idler wheel rides] gear. The intermediate gears, regardless of their size, do not alter the overall gear ratio of the chain. But, of course, the addition of each intermediate gear reverses the direction of rotation of the final gear."

Can we all stop worrying about affecting turntable speed by changing the diameter of the idler wheel now?

rdnzl

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Re: Had one given to me today. Not sure what model.
« Reply #37 on: August 13, 2018, 09:05:05 AM »
If the RPM of the motor remains the same, and the idler diameter changes, the RPM of the platter will be different.

Simple as that.

rdnzl

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Re: Had one given to me today. Not sure what model.
« Reply #38 on: August 13, 2018, 09:37:20 AM »
Do you think that the surface speed of the tires on a tractor trailer going 60 MPH is somehow a different speed than the surface speed of little 12" trailer tires on a trailer traveling at 60MPH?

It takes more RPM to achieve the same speed on those 12" tires.

The forward speed is the same. Forward velocity. But to achive that, the motor has to turn faster.


Look at how your 10 speed bicyvle works.  In low gear, you are pedaling really fast to go 10 mph. But put the chain on the smaller gears, in high, you pedal much slower, and you get the same 10mph/






ed from Baltimore

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Re: Had one given to me today. Not sure what model.
« Reply #39 on: August 13, 2018, 10:43:06 AM »
             God forgive me for jumping in here, but I think you are both "right"  not withstanding that most turntable take-aparters refer to the extra wheel between the motor shaft and the turntable platter as "idler wheel" when on most turntables it is more of the intermediate wheel.
         The usual turntable with a 2 pole motor and a small diameter shaft spinning at around 3600 RPM needs another stage of speed reduction between the motor and the   turntable platter, so the in-between wheel has two separate diameters. The 2 inch or so diameter part of the wheel is engaged to the motor shaft, while the other 1 inch or so diameter part of the wheel rubs against the turntable platter.    Changing the diameter of either part of the wheel without changing the diameter of the other part will affect the platter speed because the motor rubs on one of the wheel diameters only and the turntable rubs against the other diameter only.
         But in the Magnavox with the slower turning 1800 RPM motor the extra speed reduction of the 2-diameter intermediate wheel isn't needed. In this case a "true"  idler wheel can be used with the motor and the platter both rubbing against the same idler wheel rubber.  No matter what the diameter of the wheel is, the  speed ratio between the motor shaft and the turntable platter stays constant. If the idler wheel is tiny, it speed is closer to the motor speed, while a large idler wheel will have a slower speed closer to the turntable platter. But the overall ratio between the motor and turntable speed remains unchanged.
           Very few turntables in consoles have the 4 pole motor,
     

rdnzl

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Re: Had one given to me today. Not sure what model.
« Reply #40 on: August 13, 2018, 10:58:14 AM »
The idler is more of a drive wheel because it is the contact to the “road”. The road being the inside of the edge under the platter. Unless the motor RPM changes,a smaller tire on the wheel would result in slower turntable speed

ed from Baltimore

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Re: Had one given to me today. Not sure what model.
« Reply #41 on: August 13, 2018, 12:32:21 PM »
          Believe it or not, I am going to go out and buy one of those Spirograph toy sets with all the different sized plastic gears and do some experimenting.  I've been wanting to get one for awhile now just to be able to doodle and draw circles and things, so this is as good a time as any. It reminds me of those variable cone diameter transmissions that are on the newer cars with all the speed reductions and ratio changes. 
           I have pretty much forgotten but I assume the Magnavox turntable has a "stepped shaft" on the motor shaft and the idler wheel (or drive wheel) moves up and down to rub on different diameters  of the motor shaft to go from 16 2/3 to 78 RPM. That vertical slide shaft always seems to get gummed up and stiff and it would get hard to change speeds. It was always hard to replace the platter because the idler wheel was always pushing outward and you couldn't get your fingers under the 11 inch platter to tuck it back inwards. And the finish on the inside rim of the platter where the idler wheel would rub always had a slippery feel to it, even after cleaning with alcohol. I used to use steel wool lightly on it to remove the paint which I think they didn't intend to get that paint on that underneath area. The paint seemed to have a Teflon feel to it that prevented the idler wheel from really getting a good grip.
           Some of the Magnavox phono preamps had a bass blend circuit that might have been put in to minimize turntable rumble and feedback at high volume levels.

rdnzl

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Re: Had one given to me today. Not sure what model.
« Reply #42 on: August 13, 2018, 01:02:11 PM »
It's not about any gears or wheels that occur before contact with the platter.  It's about  the platter being driven by a wheel with a rubber "tire" on it. How X RPM's get to that point is irrelevant. It's about wear on the tire that effectively makes the diameter smaller.  If the RPM's delivered to the wheel are constant, a smaller wheel results in the platter turning slower, albeit probably not a LOT slower.


But in the case of my turntable, it could just be that something is preventing the tire from pushing hard enough to prevent slipping.

Alfista

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Re: Had one given to me today. Not sure what model.
« Reply #43 on: August 13, 2018, 01:45:12 PM »
With constant rpm's and (c=pi*d), just a bit of math will explain it.


-Tim

rdnzl

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Re: Had one given to me today. Not sure what model.
« Reply #44 on: August 13, 2018, 03:17:22 PM »
The results, professor?

The only pie I know is served with whipped cream.