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Old 11th March 2004, 02:58 AM   #1
g13092 is offline g13092  United States
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Default update existing TTs or save pennies for a new setup?

Hi all, fine forum you have going here. I've been lurking for quite a while, lusting after the fine DIY TT some have built. I don't have anywhere near the system some of you have, nor the ear, but I do want to get the most out of what I do have.

I've got two decks that I don't find much info about around the web:
1) a sony PS-4750 manual direct drive. Speed control pots are a bit dirty, otherwise never had a problem with it, but I can't get styli for this oddball pickering cartridge.
2) Pioneer PL-41, belt drive, AC synchronous motor, runs a little fast, tone arm has no anti-skate controls, in great physical shape, old ADC series II cart.

So, the old sony works, but I need to change those pots. I've been using it for transfer of vinyl to CD at my computer. I'd like to use the pioneer for my listening system, but there are a few problems (speed, tone arm). Is this thing worth tweaking? Or should I give up on it, ebay it outta here & save for a Rega?

I'm presently working on a 120hz strobe circuit so I can verify the speed. I believe that I can make a OPA548 circuit work to provide decent control of a DC motor (some form of Maxon?), so I was thinking that would be the next step once I confirm my fast-running conditions with the strobe. However, is it worth the effort given the rest of this deck?

To make it more interesting, this tone arm bugs me a bit. When I gently lower the needle to the vinyl, the lead-in practically whips the needle over the first few grooves, a revolutions into the first cut. If I set it to 0 grams, the arm will float half way between lead-in and runout. I read somewhere this is correct for an arm without anti-skate controls. However, this setting really seems to be causing the jump that occurs when I drop in the lead-in. Does this tonearm stink? If its worth while to upgrade the arm on this deck, where do I start? I am definitely on the low end $$$ wise...

Thanks in advance for any feedback any of you may have.

Mark D
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Old 11th March 2004, 09:17 AM   #2
sreten is online now sreten  United Kingdom
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I'd rebuild the PL41, with a replacement arm, new cartridge.

I'm puzzled as to how an AC sychronous would run slow.

Remember the Rega 2 is just a great arm bolted to plank of
wood (formica'd lowish density fibreboard) with a MDF platter.
the turntable part isn't that great.

sreten.
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Old 11th March 2004, 02:08 PM   #3
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I agree on the PL41, some internal damping and a Rega arm or something similar. The speed is strange, do the pl41 have a selector for 50/60 Hz , as a globaly sold model it could.
Would be inside player, close to cap on ac power.
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Old 11th March 2004, 02:16 PM   #4
sreten is online now sreten  United Kingdom
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A 50Hz synchronous motor would run seriously fast on 60Hz.

110/220V switching is possible, but a synchronous motor
cannot have a 50Hz/60Hz switch. the mmotor puuley needs
to be reduced in diamter or a variable AC frequency PS used.

sreten.
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Old 11th March 2004, 06:20 PM   #5
g13092 is offline g13092  United States
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Thanks for the input.

Quote:
I'm puzzled as to how an AC sychronous would run slow.
Actually, it's slightly fast, not seriously fast. There is a switch for 50/60Hz as there is another for the voltage selection. The 50/60hz switch is wired to another tap on a large cap, I should probably sketch out the wiring sometime for my own edification. It's set to 60, but maybe that cap is shot.

There are 2 pullies, a smaller one for 60hz & a larger for 50. I have the smaller one in place. At one time I was thinking of having the pullies turned down to fix the speed problem, but that seemed like a real ardous trial and error process, especially without the tools to do the work...

I'm still thinking conversion to a DC motor makes sense, but I need to figure out some sort of pully.

Mark D.
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Old 11th March 2004, 09:45 PM   #6
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Quote:
There are 2 pullies, a smaller one for 60hz & a larger for 50. I have the smaller one in place.
This sounds strange, do You have interchangeable pulleys? If the tt has a switch for 50/60Hz operation there would be no need for different pulley diameters. Your not talking about a stepped pulley, two different size "tracks" on the pulley wheel? If so thats for 33/45 rpm operation.
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Old 11th March 2004, 10:00 PM   #7
sreten is online now sreten  United Kingdom
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The only possible function of the 50Hz/60Hz switch is to change
the value of the phase shifting capacitor - given it has this switch
then the pulley must have 50Hz/60Hz steps on it.

There is the possibility it is an asychronous AC motor, which,
unlike a synchronous AC motor, allows minor speed adjustment
via some sort of braking / variable friction mechanism.
(Garrard 301, Garrard 401, Thorens TD124 use this system)
If so, and the mechanism is not functioning it would run too fast.

Looking at the size of the motor, asynchronous is possible.

sreten.
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Old 12th March 2004, 04:06 AM   #8
g13092 is offline g13092  United States
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Darn, sreten, where'd you find that picture? I swear I saw it once, shame on me for not bookmarking it.

The manual & a sticker on the plinth both clearly indicate the motor is a "4-pole hystersis synchronous motor". The only friction related item I could think of would be in the bearing. It appears to be designed to be filled with some oil, but is quite empty. Maybe there is less drag without the oil?

The two pullies are stepped for 33/45 operation. From the picture you can see the buttons on the upper left cover. These guide the belt from one step to the other. There is actually clip under the platter to hold the extra pulley. I have tried the other pulley, on both diameters, just to make sure I didn't miss something; it wasn't even close.
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Old 12th March 2004, 05:43 AM   #9
Werner is offline Werner  Europe
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[QUOTE]Originally posted by g13092


I'm still thinking conversion to a DC motor makes sense, but I need to figure out some sort of pully


If your PL41 is indeed the same as the pro-model pictured above then you might need an awfully big DC motor. Do you know if the platter is free-running, or braked in some way?
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Old 12th March 2004, 06:25 AM   #10
Raka is offline Raka  Europe
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I can't understand how an asynchronous motor can be used in order to get a fixed fixed speed. Can ayone elaborate on this brake system of the Garrard?
I suppose that would be kind of getting higher slip at no load condition, and something that relax the brake when higher torque is required, but seems to me difficult to implement with mechanical devices.

BTW, nice tt you have there, sreten.

Werner, you always can buy a low voltage synchronous motor and build an oscillator and amp box. It's worth the (small) effort.
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