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  #16  
Old 06-26-2012, 07:46 PM
comradesvox comradesvox is offline
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Persoanlly speaking .... Do I know anyone who was killed? no. Severely and permanently injured? Though not an accidental shock from an old radio, the answer is unfortunately yes ... And that is all I care to say about it as the story has a tragic ending.

My dad's turntable, as long as it is kept in the wood case and the plastic knobs remain affixed, I guess would be generally ok under normal use, unless someone touches those cartridge leads wile grounded. With no transformer it would be like sticking your finger into the socket from what I can tell. There is also another exposed metal part - the speed selector switch - though I am not certain if any of that mechanism can present a path to exposure. I do have the knob for it but I am missing the tension spring clip that keeps it on so I will rig something up.
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  #17  
Old 06-26-2012, 08:18 PM
comradesvox comradesvox is offline
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Schematic updates ... Tone pot is 25K. Bob, you were correct ... Looking more carefully at it this time around ... Filament pin 7 goes right to ground. Cathode pin 8 is connected to the white cartridge wire, and the 150R resistor to ground. Thanks for catching that! Visio is on my work laptop so I'll adjust and repost the diagram later. My theory about how the cathode is biased then was in error - sorry!
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  #18  
Old 06-26-2012, 09:37 PM
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Revised schematic is uploaded in my original schematic post. Thanks!
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  #19  
Old 06-27-2012, 10:33 AM
bob91343 bob91343 is offline
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Another comment. I would guess the cartridge goes to ground and the volume control, not to the cathode.
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  #20  
Old 06-27-2012, 10:56 AM
comradesvox comradesvox is offline
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Bob,

The red wire from the cartridge goes to the volume control. The white wire from the cartridge however is soldered directly to pin 8 (cathode). There are two things soldered directly to pin 8 - the white cartridge wire, and the 150R resistor. Does that sound wrong? You would know better than me.
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  #21  
Old 06-27-2012, 11:25 AM
bob91343 bob91343 is offline
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I guess that's a way to get a bit more gain without adding a cathode bypass or coupling capacitor. It puts the cartridge at a slight DC potential but who cares, with so much voltage to ground anyway?

The more I look into units like this, the more unhappy I get. Sure they work and they are cheap, but my engineering sense rebels at some of the techniques.
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  #22  
Old 06-30-2012, 01:35 PM
comradesvox comradesvox is offline
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I took the motor assembly apart to service it. There do not appear to be oiling holes for the bearings - like I have seen in similar small motors for fans, leslie speakers, etc., so I'll do the best I can to get the bearings lubricated. I usually use Singer sewing machine oil.

Since I need to order the mounting grommets too I removed one and took a closer look as per the units on the VOM page. The strange thing is that my motor has straight shafts. But, the present motor mount grommets are the ones designed for cone shafts. Was this a common practice? Should I order the straight shaft mounting grommets instead?

Also, For the mechanical parts like the shifting assembly, what sort of lubricant would you recommend?

Thanks.
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  #23  
Old 06-30-2012, 04:32 PM
bob91343 bob91343 is offline
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You can decide on lubricant based on application. If it's a high speed situation (motor shaft) use thin oil. If it's a slow moving part (control lever) silicon grease may be the right stuff. If it's exposed to the elements, molybdenum disulfide is good.

I am not aware of the motor mount differences. If there is a need for side motion to correct for tolerances, then perhaps a sloppy grommet is the kind to use. Of course, if the motor moves during operation, we have a source of wow. And if you like the way it works, you will be saying wow.
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  #24  
Old 07-01-2012, 12:41 AM
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These are the kind of grommets I use:

http://www.tubesandmore.com/products/P-R407
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  #25  
Old 07-01-2012, 06:03 AM
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Hot or "warm" chassis kitchen radio, exposed chassis screws, touching these while touching the faucet with the other hand, and you have a path of juice right across your heart. Quite possible. And how many kids sat those record players out on the edge of the porch and tried to play records while standing barefoot on the ground?
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  #26  
Old 07-02-2012, 05:22 AM
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Out of curiosity ... I was looking at the spec sheet for the 25L6GT tube and the cathode resistor is listed as 180 ohms. But the resistor in my unit is 150. My resistor is out is spec and will be replaced. I already have both a 150 and 180 to replace it with. Should I stick with a 150, or use a 180 as per the spec sheet?
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  #27  
Old 07-02-2012, 10:00 AM
bob91343 bob91343 is offline
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I would use what was in there. Those specs apply to one point in the operation of the tube and the circuits usually aren't exactly at the same conditions. Having said that, however, you won't see any significant difference. The higher value will result in slightly less plate current but you'd have to measure it to see that.
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  #28  
Old 07-06-2012, 03:09 PM
comradesvox comradesvox is offline
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Update - I got the motor mounts and cartridge in from Gary. I lubed/cleaned the motor and installed the mounts. BIG difference. I also replaced the tone cap and cathode resistor, and cleaned the volume and tone pots.
Now, I am still having intermittent issues with the volume cutting out. When this happens I can touch the red lead on the cartridge I get a hum. But when I touch the needle I get nothing. So it is probably a bad connection or the cartridge dying. I want to install the new cartridge but have never done this before. The replacement cartridge comes with small metal contacts that slip over the two cartridge pins so I guess I have to unsolder the small leads and solder them to these contacts. That looks difficult. Maybe I should mount the cartridge, slip the contacts on, and then solder? I also have no idea which needle is the 78 vs LP.
Since some of you have already done this, any tips would be greatly appreciated. Thanks.
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  #29  
Old 07-06-2012, 03:16 PM
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Also I assume polarity does not matter with the cartridge. Just want to be sure.
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  #30  
Old 07-06-2012, 05:43 PM
bob91343 bob91343 is offline
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Polarity does not matter but it's better to do the soldering when the heat can't reach the innards of the cartridge. In other words, before you slip the terminals on.

Sometimes there is a ground side of the cartridge, which has a lug that connects to the metal body. Other than that, there is no polarity.

You can tell which is 78 with a jeweler's loupe or good magnifier. It's triple the radius of the long play side, much fatter.
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