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Topic: Calling Electronics Gurus... (Read 923 times) previous topic - next topic

Calling Electronics Gurus...

Hi all... I know it's probably been close to a year since I've posted anything or even lurked.  I haven't touched my car in just as long either.  No job means no extra money, for anything.  Anyway, that's it for my exciting news, I need taken to school in an electronic fashion.

A month or so ago the stylus on my record player broke off and I just recently discovered I had some extra money in paypal, so I found a cheap replacement on eBay.  Put it on yesterday to discover that the tuntable doesn't... turn.  As far as I can remember, the only problem with it before was the needle, so I took it apart, being five years old, the warranty was long gone anyway.  After some poking and prodding I came to the conclusion that the main (only) drive motor for the turntable was getting it's ~ 12v DC, when it's supposed to (when the tonearm is moved from the rest or when the "start" button is engaged, but the motor doesn't turn.  Here's a picture of the motor in question:



What I ended up doing, which I now regret, was unsolder the resistor/thing you see attached to the small circuit board on the base of the motor and instead wiring the black & red wires directly to where it seemed like the two brushes connect at the end of the motor.  The thing (I know it's either probably a resistor or a capacitor of some kind maybe?) has four pins on it, I unoldered them all, thinking that, because I have all of about 2 45 RPM records that I could live with it going one speed.  Well, my plan sort of backfired.  Now the thing spins at approxamately 10x the normal 33rpm speed... I haven't counted but it's fast.

Ok so I figured I could remedy this by purchasing a simple rheostat/potentiometer from Radio Shack.  Bought a 3W/25ohm 330* sweep rheostat.  Multimeter tells me that it does indeed lower the voltage, but even turned all the way down, the platter still spins WAY too fast.

SO, in short, what I'm asking is... is there any sort of resistor I could put in line with the power to the motor or across the two terminals? Would that bring the whole speed down some?  As you can see I'm probably the last person who should be messing around with this, but so far I've only spent $6 and even with the cost of a resistor or something like that, it'll still be cheaper than a new turntable. 

Any ideas?

Calling Electronics Gurus...

Reply #1 –
the key is to measure the resistance of the motor.

if your motor is 500k ohms total
and....
your variable resistor is only max of (just saying) 5k ohms.

it depends on one of two options

place the variable resistor inline or series with the hot wire going to the motor

or

place the varible resistor in parallel,meaning bond one leg of the variable resistor to the hot wire and the other leg of the variable resistor to the return/gnd leg.


So lets do some math

motor is 100k ohms
variable resistor is max of 2kohms

wiring in parallel automatically tells me that both the motor and the resistor will have 12v but most of the time all current will be going down to the motor.
placing the variable resistor in series might be better.

all of  this is speculation without knowing the measuremet you took of the motor resistance.
You should have designed your new speed control variable resistor spec around this.

I would suspect you might find the better resistor specs on a dimmer switch for lighting or cieling fans.
you might even find it on an old volume control  ect.

you can always call if you get confused.  no fires allowed:D

Calling Electronics Gurus...

Reply #2 –
Any markings or numbers on the component you removed?
Maybe you can google it and find out what it is and maybe find a replacement.
 

Calling Electronics Gurus...

Reply #3 –
Ok, sorry to dig this thread up again but I'm just now getting around to taking another crack at this.  If I'm understanding it correctly, it looks like I already do have the variable resistor (potentiometer) wired in series with the hot wire of the motor.  In other words, the current going through the red wire's being 'regulated' by the var. resistor, correct?  Scott, if you said this is the most effective usage of the var. resistor then I'm guessing that trying to wire it across (one leg on red, one leg on black wire of motor) will do me no good.  So, it seems like I need to seek out a different var. resistor? 

Also I'm having a hard time measuring resistance across the motor.  When I set my (junky) multimeter to 200k ohms, it jumps the reading from '60' all the way up to '170' and the number just bounces.  Is the multimeter at fault? It seems to do this no matter what scale I set the multimeter at (200k, 20k, etc).