9V replacment for 12V motor

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caution

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Mar 25, 2014
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I'm trying to install a replacement motor that needs 9V, but the original motor was 12V.

I narrowed my options down to some silicon diodes in series to drop it 3V, or just use a 7809. I went with the latter since it would give a more stable voltage and won't get it very hot, since the motor doesn't seem to need much power. Without any resistance, the motor only draws 5mA but if I pinch the shaft pretty hard it jumps to 40 or 50.

Does this seem like overkill? Would a simple resistor of proper wattage be enough since the motor isn't a very dynamic load, or would it be too inefficient?
 

Superduper

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Using diodes in series is probably the simplest and most reliable method but you'll need anywhere from 4 to 6 of them and will need to string them up and test the drop. You can also use the 7809 or use a 9.6v zener and small power transistor to create a simple regulator of the type found in most every boombox out there from the vintage era. Designs like these have worked for the longest time and would make for a stable voltage supply to the motor. Regulators will keep voltage constant as opposed to diodes which although can be designed with a predetermined voltage drop, is still prone to voltage changes based on AC vs DC or depleting batteries, etc.

However, I do NOT think that a resistor is a proper way of managing power to a motor. How will you "size" a resistor? Based on 5mA or 50mA? First, motors need a stable voltage supply, not a stable current supply because a motors power consumption varies with oomph. This includes the period where it ramps up in speed. Motors have back or counter electromotive force which is not present when it is not on. In short, motors have very little internal resistance when stopped or being lugged but have high internal resistance when spun up and cruising. Resistors do not vary or regulate voltage. Instead, they limit current. When a motor needs a momentary surge in current to overcome humps in the rotational forces needed to "reset" a deck, etc., the worse thing you can do is limit that current. Anyone that has spun a flywheel by hand to "reset" one knows what I'm talking about.
 

Fatdog

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May 3, 2009
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Hey Norm, do you still have any of those motors I sent you a long time ago? I think those were all 12V motors. I don't know if they have the same specs Eric needs though.
 

caution

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I just finished wiring up the regulator and I'm having the same issue I had before, so I actually have a good 12V motor after all. It is running suuuuper fast. So fast that it's still too fast even after cranking the speed adjustment pot all the way down. It may be that the belt was never put back on properly when someone worked on this before.

It's Simon and Alvin on every setting.
 

caution

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Heh. I was about to try that, but all of a sudden it started playing at the proper speed. Then I noticed the takeup reel stopping and starting, eating my test tape, so I think it's the belt. I checked and it had tension, but not enough I guess.

I'm going to pay more for a belt than I did for the box!
 

Superduper

Member (SA)
I know what you mean. Risky business playing those ultra expensive test tapes on unknown decks. If the heads aren't properly degaussed, could permanently alter the frequency response of the tape or if it's not mechanically right, could wrinkle or eat the tapes before you know it.

The high speed tape playback probably was the result of the pinch roller and capstan not gripping the tape properly either because the roller surface has turned to plastic or not getting enough pressure from the spring. Takeup reels by design generally preload the tape to keep tension and prevent the tape from feeding faster than the take-up reel can, well, take up the slack. So if not gripped properly, the take-up reel could pull and slip the tape across the roller and capstan. On the other hand, if the reel belt is not working properly, then the pinch roller could feed faster than the reel and it bunches up and becomes lunch.

Sounds like a full deck service should be performed first because it's obviously not working properly mechanically.
 

caution

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Mar 25, 2014
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Indeed, this one was a big magnetized. I lubed it a bit and played the Wall in its entirety so it may be fine for now.
And I want my kitchen table back :lol:
 

hopey

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Dec 28, 2014
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I had the same problem must be Voltage drop across a set of contacts for the 12v supply to the motor. I haven't resolved my issue yet but was uncovered when I was using a voltage divider to drop the 12 to 9v and had the same problem with supply volts.
 

PostEnder

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Nov 21, 2012
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caution said:
I just finished wiring up the regulator and I'm having the same issue I had before, so I actually have a good 12V motor after all. It is running suuuuper fast. So fast that it's still too fast even after cranking the speed adjustment pot all the way down. It may be that the belt was never put back on properly when someone worked on this before.


It's Simon and Alvin on every setting.
Hi. By stating: "It's Simon and Alvin on every setting," you mean that playback on the cassette deck of your sound system (an Aiwa or maybe a Yamaha?) was so fast that the audio (music or not) sounded like commentary from The Chipmunks? (But what about Theodore?)

And by "the Wall," I suppose you mean the legendary -- and, from a Wikipedia check minutes ago, the very long -- 1979 album The Wall by Pink Floyd?
 
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