Have 12v AC, converted to 12v DC, motor slow

In summary, the conversation is about using a stater that produces AC voltage to run an 80 watt 12V DC cooling fan. The person has installed a voltage regulator to control the voltage, but after testing, the fan is spinning slower than when powered by a 12V battery. The conversation then delves into the possible reasons for this, including the voltage drop across diodes and the fact that the output voltage of a rectified signal is not a flat DC voltage. The idea of using a capacitor instead of a battery is also discussed, with suggestions on the size and type of capacitor needed. The conversation also touches on the frequency and phase output of the stator, as well as the operation of the regulator. Suggestions are made
  • #1
Mikel_NY
41
0
Hello All,

I have a stater that puts out AC voltage, at max RPM it produces 40 volts. I have a voltage regulator installed parallel and it controls volts to max 14+v AC.

I need to run an 80 watt 12V DC cooling fan so I installed a little block that has 4 diodes inside. Possibly this is called a rectifier, I am not sure. I was told how to install with AC on two posts and DC load on the two posts that are labeled "+" and "-".

After install and test I have found that the fan is spinning slower than if I powered the fan from a 12V battery. ( I would like not to run a battery )


Would a DC motor run slower with a 12V AC converted to DC than just wiring it to a 12V DC car battery?




Please explain in simple terms as this is not my field of expertise.



This is a snowmobile motor with a 230 watt stater plate and I am cooling it with a radiator and fan similar to a car setup. the fan is 12v DC 80 watt.

Thank you in advance for any input to help with original fan speed to gain more airflow to cool motor.
 
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  • #2
Welcome to PF.
How much slower?

The assumed “bridge rectifier” you are using will have silicon diodes that will drop about 1 volt each when conducting. That reduces your 14V to 12V or less.

If you have wired it wrongly then you might be getting 12V for only half the time.

AC is measured as RMS. So the peak voltage of AC is √2 = 1.4142 times the equivalent DC.

Does your regulator set the peak at 14V for battery charging? You may need a small battery to maintain the 14V all the time.
 
  • #3
There's two main reasons I can think of that would affect your motor:

  1. The output voltage of a rectified signal is not a flat DC voltage, it is actually just a sin wave except the negative part of the wave is reversed to be positive. The average voltage of this wave is lower than your RMS voltage. This picture represents that well (taken from isaac42's post on HarmonyCentral Forums). Just pretend the 24V RMS is your 14V and the ratio of 24V to 15.3V still stands.:

    2011521194313_rectified%20AC.PNG

    Source: http://www.harmonycentral.com/forum/forum/Forums_General/acapella-94/293518-


  2. There is a voltage drop across diodes, usually about 0.7 V. This will also reduce your effective DC voltage.
 
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  • #4
the outflow of air is noticeably less using the converted AC power than just using the batter power.

Knowing VERY little on this subject ... could a cap be installed to help in this problem?(I only know that a cap is like a very small battery. it stores power to be discharged later when primary voltage drops.)I would like to eliminate a battery for weight saving, can it be done in a different way? maybe I just need to run a battery.
What is the best way (and LIGHTEST) to run a 80 watt DC motor (fan) the fastest for cooling with a given 12-14 Volt AC supply rated for 230 watts? ( tomorrow I will meter the AC volt at idle and then after the gismo measure the "DC" voltage and see what drop there is.)
 
  • #5
here is a picture of how my "gismo" was explained to me. It is the diamond shape in the middle.

would guess the left image is a transformer which I don't have because my stater is already outputting 12-14V AC.In the picture, on the right there is a straight line over a curve (frown) line. this I think is a cap. If this is true, homework would I size one if it would help the fan spin faster?

If one is installed ... is there a cycle/service life for a cap?
 

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  • #6
Does the stator produce 1, 2 or 3 phase output?
Is it wired as a 3 phase star with the centre connected to the chassis, or maybe a floating delta?

If your 14V regulator is for battery charging it will be regulating the peak AC, not the RMS

To use a capacitor instead of a battery;
An 80W fan on 12V will draw a current of 80W/12V = 6.67A
What is the frequency of the AC? If 100Hz then period T = 10msec.
C = I * T / dV; For a 1.0V droop in the capacitor during the cycle;
C needed will be C= 6.67A * 0.01sec / 1.0V = 0.0667F = 66.7mF, rated at 15V or better.

Maybe buy ten x 10,000 uF capacitors. US$2 each.
Wire them in parallel to give 0.1F that will handle the current at the stator output voltage peaks.
 
  • #7
Are you sure that regulator gives 14volts AC not DC ?
I'm guessing that your snowmobile regulator is similar to the motorcycle regulators I've seen. Their operation verges on brutal.
What they do is first convert to DC then measure the voltage across the battery and figure out how much of the sinewave needs to be let through to keep about 14.5 volts there.
At some point in the sinewave the regulator throws a dead short across the alternator which removes the remainder of the alternator's ac sine wave.
In Kavik's sketch above, the regulator would chop off the wave as required to maintain battery voltage about 14.5 .

With no battery present to absorb current i'd be at a loss to predict how your regulator would work. It'd probably chop the wave early which would give you lower average voltage across the motor.
In Kavik's sketch, draw a line straight down from each sinewave peak and erase the right half of the sine wave.

That's how the regulators that I've seen work. That'd give you lower average voltage at the fan.

You might try a large capacitor across your fan.

80 watts at 12 volts is 6.67 amps.
Around 5000 microfarads of capacitor should make the regulator think there's at least a tiny battery out there. The more capacitance the better. You could try a few of these.
https://www.amazon.com/s/ref=nb_sb_...=6800&rh=n:306788011,k:6800&tag=pfamazon01-20
41fAiiO2ukL._SP160,160,0,T_.jpg


EDIT: Ahh i see Baluncore already answered. We're thinking pretty much alike.
 
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  • #8
I would say that the regulator is outputting AC because the fan would just twitch when turned on, it would not spin until I installed the square block and I have the meter set on AC to read the voltage output.

The regulator is wired parallel from the factory if that bit of information helps. I am clueless on Caps ... I like Jims post showing me what to buy, I like Baluncore's post telling me exactly what I need.

Just looking at the numbers, they are not the same. Am I wrong here? I like to ask ten questions and get this correct the first time than try once and fry some wiring.I think I can test for frequency, I do not know if the stator is single or 3 phase. If you describe a test method, I can report back. I know that the stator is a 6 pulse and there is 2 wires coming out, Positive and ground.
 
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  • #9
Balun and i assumed different frequencies, he used 100 hz and i guessed at 1000.

He used mf for millifarads, i used microfarads which also starts with "m".

We're not so far apart.

You just need plenty of capacitance. British motorcycle racers in the late fifties used a large capacitor to replace their battery. Manufacturers thoughtfully oriented the rotor magnets to provide a pulse of current just when the ignition coil needed it. If you ever kick-started a 500cc single you really appreciate that !

Anyhow - a few tens of thousands of microfarad should do the job. More won't hurt.

You might think of a capacitor as a tank that stores electric charge.
The unit of capacitance is the farad.
A farad is a lot of capacitance. We usually work in millionths of a farad, microfarads.
Think of a farad as a 55 gallon drum compared to a microfarad , which holds just a thimblefull.

Do you have a friend who'd loan you a car stereo capacitor? Walmart sells a 1.5 farad (1.5 million microfarads) for $25.
http://www.walmart.com/ip/Power-Aco...5F-1.5-Farad-Digital-Power-Capacitor/21607606

here's some 10,000 microfarad like Baluncore suggested

http://www.futurlec.com/Capacitors/C10000U25Epr.shtml

these are 25 volt rated . 16 volt would probably be okay but i like a margin of safety.
Ten of them in parallel would be 1/10th of a farad.

capacitor goes on DC side of your rectifier - right across fan wires. Observe polarity : + to +
 
  • #10
I dialed meter to Hz and hooked probs to AC regulated wires. The voltage at idle (1500) is 12.3, the meter on Hz read about 150. When motor was reved to say 4000 the meter switched range to "K" and meter read ".5xx".

I than moved the probes to the "DC" output and it was 9.3V DC, no AC measurable.




DO the caps, when installed, boost voltage average like the average line in pic of post #3?
 
  • #11
You have two terminals from the stator, that is single phase. If the chassis wire remains connected to the chassis, then the bridge rectifier will produce +ve and –ve to the capacitors and fan. Both the +ve and –ve will be swinging together in voltage relative to the chassis connection.

High RPM gives about 500 Hz so that will give a ripple frequency of 1kHz, T = 1 msec.
C = I * T / V = 6.67A * .001sec / 1.0V = 0.006670F = 6670 uF
Voltage will be lower at idle when cooling may be not needed so much.

For the same fan voltage at idle,
Idling, at 150Hz, will give a full wave rectified ripple frequency of 300 Hz. T = 3.3msec
C = I * T / V = 6.67A * .0033sec / 1.0V = 0.022011 F = 22000 uF.

Yes, the caps raise the average voltage. The caps are charged only during the voltage peak from the rectified AC, they must store enough energy to run the fan through the rest of the half cycle. If the current flows for half the time, the current into the cap will be twice the average current out. If the current flows for 1/5 of the time, the current into the cap will be five times the average current out. The peak current could be maybe 35 amp. That peak current must be handled by the bridge. The cap(s) will need to be low ESR. I prefer multiple capacitors to distribute the peak current amongst several smaller units.
 
  • #12
Can the regulator handle a 35 amp charge current? Is it a permanent magnet alternator designed only to charge the battery?

If the alternator has no controlled field winding then you might consider removing the series regulator for battery charging, then use a single diode to the fan.

Also consider rectifying a chassis referenced AC, but half wave, with only two diodes into separate positive and negative rails, each with it's own capacitor(s). That will double the fan voltage.
 
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  • #13
the motor will max out in the 8500 RPM range, I only rev the motor on the stand to said 4000 rpm.

The stator is stationary, there is a flywheel that rotates around the stator with permanent magnets attached to flywheel.

The regulator is standard factory issue and installed as it comes from a stock factory install. that being said, it was not designed to run a DC motor. Just a few light bulbs ( about 150 watts total ) and heated hand grips that could be 45 watts.

They sell a 3 wire regulator that is designed to charge a battery if sled was ordered with electric start.

I would like to run the fan in my application as fast as possible ( w/o burning it out ) for max cooling without having the "heavy" battery. I'm trying to lighten the chassis as much as possible.
One of the two wires coming out of the stator goes to ground. I also have a ground wire from engine block to regulator body.



So with the new information of the motor max RPM in the 8300 rpm range and not testing it but measured at idle the Hz was 150, and mid 400 RPM Hz want to 500 ... DO I need to measure the Hz at top RPM to size the caps correctly?

the square block is a Motorola MDA 980-2, 8723 W Bridge Rectifier. I see a 2V drop on the DC side vs. the AC side.
 
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  • #14
MDA980 is a 1970's 12 amp rectifier.

Since the original alternator only needed to run lights and heaters there was no need to turn it into DC. So they might not have.

On motorcycles one disconnects the regulator and measures stator AC voltage with nothing but a meter connected to it. That's called "open circuit stator test".
That way you have nothing but magnets and wire active, no electronics to fool you.
Around twelve volts idle, forty or fifty at midrange rpm . Voltage should be linear with RPM.
Don't know what voltage to expect on yours. But if open circuit voltage doesn't go up with rpm it sounds like stator troubles.

If it voltage goes up nicely with rpm, doubling when rpm doubles, then stator and magnets are probably fine. Reconnect regulator and continue testing.

old jim
 
  • #15
ps i see no need for voltage measurement at top rpm. I don't like to run engines that fast unloaded.
 
  • #16
here is my wire schematic. the only part that is confusing is the stator plate is labeled engine ground and the regulator is chassis ground. should this be separate? I don't see a closed circuit if the two grounds are not connected.
I was asking If Hz measurments are needed at top RPM to size the caps correct. just guessing.
 

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  • #17
I have tested open voltage and it climbs with RPM to a may of 40V AC. the book says up to 45V AC is good.
 
  • #18
I did all testing with book specs to stator as open voltage and wire to wire resistance. All very close to book. book also says 20% error is ok and testing should be at 68 degrees F.
 
  • #19
jim hardy said:
MDA980 is a 1970's 12 amp rectifier.

Is this "big enough" for running a 12V 80 watt DC motor (rad fan) continuous for 20 minutes at a time in under hood heat of say 200 degrees F?
 
  • #20
T to Brn must be the lighting coils? That's where you'd make the open circuit stator test.

I'd wager that W-WR is trigger coil for CDI and G-R-BR is power for CDI.

Surely engine is bolted to chassis so grounds are common - is there not a braided link like in modern cars?
 
  • #21
sounds like your stator and magnets are fine

yes 12 amps is plenty for that 80 watt fan, about seven amps . Feel of it - if it's getting hot mount it on a piece of aluminum in the airflow.

good work you have eliminated a lot of potential unknowns.

With Balun's excellent tutelage you'll be going soon.

Have fun !
 
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  • #22
yes the Brown (ground) on the right and the Left wire that goes over the top of the stator is the Yellow wire (positive).

I have the engine bolted to the chassis and the case of the regulator has a 12g wire bonded to the engine case.that print is out of the Polaris book. when the motor is installed in a sled. the motor is rubber mounted, the carbs are rubber mounted and the exhaust is rubber mounted so I can't see where the original application's engine and chassis ground met. there was no bond wire I can think of.

With that being said ... I do not have this motor in a stock chassis. the motor is now hard mounted to chassis and grounded to chassis.
 
  • #23
I thank both of you for your help. I just need a final answer as to sizing and quantity of the caps. then its off to the races so to speak.
 
  • #24
It's not that easy. We need to identify the existing regulator capabilities. Is it a linear or switching regulator?

I suspect your existing regulator may not handle the 6.67A fan current when the rectified stator voltage is high.
I expect you will need a switching regulator for the 14V, 6.67A fan supply. Unfortunately, cheap buck regulators only work up to about 32V, like this example;
http://www.dx.com/p/10a-dc-cnoverte...ge-regulator-power-supply-214277#.VAPZRaOuDs4
The advantage is the high conversion efficiency. 6.67A out at 14V will only require 3.12A input at 30Vin.
 
  • #25
It is a shunt regulator.

Most of these types use a zener, or amplified zener to chop off part of only one cycle of the waveform.
The reverse cycle should be the full voltage from the stator.
It is what is called a half wave regulator.
Simple and few part count, with the excess current being dumped to grd.

So what is going into the regulator is one half of a sine wave and a chopped sine wave on alternative parts of the cycle.

I doubt if this type of regulator is any different than what is used, or has been used for the last 50 years or more for running the always-on headlamp, taill lights and brake lights with no battery on snow mobiles. In fact, even snowmobiles with a battery and a DC charge circuit, the lights run on this AC waveform, but I have no idea of how the DC is made compatable with the AC so that cross currents do not flow.

Using a bridge rectifier complicates things somewhat as the GRD of the motor does not match the GRD of the stator, engine, or chassis.
 
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  • #26
Hmmmm well i learned something.

What i expected to find is in figure 22 here:
http://www.schematicsforfree.com/archive/file/Automotive/Other/Charging%20and%20ignition%20systems%20for%20atv%27s%20nbr18.pdf

but it seems you have something closer to figure 16, with your bridge tacked on behind.

I'll guess their battery equipped snowmobiles are closer to figure 19 which does what you said - clips one half of the wave and let's the other fall where it will.

The reason such simple regulators work with permanent magnet alternators is this:
Any current flowing in the stator makes a magnetic field that opposes the one from rotor's permanent magnets .
That makes an alternator inherently current limiting - when voltage rises so does stator current which weakens total magnetic field, pushing voltage back down.
That's why the resistor in figure 19, so voltage won't rise when headlight is switched off.
Whatever current is left over after lighting on positive half cycles can go into battery. Regulator makes sure it won't overcharge battery.

gotta go, more later.

see also this guy's page:

http://mastercircuits.blogspot.com/2012/07/half-wave-to-full-wave-conversion.html
 
  • #27
from post #16: here is my wire schematic. the only part that is confusing is the stator plate is labeled engine ground and the regulator is chassis ground. should this be separate? I don't see a closed circuit if the two grounds are not connected.

I can't quite make out the symbols inside triangles.

Looks like regulator is across main winding , brown and yellow, as expected for a shunt regulator.

Brown looks like it's grounded to engine at triangle adjacent stator? Is that an "E" inside triangle?
It's also grounded at voltage regulator by triangle with a "C" inside?

Are those two grounds maybe one at engine and one on chassis?
If so the brown wire ties engine to chassis.

Now one wonders how alternator would behave if it powered the fans with no regulator. Would alternator's inherent limiting keep voltage manageable ?

Do your fans have electronics inside them or are they just plain old brush type motors?
Clearly the engine will run without a regulator else you couldn't have made that open circuit stator test.

Sorry, i wasn't going to meddle further. Just i sensed we were all missing something about the regulation scheme. It's halfway between my lawnmower(none) and a Suzuki 650(three phase full wave). Your term "half wave regulator" was new to me, i never encountered one before.
And living in South Florida nearly all my life i never encountered snowmobiles.

What other interesting tidbits have you for us?

Have at it you younger and quicker minds.

old jim
 
  • #28
I thank both of you for your help. I just need a final answer as to sizing and quantity of the caps. then its off to the races so to speak.

Balun has sized them at 22,000 uf. I sized them at "a few tens of thousands of microfarads." We agree there.
Two or three 10,000's should do. More won't hurt.
Capacitors have a rating for AC current . It's called "Ripple Current".
Basically the larger the case size the more AC current they can handle - there's more surface area to get rid of the heat. That's why he suggested using several normal caps instead of one humungous one like i linked.
Feel them and if they're getting hot to touch we'll have to address capacitor heating.
Just for ballpark: this series of (very good) capacitors has ripple current rating of about four amps.
http://www.vishay.com/docs/28336/120atc.pdf
Without calculating anything, conservative common sense says :
Seven amps divided between just two of their 6800's would be fine.. four or five would be better to get more capacitance.
You'd want four or more of this slightly less expensive but more readily available capacitor with only about two amp ripple rating..
MAL213816682E3
http://www.digikey.com/product-search/en?vendor=0&keywords=+MAL213816682E3+&stock=1

That's one example of why i remarked about Balun's excellent tutelage.

Sorry if this seems like rambling. I try to explain "Why" as well as "what"; after all this is a Physics forum.. physics has a practical side though.

over and out...

old jim
 
  • #29
jim hardy said:
Brown looks like it's grounded to engine at triangle adjacent stator? Is that an "E" inside triangle?
It's also grounded at voltage regulator by triangle with a "C" inside?

Yes, E inside triangle is engine ground and C inside triangle is chassis ground.


jim hardy said:
Now one wonders how alternator would behave if it powered the fans with no regulator. Would alternator's inherent limiting keep voltage manageable ?

The fan would not turn. I only saw the fan quiver when switched on/off when connected directly to Yellow Brown from stator.

jim hardy said:
Do your fans have electronics inside them or are they just plain old brush type motors?
Clearly the engine will run without a regulator else you couldn't have made that open circuit stator test.

I believe the fan is a plain cheap brush motor, nothing fancy.

The engine only spins the stator, I see two other circuits that power the CDI and sink the rotation timing.

The book has a test to unhook regulator and test fro AC up to 45V. mine tested to 41V AC. book says that is good.

the stock wire harness has the regulator wired in parallel w/ the Yellow Brown wires off the lighting side of the stator. the motor knows nothing if it is working or not.

I have the tach hooked to the Yellow brown wires to count the pulses and show RPM. If regulated or not that still reads pulses. I have another set of Yellow brown going to the bridge rectifier. the fan also hooked to the other two posts of BR.

With this setup, the motor runs (up to 5000 rpm tested), the light bulbs don't blow (new regulator) and the fan spins ( bridge rectifier).My issue now is fan speed ... I think the fan is spinning slower than when hooked directly to the battery.
 
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  • #30
jim hardy said:
Sorry if this seems like rambling. I try to explain "Why" as well as "what"; after all this is a Physics forum.. physics has a practical side though.


I find none of this "rambling"! I find this a type of school ... you know this, I don't. This is awesome to me that there are people that are good at something to share knowledge and be patient enough for me to learn another fraction of something I didn't know yesterday.
 
  • #31
jim hardy said:
Two or three 10,000's should do. More won't hurt.
Capacitors have a rating for AC current . It's called "Ripple Current".

Do i understand that I will be installing the caps on the DC side, parallel with the fan?

Now I read that caps have a rating for AC ripple ... Do I install two different types before and after the rectifier? ... I'm only asking for clarification.
 
  • #32
The fan would not turn. I only saw the fan quiver when switched on/off when connected directly to Yellow Brown from stator.

Correct. Fan is probably a permanent magnet motor. They won't run on AC like the "universal" motor in your Skilsaw or drill.
Yellow to Brown is AC . And it's regulated.

What i'd like to try is:
Regulator and lights disconnected.
Yellow - Brown connected to AC terminals on bridge
Fan connected to DC terminals on bridge, voltmeter same place to read fan voltage. Note fan won't be grounded.

Then start engine, observe fan voltage (and current if you have an ammeter?) at idle
then gingerly increase engine speed until you see almost 15 volts on fan, note RPM. (and fan current?). Don't overrev engine though.
Now we'd know the raw capability of the alternator .
And can figure out a test to determine whether that half wave regulator is appropriate.
Baluncore questioned that way back in the thread.

Do i understand that I will be installing the caps on the DC side, parallel with the fan?
Yes. Exactly.

Now I read that caps have a rating for AC ripple ... Do I install two different types before and after the rectifier? ... I'm only asking for clarification.
No. Not yet, anyhow...
Rectified AC makes a lumpy wave with peaks as in Kavik's post #3. The capacitor's job is to store current between the peaks , to fill in the valleys if you will pardon that informal phrase.
So current rushes into the capacitor on the peaks and dribbles back out between peaks. That's AC. And it's called "ripple". (see http://en.wikipedia.org/wiki/Ripple_(electrical) )
So your capacitors go on the DC side.

Try adding some capacitors across fan with the setup you have now, regulator in place. You might find some in a junk computer power supply or TV set from a trashpile just for an experiment.

I hope this project helps you understand the snowmobile alternator. It's an interesting device, closely related to the synchronous machine. We learn fastest by doing.

old jim
 
  • #33
jim hardy said:
And can figure out a test to determine whether that half wave regulator is appropriate.
Baluncore questioned that way back in the thread.
I'm eager to see where this comment will be going, sounds like "2 ways to skin a cat". ( my interest is peaked). Are you thinking that the factory regulator is the cheapest production item that served the original application and there may be a better regulator for my current application depending on if the stator makes enough juice? (just a guess)

Edit: Just reread Baluncore's post 24, (that must be where I got the idea from)

Jim, I will do some more testing tomorrow and report results.
 
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  • #34
Edit: Just reread Baluncore's post 24, (that must be where I got the idea from)

Yep. I'm unabashed - will steal a good idea anywhere. Balun is a cornucopia of them.

Thanks Balun - as usual i was a day behind...

old jim
 
  • #35
jim hardy said:
What i'd like to try is:
Regulator and lights disconnected.
Yellow - Brown connected to AC terminals on bridge
Fan connected to DC terminals on bridge, voltmeter same place to read fan voltage. Note fan won't be grounded.

base test:
Fan RPM of--1840 --(Directly hooked to a 12V battery at battery V of 12.4 sat overnight, not fresh charge)

Fan RPM is--1680 at 12.30-12.40V AC / 9.7VDC engine at 1500rpm idle regulator hooked up

Fan at 15V DC, unhooked reg/no bulbs tach 2800, fan speed 2250

Fan speed: 1720-1730 @ 10.3VDC engine idle 1500, reg plugged in, one 35V 10,000 mfd installed DC side in parallel
same engine speed, fan RPM increase of 40 RPM w/ cap.


Sorry, no amp readings at this time. Please ask or reask a question if more information is needed.

Mikel
 
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