Author Topic: Cassette Tape motor  (Read 5852 times)

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Offline MattHollands

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Cassette Tape motor
« on: March 26, 2015, 09:42:54 am »
Hi!

I am building a tape delay pedal. I am using parts from an old cassette player. Now, a cassette player wants to run at constant speed so uses a capstan motor, therefore I can't influence the speed of the motor much. But I need to be able to vary the speed of the motor greatly. So as I see it I have two options:

1) Modify the capstan motor so that it is effectively a DC motor and I can control the speed
2) Buy a different motor

Option 1 is good because I can guarantee that it will fit the mechanics etc. However, I have no clue how I could do this.

Option 2 is good because it's probably easier. It's bad because I can't find a motor of exactly the right size and I have had bad experiences with noisy motors before.

So I have a couple of questions:

1. Does anyone have any idea how to influence the speed of a capstan motor, beyond the small potentiometer it has inside it which doesn't give much variation.

2. If I were to replace the motor with this motor (http://www.rapidonline.com/electrical-power/trumotion-low-inertia-motor-6v-2700rpm-180710), do you think I would have an issue with noise beyond what I get from the original motor? Is there a way of knowing if this motor will be particularly noisy? (I am talking about electrical noise). This is an audio application and therefore I need to be careful

I realise this requires a little bit of speculation on your part as I can't give the specs of the original motor as I can't find it online, sorry   |O. I hope you can help.



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Offline MattHollands

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Re: Cassette Tape motor
« Reply #1 on: March 26, 2015, 10:36:10 am »
You know what, I'm really stupid. This motor has "Cassette tape recorder" listed as a typical application. Sorry to waste time and server space!  |O

p.s. If anyone can think of a reason I shouldn't use this motor, still let me know please  :-+
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Online IanB

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Re: Cassette Tape motor
« Reply #2 on: March 26, 2015, 11:17:34 am »
I do not know what the "capstan motor" is, or what it looks like. But I do know that when I have taken apart simple tape decks in the past there has been a small motor and a pinch roller that the tape passes through. The motor has just been an ordinary small DC motor. The easiest way for the tape deck to regulate the tape speed would be an electronic feedback mechanism varying the motor voltage. Do you know for certain that your existing motor is not a simple DC motor already?
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Offline MattHollands

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Re: Cassette Tape motor
« Reply #3 on: March 26, 2015, 11:26:13 am »
This is quite a nice tape deck I am using and I'm 95% certain it isn't just a normal DC motor.

For one thing, it has a hole in the top of the motor through which you can stick a screw driver and turn a pot which varies the speed very slightly.

Furthermore, when I powered it from my variable power supply, if I resisted the shaft I could feel the torque (and see the current) increase in order to maintain the turning speed.

Finally, I also put some resistors in series with the motor and found that a resistance of 0-30ohms made very little difference in speed, and then at 40 ohms it just shut off and did nothing. This suggests to me that there is some electronics attempting to regulate the speed.

I could well be wrong as this is the first time I have come across this.
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Offline Richard Crowley

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Re: Cassette Tape motor
« Reply #4 on: March 26, 2015, 11:30:53 am »
I am building a tape delay pedal. I am using parts from an old cassette player. Now, a cassette player wants to run at constant speed so uses a capstan motor, therefore I can't influence the speed of the motor much. But I need to be able to vary the speed of the motor greatly. So as I see it I have two options:

1) Modify the capstan motor so that it is effectively a DC motor and I can control the speed
2) Buy a different motor

1) You have revealed nothing about the "capstan motor" you are starting with.  Most of the later cassette gear used DC servo motors for the capstan, and varying the speed is simply a matter of diddling the servo speed control circuit.  Unless you have a really rare, vintage mains-powered cassette machine there, there is an excellent chance that you already have a simple DC motor that can easily have its speed controlled.

2) Kludging a different motor into the mechanism is typically a very difficult mod unless you are lucky enough to find a replacement motor with identical specs (body diameter and shaft diameter),
 

Offline MattHollands

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Re: Cassette Tape motor
« Reply #5 on: March 26, 2015, 11:42:45 am »
1) You have revealed nothing about the "capstan motor" you are starting with.  Most of the later cassette gear used DC servo motors for the capstan, and varying the speed is simply a matter of diddling the servo speed control circuit.  Unless you have a really rare, vintage mains-powered cassette machine there, there is an excellent chance that you already have a simple DC motor that can easily have its speed controlled.

The tape cassette I am using is this one, http://www.amazon.com/Panasonic-RQ2102-Cassette-Recorder/dp/B00004T1XK, a relatively old, mains powered cassette player. I have stated in my previous post that the motor has an internal pot which allows me to control the speed but this is inside the motor, ie I would have to take the motor apart in order to change this (which I have tried and failed). This leads me to believe that this is not a simple DC motor as it clearly has some circuitry inside. Furthermore, it is difficult for me to tell you too much about this motor as I cannot find any data about it online.

2) Kludging a different motor into the mechanism is typically a very difficult mod unless you are lucky enough to find a replacement motor with identical specs (body diameter and shaft diameter),

This is something that did worry me. The original motor was 32mm outer diameter, 25mm separated mounting holes and 2mm shaft. The replacement motor is 32mm outer diameter, 22mm mounting holes and 2mm shaft. I am hoping that this is close enough and I can simply drill one or two new holes to mount the motor.
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Offline Richard Crowley

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Re: Cassette Tape motor
« Reply #6 on: March 26, 2015, 12:02:09 pm »
Well, the capstan motor IS a simple DC motor, with some additional circuit tacked on for speed control.
With no reference to whatever you had written before, we don't know what "tried and failed" means here?

There are hundreds (even thousands) of variations on small DC motors.
Finding a replacement motor of the same physical dimensions could take months or years, and there is an excellent chance that you would never find one.

IME, you have a chance at bypassing the speed servo circuit inside the existing motor.
But you have no practical chance of finding a physical replacement for the motor.  Good luck.
 

Offline MattHollands

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Re: Cassette Tape motor
« Reply #7 on: March 26, 2015, 12:22:38 pm »
With no reference to whatever you had written before, we don't know what "tried and failed" means here?

There are hundreds (even thousands) of variations on small DC motors.
Finding a replacement motor of the same physical dimensions could take months or years, and there is an excellent chance that you would never find one.

IME, you have a chance at bypassing the speed servo circuit inside the existing motor.
But you have no practical chance of finding a physical replacement for the motor.  Good luck.
'Tried and failed' means I attempted to open the motor up but did not manage to do so. It is not clear to me how to open it up. It appears that the top plate is soldered on to the rest of the enclosure, but I can't get the solder to melt. I suspect I don't have a high enough thermal capacity iron. Either way I can't figure out how to open it, let alone bypass the control circuitry.

I don't think the motor needs to be exactly the same dimensions. The motor is connected to the mechanism by a belt and is mounted on a plate with a cm or so of space around it. This makes me think it will be pretty trivial to mount another motor there but maybe I will live to regret these words and I'm sure you have more experience than me.
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Offline BradC

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Re: Cassette Tape motor
« Reply #8 on: March 26, 2015, 12:35:08 pm »
My tape delay has a high quality DC motor driven with a simply DC circuit, the key is it is belt coupled to the capstan which has a *huge* flywheel on the back of it. Most tape delays I've seen over the years have a similar system.

The old Binson drum delay used a variable speed motor coupled to the drum with a rubber idler wheel. There are may ways to skin the cat.

Personally I'd be attacking your motor with a tin opener and/or hacksaw to get access to the speed control electronics. Most cassette motors I've looked at (and I've not looked closely for probably 20 years but I suspect they won't have changed a winning formula much) just had a small speec control circuit built into the motor. The motors are a pretty standard DC motor, but they have a high mass armature to remove the requirement for a separate flywheel.
 

Offline mrkev

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Re: Cassette Tape motor
« Reply #9 on: March 26, 2015, 12:52:43 pm »
As far as I know, there were two ways how to do this. One used AC motors that were actually synchronized by the frequency of mains (and yes, therefore connected directly to wall voltage). Those were often used in gramophones and really old cassette players.
 
The other (and obviously used in your case) method is to make some form of negative feedback. I would definitely try to get inside that motor and bypass/reuse the electronics. Even if that means gently sawing or grinding away the solder around top plate.
 

Offline amyk

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Re: Cassette Tape motor
« Reply #10 on: March 26, 2015, 03:24:19 pm »
Most cassette motors I've looked at (and I've not looked closely for probably 20 years but I suspect they won't have changed a winning formula much) just had a small speec control circuit built into the motor. The motors are a pretty standard DC motor, but they have a high mass armature to remove the requirement for a separate flywheel.
The speed regulator is quite simple and works without any additional sensors too, by sensing the back-EMF of the motor: look up datasheets on ICs such as AN6610, TDA1151, LA5586, UTC6650, etc.

http://www.precisionmicrodrives.com/application-notes-technical-guides/application-bulletins/ab-026-sensorless-speed-stabiliser-for-a-dc-motor


Agree with the suggestions to open up the motor - you'll find that it's a regular brushed DC motor with a regulator PCB attached to it. I doubt the top plate of the casing is soldered on - most are crimped. Posting some pictures would help...
 

Offline Richard Head

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Re: Cassette Tape motor
« Reply #11 on: March 26, 2015, 08:03:17 pm »
Going back 25 years the tape motors had a small linear voltage regulator circuit located on the internal round PC board located at the rear of the motor. There was a trimpot on the board that was accessible through a small hole in the rear plate. The trimpot was used to adjust the speed over a limited range. To set the speed correctly I used an 8khz head alignment test tape and connected the audio output to a frequency counter then trimmed for exactly 8khz.
All the motors I remember encountering were able to be dismantled by removing three tiny screws located around the periphery of the motor housing. Then you levered the steel end cap out revealing the PC board. I think the brushes came out also. Got to be careful here.
 

Offline G7PSK

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Re: Cassette Tape motor
« Reply #12 on: March 26, 2015, 08:45:39 pm »
The motor is a DC motor with an internal speed control, take it apart and remove the offending parts and then use your own external speed control.
 

Offline MattHollands

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Re: Cassette Tape motor
« Reply #13 on: March 26, 2015, 09:25:05 pm »
OK looks like everyone thinks I should open it up. There definitely aren't any screws in it. So not sure what to do.
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Offline MattHollands

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Re: Cassette Tape motor
« Reply #14 on: March 26, 2015, 09:42:28 pm »
OK! I Drilled out the little tab on the top of the motor and got the plate off! Let's have a look inside
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Offline MattHollands

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Re: Cassette Tape motor
« Reply #15 on: March 26, 2015, 11:05:03 pm »
Cool! I've managed to do it! I opened the motor up and found a circuit board inside. The component side was covered by a plastic case but the brushes were exposed. So I found the spot on the pcb where the brushes were soldered, scratched away traces so they were isolated and soldered in some short wires from the motor connector to the brushes.

...And it works! Awesome.

Thanks for pushing me in the right direction guys :) I've attached a photo
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