Re: Tape player speed

[Mike Curtis <wd6ehr@xxxxxxxxxx>]

<Can a tape speed control be added to tape players with a power line
rheostat?>

No.  You'll have to go inside the unit, and should have a little technical
experience, i.e. measuring current, a little basic Ohms law, soldering, etc.


I haven't actually tried this, but am familiar with general principles.  I
have a Tascam 244 Portastudio that has a speed variation control, and I use
it when I need pitch/key corrections.

Better quality tape machines use a syncronous motor.  Some are synchronized
to a quartz oscillator.  Some are synchronized to the AC mains.  These will
run at a constant speed regardless of the voltage/current, until current is
dropped to a level where operation becomes erratic.  The oscillator types
can change speed by changing the frequency of the oscillator, but crystal
oscillators are quite difficult to get significant frequency change.  So the
basic answer for synchronous capstan motors is, you're stuck.

Decks using synchronous motors that have speed controls will use some type
of variable frequency oscillator, or perhaps a phase lock loop synthesizer. 
It is also possible to use a pair of crystal oscillators, varying the
frequency of one or both, and heterodyning the outputs to produce a lower
frequency, and multiplying up if required.  This increases the percentage of
variation.

Many decks these days use DC motors, especially less expensive ones and auto
stereos.  If you measure the current through the motor, and the speed change
with a small value resistor (try 10 ohms), you should be able to determine a
resistor that will allow you to slow the motor.  You probably won't need a
lot if you just want to bring the machine into tune, but if you want to
change keys, i.e F instead of G, you'll need a bit more.

If the motor has a series resistor, you can cut the value of it and add a
rheostat with about double the value you reduced it by, and have the ability
to raise or lower pitch.  Again, this should be done by experimentation.

Alternatively, you could increase the resistor value and parallel a rheostat
with double the value needed to make the parallel combination equal to the
original resistor (i.e. center position is "normal")

Naturally, you would use linear taper rheostats.

Depending on the value of current through the motor and the actual voltage
drop across the resistor, you _might_ be able to use a carbon potentiometer. 
But you'll probably be better off using a wire wound rheostat.  A
loudspeaker "volume control", rear speaker fader, or one leg of an L pad
might be other options if the value is close enough.

You don't want to put this on the 120 volts!  Trust me on this one =8*(O)

It's also not a good idea to put it in the 12 volt line of the stereo.  The
internal amplifiers and such require this power, and won't function properly
with less than 12 volts.  Also, as the amplifier uses more and less power,
it will vary the speed.

It seems that I've seen tape machines that allow you to speed up tapes while
maintaining pitch, and that these have a "pitch" control.  If you want to
change key but maintain the speed, a machine with this capability would be
perfect.


 -- mike