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MMD > Archives > August 1998 > 1998.08.18 > 25Prev  Next


Ampico B Spool Motor Governor Resistor
By Craig Brougher

I'd like to clarify further what Robbie Rhodes just explained yesterday
about the governor resistor and the series power resistor, versus the
expected temperature on the tempo lever of an Ampico B.

Robbie is absolutely correct.  However, I thought that possibly some
people might get the wrong idea, and the last thing I want to happen is
that someone will say:  "Hmmm, my tempo lever seems to be getting very
warm lately.  Well, from what I've heard, this is really a good sign.
I've read that happiness is a warm tempo lever so I appreciate it.
Gosh, we're getting first degree burns.  What else then should we
think, except that we should be unremittingly joyful -- not just happy!
I have several other friends who have Ampico B's, and they can hold
their fingers on the lever MUCH longer than I can!  So it just goes to
show them that I have more reasons to be happy than they do!"

I am just having some fun here, but I want to stress the fact that
"Warm Tempo Levers" do NOT mean a darn thing.  Everybody has their own
definition of "warm."  You never know -- there may actually be Eskimos
who own Ampicos, and when their fingers freeze to the lever for only a
minute or two, they think they have a problem.

The original governor resistor (suppresser resistor) probably doesn't
"burn out." I suspect it has to do with the lousy way they used to
attach the nickel element to the tinned leads.  Those corrode, and then
loosen gradually, until you don't have a connection, anymore.  Today,
they are resistance-welded pretty well!  But then, they didn't know how
to resistance weld a tungsten/nickel alloy to copper, except -- sorta.

The real heat in this lever is generated directly by the fire in the
contact points, once the resistance becomes "too high."  In other words,
"OPEN."  So, if your Ampico B tempo lever starts getting rather hot
after about 5 rolls, You should have it checked.  I'd say more like 10
long rolls and then you start realizing a bit of warmth, but, you know,
I may be a cold guy by nature!

That governor resistor is supposed to be scaled so that the motor will
not "creep" when you set the tempo to zero.  It is there simply to
"quench" the sparking, and to prevent motor current from ever dropping
all the way to zero, before it is re-energized.  As Robbie just said,
the lower you can get the resistance, the better.

That IS the principle, but below a certain ohmage, the motor will begin
to creep.   So there is a nice "window" in which you can be safe that
the motor cannot run through the resistor current alone.

Forget ordering special governor 360-ohm resistors.  You can't find
them anymore unless you are in an "electronics" town with lots of
surplus electronics stores.  Just buy four 1500-ohm 10-watt resistors
and parallel them together.  That will make you about the right ohmage,
and wattage.  Cement them together with EC ceramic resistor cement (if
you want to get fancy).  There is enough room in the governor box for
this.  Don't buy stuff like Dale aluminum-cased resistors, because you
could have a serious short after a number of years, hanging it from its
wires.

Now, here's what happens if you don't fix the problem quickly.  Your
roll drive motor will start overheating its armature because of its own
brushes doing too much sparking as a result of back emf, etc.  They
start varnishing and burning the armature, which causes more and more
heat and fire.  Remember, the governor points are furiously opening and
closing probably hundreds of times/minute, with too much voltage on
them.  That gets the armature hot, and it sometimes causes a solder
joint to fail at the armatures, or might actually short a turn.  That
means the motor won't start up, sometimes.

So as far as the suppresser (governor) resistor causing the heat you are
feeling in the tempo lever, it's immaterial -- when actually the lack
of a resistor (too high resistance) will give you a tempo lever that
you can really brag to your friends about!

Regarding the series resistor back under the rim of the piano, that
resistor is necessary for several reasons, but mainly current limiting.
Since it has several other taps, with 25 and 50 ohm spaces on it, if
one section burns out, you still can make combinations to get the
ohmage you need.  It's a big one, though, and expensive if you would
have to buy one; probably impossible to find!  And remember that two 25
ohms in series make 50, and two 50 ohm segments in parallel make 25.
You will have two segments of each to play with.

Craig Brougher


(Message sent Tue 18 Aug 1998, 22:02:07 GMT, from time zone GMT-0500.)

Key Words in Subject:  Ampico, B, Governor, Motor, Resistor, Spool

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