Re: Progress at last! -no

[Jon Elson <jmelson-at-artsci.wustl.edu>]

JohnDRoc-at-aol.com wrote:

> Ok, I take it back.  No progress.  What are the chances of a tach working in
> only one direction?  I've tried all amps and all DAC's on the Y axis and it
> still doesn't work!  It goes to the + direction fine, then it takes off in
> the - direction until following error or it hits the end.  Can't be good for
> it.  If this keeps up, I'm going to apply at McDonald's.

Ugh.  You may have loused up your amps by twiddling sealed adjustments.
Servo Dynamics probably had a real good reason for sealing those pots
after setting them.  They probably compensate for various component
variations in the amp.  You might be able to set them back by looking
at the glop that they were sealed with.

Yes, it is quite possible the tach is defective.  It is not unheard of for
them to have broken wires in the windings if mishandled, and since
your machine was moved, it could have gotten bumped in the
wrong place.  DC tachs are simple.  Connect a voltmeter (a scope
is better, but I gather you don't have one) to it, and turn it slowly
one way, then the other.  See if you get a moderately steady
output.  If the brushes (often fine, silver-plated wires that lightly
ride on the commutator segments) are worn out, you definitely
can get this kind of one-way operation.  I have taken apart a
few tachs, and have sometimes been able to make at least
temporary repairs.  Small Maxon and Escap (sometmes labeled
PortEscap) motors are made the same way as their tachs, and
have made good tachs for a number of people.  If your tach
is dead, I might be able to recommend a source for low-cost
replacements.

If all amps work on another axis, then the problem HAS to be
either the tach or the motor (or the cables).  And, a bad motor
would be more likely to blow fuses (or amps) and just sit, rather
than runaway.

Jon