On 2010-02-21, Ned Simmons wrote:
One axis on my grinder readout is reading erratically and the symptoms
point to the pulse detection/shaping circuit being out of alignment.

[ ... ]

I've fixed similar problems in optical encoders, but this is a
Magnescale and most of the circuitry is encapsulated in what appear to
be three mini circuit boards - the long brown devices in the photo.

Hmm ... if those are what they *look* like, I would expect only
resistors and capacitors under the coating -- mounted on a ceramic
substrate with silk-screened metalization for conductors. And the
coating is fired on, like for a ceramic capacitor, so solid state
devices (transistors, diodes, etc) would not survive the firing
temperatures.

Just how old *is* this thing? The connectors look more modern
than the packaged circuits would suggest.

I
haven't been able to make an informed guess as to the function of the
several pots. The optical encoders I've fussed with have one pot for
each channel that tweaks the switching point of a comparator; this is
clearly more complicated.

http://www.suscom-maine.net/~nsimmon...caleHead01.jpg

Hmm ... it might be that the comparator is at the other end of
the cable, and the circuitry is mostly to match impedance so there is no
reflection of pulses.

Looking at the wires, and assuming that ground is carried by the
shield, while the red is likely the power to drive the LEDs -- also out
of period with the packaged circuits, which were from the 1950s IIRC.
Anyway -- that leaves five signal wires -- two for the 'A' sensor (one
inverted) and two for the 'B' (also one inverted), which produces the
quadrature. That leaves one for the Index (saying that it has reached a
certain reference point on the scale. This matches the five pots, which
probably set amplitudes for each signal fed downstream to the
comparators. Some systems expect to be fed sine waves from the sensors
and use intermediate voltages to interpolate extra resolution.

Before I hook this up to a scope and start fiddling with the pots,
does anyone have or know of an aligment procedure for this unit or any
similar?

That I don't -- sorry.

I would first try checking the waveform from a constant speed
for each of the signals (expecting one of them (the index) to only
change once at an extreme end of the scale.) If any one has a lower
amplitude sine wave than the others, it is suspect. But it might be the
comparators back at the electronics/display box instead.

Good Luck,
Don.

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